From e725523898b1b6e310b38fb069b4890878d6eb50 Mon Sep 17 00:00:00 2001
From: Eric Coissac <eric@coissac.eu>
Date: Wed, 8 Jul 2026 12:39:21 +0200
Subject: [PATCH 1/3] feat: add entropy-driven k-mer complexity filtering

Introduces a MinComplexity filter driven by new CLI arguments, enabling sequence-aware threshold checks during index reconstruction and partitioning. Adds the kmer_entropy module for normalized complexity scoring, updates the KmerFilter trait to evaluate per-kmer context, and refactors test modules for better organization.
---
 src/obikmer/src/cmd/filter.rs              |  21 ++-
 src/obikmer/src/main.rs                    |   2 +-
 src/obikpartitionner/Cargo.toml            |   2 +-
 src/obikpartitionner/src/dump_layer.rs     |  38 ++---
 src/obikpartitionner/src/filter.rs         |  62 ++++++--
 src/obikpartitionner/src/rebuild_layer.rs  |   4 +-
 src/obiskbuilder/src/iter.rs               | 156 +--------------------
 src/obiskbuilder/src/kmer_entropy.rs       |  49 +++++++
 src/obiskbuilder/src/lib.rs                |   2 +
 src/obiskbuilder/src/tests/iter.rs         | 152 ++++++++++++++++++++
 src/obiskbuilder/src/tests/kmer_entropy.rs |  54 +++++++
 11 files changed, 350 insertions(+), 192 deletions(-)
 create mode 100644 src/obiskbuilder/src/kmer_entropy.rs
 create mode 100644 src/obiskbuilder/src/tests/iter.rs
 create mode 100644 src/obiskbuilder/src/tests/kmer_entropy.rs

diff --git a/src/obikmer/src/cmd/filter.rs b/src/obikmer/src/cmd/filter.rs
index 993f80a..ae3f45f 100644
--- a/src/obikmer/src/cmd/filter.rs
+++ b/src/obikmer/src/cmd/filter.rs
@@ -2,14 +2,14 @@ use std::path::PathBuf;
 
 use clap::Args;
 use obikindex::{KmerIndex, MergeMode};
-use obikpartitionner::filter::{MaxTotalCount, MinTotalCount};
+use obikpartitionner::filter::{MaxTotalCount, MinComplexity, MinTotalCount};
 use obisys::Reporter;
 use tracing::info;
 
 use super::predicate::FilterArgs as KmerFilterArgs;
 
 #[derive(Args)]
-pub struct FilterArgs {
+pub struct FilterCmdArgs {
     /// Source index directory
     pub source: PathBuf,
 
@@ -28,6 +28,18 @@ pub struct FilterArgs {
     #[arg(long)]
     pub max_total_count: Option<u32>,
 
+    /// Minimum normalized entropy (complexity) to keep a k-mer — same metric
+    /// as `obikmer index`'s --theta, applied here to k-mers already committed
+    /// to the source index (reconstructed from unitigs.bin). K-mers scoring
+    /// below this are removed.
+    #[arg(long)]
+    pub min_complexity: Option<f64>,
+
+    /// Maximum sub-word size for the complexity computation (see `obikmer
+    /// index`'s --level-max). Only used when --min-complexity is set.
+    #[arg(long, default_value_t = 6)]
+    pub complexity_level_max: usize,
+
     /// Output as presence/absence instead of counts
     #[arg(long)]
     pub presence: bool,
@@ -37,7 +49,7 @@ pub struct FilterArgs {
     pub force: bool,
 }
 
-pub fn run(args: FilterArgs) {
+pub fn run(args: FilterCmdArgs) {
     let src = KmerIndex::open(&args.source).unwrap_or_else(|e| {
         eprintln!("error opening source index: {e}");
         std::process::exit(1);
@@ -62,6 +74,9 @@ pub fn run(args: FilterArgs) {
     if let Some(v) = args.max_total_count {
         filters.push(Box::new(MaxTotalCount { total: v }));
     }
+    if let Some(theta) = args.min_complexity {
+        filters.push(Box::new(MinComplexity { level_max: args.complexity_level_max, theta }));
+    }
 
     let mut rep = Reporter::new();
     KmerIndex::rebuild(&args.output, &src, &filters, mode, args.force, &mut rep)
diff --git a/src/obikmer/src/main.rs b/src/obikmer/src/main.rs
index a0b270b..cb701e6 100644
--- a/src/obikmer/src/main.rs
+++ b/src/obikmer/src/main.rs
@@ -21,7 +21,7 @@ enum Commands {
     /// Merge multiple built indexes into one
     Merge(cmd::merge::MergeArgs),
     /// Apply row-level selection (σ) to an index: retain only k-mers matching the predicates
-    Filter(cmd::filter::FilterArgs),
+    Filter(cmd::filter::FilterCmdArgs),
     /// Project and/or aggregate genome columns into a new or in-place index
     Select(cmd::select::SelectArgs),
     /// Query an index with sequences and annotate matches
diff --git a/src/obikpartitionner/Cargo.toml b/src/obikpartitionner/Cargo.toml
index 1c42cab..a0c810e 100644
--- a/src/obikpartitionner/Cargo.toml
+++ b/src/obikpartitionner/Cargo.toml
@@ -6,7 +6,6 @@ edition = "2024"
 [dev-dependencies]
 tempfile = "3"
 obikseq   = { path = "../obikseq",   features = ["test-utils"] }
-obiskbuilder = { path = "../obiskbuilder" }
 obiread   = { path = "../obiread" }
 obikrope  = { path = "../obikrope" }
 
@@ -14,6 +13,7 @@ obikrope  = { path = "../obikrope" }
 niffler = "3.0.0"
 remove_dir_all = "0.8"
 obikseq      = { path = "../obikseq" }
+obiskbuilder = { path = "../obiskbuilder" }
 obiskio      = { path = "../obiskio" }
 obidebruinj  = { path = "../obidebruinj" }
 obilayeredmap = { path = "../obilayeredmap" }
diff --git a/src/obikpartitionner/src/dump_layer.rs b/src/obikpartitionner/src/dump_layer.rs
index e1c8226..00a8507 100644
--- a/src/obikpartitionner/src/dump_layer.rs
+++ b/src/obikpartitionner/src/dump_layer.rs
@@ -62,7 +62,7 @@ impl KmerPartition {
                 for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
                     if let Some(slot) = mphf.find(kmer) {
                         let row = mat.row(slot);
-                        if passes_all(filters, &row, n_genomes) {
+                        if passes_all(filters, kmer, &row, n_genomes) {
                             cont = cb(kmer, row);
                             if !cont { break; }
                         }
@@ -75,7 +75,7 @@ impl KmerPartition {
                 for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
                     if let Some(slot) = mphf.find(kmer) {
                         let row: Box<[u32]> = mat.row(slot).iter().map(|&b| b as u32).collect();
-                        if passes_all(filters, &row, n_genomes) {
+                        if passes_all(filters, kmer, &row, n_genomes) {
                             cont = cb(kmer, row);
                             if !cont { break; }
                         }
@@ -83,16 +83,17 @@ impl KmerPartition {
                 }
                 cont
             } else {
-                // No data matrix: implicit presence — all values are 1.
-                // The filter result is identical for every kmer, so evaluate once.
+                // No data matrix: implicit presence — all values are 1. `row`
+                // is identical for every kmer, but a filter can still depend
+                // on the kmer's own sequence (e.g. MinComplexity), so this
+                // cannot be evaluated once for the whole layer — filters must
+                // still be tested per kmer.
                 let all_present: Box<[u32]> = vec![1u32; n_genomes].into();
                 let mut cont = true;
-                if passes_all(filters, &all_present, n_genomes) {
-                    for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
-                        if mphf.find(kmer).is_some() {
-                            cont = cb(kmer, all_present.clone());
-                            if !cont { break; }
-                        }
+                for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
+                    if mphf.find(kmer).is_some() && passes_all(filters, kmer, &all_present, n_genomes) {
+                        cont = cb(kmer, all_present.clone());
+                        if !cont { break; }
                     }
                 }
                 cont
@@ -140,7 +141,7 @@ impl KmerPartition {
                 for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
                     if let Some(slot) = mphf.find(kmer) {
                         let row = mat.row(slot);
-                        if passes_all(filters, &row, n_genomes) {
+                        if passes_all(filters, kmer, &row, n_genomes) {
                             cont = cb(part, layer, kmer, row);
                             if !cont { break; }
                         }
@@ -153,7 +154,7 @@ impl KmerPartition {
                 for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
                     if let Some(slot) = mphf.find(kmer) {
                         let row: Box<[u32]> = mat.row(slot).iter().map(|&b| b as u32).collect();
-                        if passes_all(filters, &row, n_genomes) {
+                        if passes_all(filters, kmer, &row, n_genomes) {
                             cont = cb(part, layer, kmer, row);
                             if !cont { break; }
                         }
@@ -161,14 +162,15 @@ impl KmerPartition {
                 }
                 cont
             } else {
+                // Same as iter_partition_kmers: row is constant but a filter
+                // may still depend on the kmer's own sequence, so this must
+                // be tested per kmer, not once for the whole layer.
                 let all_present: Box<[u32]> = vec![1u32; n_genomes].into();
                 let mut cont = true;
-                if passes_all(filters, &all_present, n_genomes) {
-                    for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
-                        if mphf.find(kmer).is_some() {
-                            cont = cb(part, layer, kmer, all_present.clone());
-                            if !cont { break; }
-                        }
+                for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
+                    if mphf.find(kmer).is_some() && passes_all(filters, kmer, &all_present, n_genomes) {
+                        cont = cb(part, layer, kmer, all_present.clone());
+                        if !cont { break; }
                     }
                 }
                 cont
diff --git a/src/obikpartitionner/src/filter.rs b/src/obikpartitionner/src/filter.rs
index 00f3b03..db69aba 100644
--- a/src/obikpartitionner/src/filter.rs
+++ b/src/obikpartitionner/src/filter.rs
@@ -1,17 +1,24 @@
 use obicompactvec::FilterMask;
+use obikseq::CanonicalKmer;
 
-/// Trait for kmer row filters.
+/// Trait for kmer filters.
 ///
+/// `kmer` is the k-mer's own canonical sequence, reconstructed from the
+/// source index's `unitigs.bin` (always present — see `rebuild_layer.rs`);
 /// `row` contains raw per-genome counts (or 0/1 for presence/absence data).
-/// `n_genomes` equals `row.len()`.
+/// `n_genomes` equals `row.len()`. Most filters only need `row` — `kmer` is
+/// there for filters that reason about the k-mer's sequence itself (e.g.
+/// [`MinComplexity`]).
 pub trait KmerFilter: Send + Sync {
-    fn passes(&self, row: &[u32], n_genomes: usize) -> bool;
+    fn passes(&self, kmer: CanonicalKmer, row: &[u32], n_genomes: usize) -> bool;
 
     /// Express this filter as a [`FilterMask`] column-operation expression.
     ///
     /// Returns `Some(expr)` if the filter can be evaluated solely from matrix
     /// column aggregates (no per-kmer row scan needed).  Returns `None` if the
-    /// filter requires row-level inspection.
+    /// filter requires row-level inspection — always the case for a filter
+    /// that needs the k-mer's sequence, since a `FilterMask` only expresses
+    /// per-genome column aggregates, never per-slot sequence data.
     ///
     /// `threshold` semantics in the returned mask use `>= threshold`, matching
     /// [`obicompactvec::MatrixGroupOps`].  Implementations must add 1 to any
@@ -23,8 +30,13 @@ pub trait KmerFilter: Send + Sync {
 
 /// True when `row` passes every filter in `filters`.
 /// Returns `true` if `filters` is empty.
-pub fn passes_all(filters: &[Box<dyn KmerFilter>], row: &[u32], n_genomes: usize) -> bool {
-    filters.iter().all(|f| f.passes(row, n_genomes))
+pub fn passes_all(
+    filters: &[Box<dyn KmerFilter>],
+    kmer: CanonicalKmer,
+    row: &[u32],
+    n_genomes: usize,
+) -> bool {
+    filters.iter().all(|f| f.passes(kmer, row, n_genomes))
 }
 
 // ── Quorum filters ─────────────────────────────────────────────────────────────
@@ -40,7 +52,7 @@ pub struct MinGenomeFraction {
 }
 
 impl KmerFilter for MinGenomeFraction {
-    fn passes(&self, row: &[u32], n_genomes: usize) -> bool {
+    fn passes(&self, _kmer: CanonicalKmer, row: &[u32], n_genomes: usize) -> bool {
         let p = present_count(row, self.threshold);
         p as f64 / n_genomes as f64 >= self.frac
     }
@@ -63,7 +75,7 @@ pub struct MaxGenomeFraction {
 }
 
 impl KmerFilter for MaxGenomeFraction {
-    fn passes(&self, row: &[u32], n_genomes: usize) -> bool {
+    fn passes(&self, _kmer: CanonicalKmer, row: &[u32], n_genomes: usize) -> bool {
         let p = present_count(row, self.threshold);
         p as f64 / n_genomes as f64 <= self.frac
     }
@@ -86,7 +98,7 @@ pub struct MinGenomeCount {
 }
 
 impl KmerFilter for MinGenomeCount {
-    fn passes(&self, row: &[u32], _n_genomes: usize) -> bool {
+    fn passes(&self, _kmer: CanonicalKmer, row: &[u32], _n_genomes: usize) -> bool {
         present_count(row, self.threshold) >= self.count
     }
 
@@ -107,7 +119,7 @@ pub struct MaxGenomeCount {
 }
 
 impl KmerFilter for MaxGenomeCount {
-    fn passes(&self, row: &[u32], _n_genomes: usize) -> bool {
+    fn passes(&self, _kmer: CanonicalKmer, row: &[u32], _n_genomes: usize) -> bool {
         present_count(row, self.threshold) <= self.count
     }
 
@@ -129,7 +141,7 @@ pub struct MinTotalCount {
 }
 
 impl KmerFilter for MinTotalCount {
-    fn passes(&self, row: &[u32], _n_genomes: usize) -> bool {
+    fn passes(&self, _kmer: CanonicalKmer, row: &[u32], _n_genomes: usize) -> bool {
         row.iter().sum::<u32>() >= self.total
     }
 
@@ -147,7 +159,7 @@ pub struct MaxTotalCount {
 }
 
 impl KmerFilter for MaxTotalCount {
-    fn passes(&self, row: &[u32], _n_genomes: usize) -> bool {
+    fn passes(&self, _kmer: CanonicalKmer, row: &[u32], _n_genomes: usize) -> bool {
         row.iter().sum::<u32>() <= self.total
     }
 
@@ -212,7 +224,7 @@ impl GroupQuorumFilter {
 }
 
 impl KmerFilter for GroupQuorumFilter {
-    fn passes(&self, row: &[u32], _n_genomes: usize) -> bool {
+    fn passes(&self, _kmer: CanonicalKmer, row: &[u32], _n_genomes: usize) -> bool {
         if !self.ingroup_idx.is_empty() {
             let n = self.ingroup_idx.iter()
                 .filter(|&&i| row.get(i).copied().unwrap_or(0) > self.threshold)
@@ -260,3 +272,27 @@ impl KmerFilter for GroupQuorumFilter {
         Some(FilterMask::And(parts))
     }
 }
+
+// ── Complexity filter (post-hoc, sequence-based) ──────────────────────────────
+
+/// Reject k-mers with normalized entropy below `theta` — the same complexity
+/// metric `obikmer index`'s `--theta`/`--level-max` apply *during* superkmer
+/// construction (see [`obiskbuilder::KmerEntropy`]), applied here after the
+/// fact, to k-mers already committed to a built index.
+///
+/// Unlike every other filter in this module, this one needs the k-mer's own
+/// sequence, not its per-genome row — `column_mask_expr` is never overridden
+/// (stays `None`), so this filter always forces the row-level scan path in
+/// `rebuild_layer.rs` (which reconstructs the sequence from `unitigs.bin`
+/// regardless, so no extra I/O beyond what filtering already requires).
+pub struct MinComplexity {
+    pub level_max: usize,
+    pub theta: f64,
+}
+
+impl KmerFilter for MinComplexity {
+    fn passes(&self, kmer: CanonicalKmer, _row: &[u32], _n_genomes: usize) -> bool {
+        use obiskbuilder::KmerEntropy;
+        kmer.entropy(self.level_max) >= self.theta
+    }
+}
diff --git a/src/obikpartitionner/src/rebuild_layer.rs b/src/obikpartitionner/src/rebuild_layer.rs
index b8893ef..e608c49 100644
--- a/src/obikpartitionner/src/rebuild_layer.rs
+++ b/src/obikpartitionner/src/rebuild_layer.rs
@@ -126,7 +126,7 @@ fn iter_src_kmers_masked(
                 Some(m) => m.get(slot),
                 None => {
                     let row = src_data.fill_row_by_slot(slot, n_genomes);
-                    filters.iter().all(|f| f.passes(&row, n_genomes))
+                    filters.iter().all(|f| f.passes(kmer, &row, n_genomes))
                 }
             };
             if passes { cb(kmer); }
@@ -165,7 +165,7 @@ fn iter_src_layers(
                 cb(kmer, row.into_boxed_slice());
             } else {
                 let row = src_data.fill_row_by_slot(slot, n_genomes);
-                if filters.iter().all(|f| f.passes(&row, n_genomes)) {
+                if filters.iter().all(|f| f.passes(kmer, &row, n_genomes)) {
                     cb(kmer, row.into_boxed_slice());
                 }
             }
diff --git a/src/obiskbuilder/src/iter.rs b/src/obiskbuilder/src/iter.rs
index 0839b97..8a33123 100644
--- a/src/obiskbuilder/src/iter.rs
+++ b/src/obiskbuilder/src/iter.rs
@@ -149,157 +149,5 @@ impl Iterator for SuperKmerIter<'_> {
 // ── tests ─────────────────────────────────────────────────────────────────────
 
 #[cfg(test)]
-mod tests {
-    use super::*;
-    use obikrope::Rope;
-
-    fn setup() {
-        obikseq::params::set_k(K);
-        obikseq::params::set_m(5);
-    }
-
-    fn make_rope(data: &[u8]) -> Rope {
-        let mut r = Rope::new(None);
-        r.push(data.to_vec());
-        r
-    }
-
-    fn run_nofilter(data: &[u8], k: usize) -> Vec<Vec<u8>> {
-        let rope = make_rope(data);
-        SuperKmerIter::new(&rope, k, 1, 0.0)
-            .map(|rsk| rsk.superkmer().to_ascii())
-            .collect()
-    }
-
-    // k=11, m=5 — valeurs minimales du projet (k ∈ [11,31])
-    const K: usize = 11;
-
-    /// Collect the set of canonical k-mers from a raw ASCII sequence (no NUL).
-    fn direct_canonical_kmers(seq: &[u8]) -> std::collections::HashSet<Vec<u8>> {
-        (0..seq.len().saturating_sub(K - 1))
-            .map(|i| obikseq::SuperKmer::from_ascii(&seq[i..i + K]).to_ascii())
-            .collect()
-    }
-
-    /// Collect the set of canonical k-mers emitted by SuperKmerIter over a rope.
-    fn iter_canonical_kmers(rope: &Rope) -> std::collections::HashSet<Vec<u8>> {
-        SuperKmerIter::new(rope, K, 1, 0.0)
-            .flat_map(|rsk| {
-                rsk.superkmer()
-                    .iter_canonical_kmers()
-                    .map(|km| km.to_ascii())
-                    .collect::<Vec<_>>()
-            })
-            .collect()
-    }
-
-    #[test]
-    fn coverage_single_segment() {
-        setup();
-        let seq = b"ACGTACGTACGTACGTACGT";
-        let rope = make_rope(&[seq.as_ref(), b"\x00"].concat());
-        let direct = direct_canonical_kmers(seq);
-        let from_iter = iter_canonical_kmers(&rope);
-        let missing: Vec<_> = direct.difference(&from_iter).collect();
-        assert!(
-            missing.is_empty(),
-            "k-mers perdus dans segment unique : {missing:?}"
-        );
-    }
-
-    #[test]
-    fn coverage_two_segments() {
-        setup();
-        let seg1 = b"ACGTACGTACGTACGTACGT";
-        let seg2 = b"TGCATGCATGCATGCATGCA";
-        let rope = make_rope(&[seg1.as_ref(), b"\x00", seg2.as_ref(), b"\x00"].concat());
-        let mut direct = direct_canonical_kmers(seg1);
-        direct.extend(direct_canonical_kmers(seg2));
-        let from_iter = iter_canonical_kmers(&rope);
-        let missing: Vec<_> = direct.difference(&from_iter).collect();
-        assert!(
-            missing.is_empty(),
-            "k-mers perdus dans deux segments : {missing:?}"
-        );
-    }
-
-    #[test]
-    fn coverage_minimizer_boundary() {
-        setup();
-        // sequence assez longue pour forcer plusieurs changements de minimiseur
-        let seq: Vec<u8> = (0..80).map(|i| b"ACGT"[i % 4]).collect();
-        let rope = make_rope(&[seq.as_slice(), b"\x00"].concat());
-        let direct = direct_canonical_kmers(&seq);
-        let from_iter = iter_canonical_kmers(&rope);
-        let missing: Vec<_> = direct.difference(&from_iter).collect();
-        assert!(
-            missing.is_empty(),
-            "k-mers perdus à la frontière de minimiseur : {missing:?}"
-        );
-    }
-
-    #[test]
-    fn single_segment_one_superkmer() {
-        setup();
-        let out = run_nofilter(b"ACGTACGTACGTACGTACGT\x00", K);
-        assert!(!out.is_empty());
-        let total: Vec<u8> = out.into_iter().flatten().collect();
-        assert!(total.len() >= K);
-    }
-
-    #[test]
-    fn segment_shorter_than_k_emits_nothing() {
-        setup();
-        let out = run_nofilter(b"ACGTACGT\x00", K);
-        assert_eq!(out, Vec::<Vec<u8>>::new());
-    }
-
-    #[test]
-    fn empty_input_emits_nothing() {
-        setup();
-        let out = run_nofilter(b"", K);
-        assert_eq!(out, Vec::<Vec<u8>>::new());
-    }
-
-    #[test]
-    fn two_segments_both_emitted() {
-        setup();
-        let out = run_nofilter(b"ACGTACGTACGTACGT\x00TGCATGCATGCATGCA\x00", K);
-        assert!(!out.is_empty());
-    }
-
-    #[test]
-    fn low_complexity_kmer_is_rejected() {
-        setup();
-        let out_pass = run_nofilter(b"AAAAAAAAAAAACGTACGTACGT\x00", K);
-        assert!(!out_pass.is_empty());
-
-        let rope = make_rope(b"AAAAAAAAAAAAAAAAAAAA\x00");
-        let out_reject: Vec<Vec<u8>> = SuperKmerIter::new(&rope, K, 6, 0.9)
-            .map(|rsk| rsk.superkmer().to_ascii())
-            .collect();
-        assert!(out_reject.is_empty());
-    }
-
-    #[test]
-    fn multi_slice_rope() {
-        setup();
-        let data = b"ACGTACGTACGTACGTACGT\x00";
-        let mid = data.len() / 2;
-        let mut rope = Rope::new(None);
-        rope.push(data[..mid].to_vec());
-        rope.push(data[mid..].to_vec());
-        let out: Vec<Vec<u8>> = SuperKmerIter::new(&rope, K, 1, 0.0)
-            .map(|rsk| rsk.superkmer().to_ascii())
-            .collect();
-        assert!(!out.is_empty());
-    }
-
-    #[test]
-    fn yields_minimizer_value() {
-        setup();
-        let rope = make_rope(b"ACGTACGTACGTACGTACGT\x00");
-        let results: Vec<RoutableSuperKmer> = SuperKmerIter::new(&rope, K, 1, 0.0).collect();
-        assert!(!results.is_empty());
-    }
-}
+#[path = "tests/iter.rs"]
+mod tests;
diff --git a/src/obiskbuilder/src/kmer_entropy.rs b/src/obiskbuilder/src/kmer_entropy.rs
new file mode 100644
index 0000000..f8fcd7f
--- /dev/null
+++ b/src/obiskbuilder/src/kmer_entropy.rs
@@ -0,0 +1,49 @@
+//! Normalized entropy of an isolated, already-built k-mer.
+//!
+//! [`SuperKmerIter`](crate::SuperKmerIter) uses [`RollingStat`] to reject
+//! low-complexity k-mers *during* superkmer construction, incrementally, over
+//! a streaming window. [`KmerEntropy`] exposes the same metric for a single
+//! k-mer taken in isolation (e.g. one already reconstructed from an index's
+//! `unitigs.bin`, with no surrounding sequence) — built on the identical
+//! `RollingStat` code path, not a re-derived formula, so a `theta` threshold
+//! chosen for index-build-time filtering means the same thing when applied
+//! after the fact (e.g. `obikmer filter`).
+
+use obikseq::CanonicalKmer;
+
+use crate::rolling_stat::RollingStat;
+
+/// Extension trait: compute the normalized entropy of a single canonical
+/// k-mer, independent of any surrounding sequence.
+pub trait KmerEntropy {
+    /// Normalized entropy across sub-word orders `1..=level_max` (the
+    /// minimum is taken across orders, same as
+    /// [`RollingStat::normalized_entropy`]). Lower means less complex;
+    /// `theta` in `index`/`filter` rejects k-mers with a score `< theta`.
+    ///
+    /// # Panics
+    ///
+    /// Requires both `obikseq::params::k()` *and* `params::m()` to already be
+    /// set (via `set_k`/`set_m`), even though the minimizer length plays no
+    /// conceptual role in an entropy score: `RollingStat` is a shared,
+    /// general-purpose struct (also used for minimizer selection during
+    /// superkmer decomposition) and unconditionally sizes an `m`-dependent
+    /// mask in its constructor. Callers must call `set_m` with a valid value
+    /// (e.g. the index's own stored `minimizer_size`) even when only the
+    /// entropy score is needed.
+    fn entropy(&self, level_max: usize) -> f64;
+}
+
+impl KmerEntropy for CanonicalKmer {
+    fn entropy(&self, level_max: usize) -> f64 {
+        let mut stat = RollingStat::new(level_max);
+        for &base in self.to_ascii().iter() {
+            stat.push(base);
+        }
+        stat.normalized_entropy().unwrap_or(1.0)
+    }
+}
+
+#[cfg(test)]
+#[path = "tests/kmer_entropy.rs"]
+mod tests;
diff --git a/src/obiskbuilder/src/lib.rs b/src/obiskbuilder/src/lib.rs
index 6dc2de1..61d8cf1 100644
--- a/src/obiskbuilder/src/lib.rs
+++ b/src/obiskbuilder/src/lib.rs
@@ -6,6 +6,7 @@
 #![deny(missing_docs)]
 
 pub mod iter;
+pub mod kmer_entropy;
 pub mod stream_iter;
 mod scratch;
 
@@ -14,6 +15,7 @@ pub(crate) mod entropy_table;
 pub(crate) mod rolling_stat;
 
 pub use iter::SuperKmerIter;
+pub use kmer_entropy::KmerEntropy;
 pub use scratch::SuperKmerScratch;
 pub use stream_iter::SuperKmerStreamIter;
 
diff --git a/src/obiskbuilder/src/tests/iter.rs b/src/obiskbuilder/src/tests/iter.rs
new file mode 100644
index 0000000..4e03036
--- /dev/null
+++ b/src/obiskbuilder/src/tests/iter.rs
@@ -0,0 +1,152 @@
+use super::*;
+use obikrope::Rope;
+
+fn setup() {
+    obikseq::params::set_k(K);
+    obikseq::params::set_m(5);
+}
+
+fn make_rope(data: &[u8]) -> Rope {
+    let mut r = Rope::new(None);
+    r.push(data.to_vec());
+    r
+}
+
+fn run_nofilter(data: &[u8], k: usize) -> Vec<Vec<u8>> {
+    let rope = make_rope(data);
+    SuperKmerIter::new(&rope, k, 1, 0.0)
+        .map(|rsk| rsk.superkmer().to_ascii())
+        .collect()
+}
+
+// k=11, m=5 — valeurs minimales du projet (k ∈ [11,31])
+const K: usize = 11;
+
+/// Collect the set of canonical k-mers from a raw ASCII sequence (no NUL).
+fn direct_canonical_kmers(seq: &[u8]) -> std::collections::HashSet<Vec<u8>> {
+    (0..seq.len().saturating_sub(K - 1))
+        .map(|i| obikseq::SuperKmer::from_ascii(&seq[i..i + K]).to_ascii())
+        .collect()
+}
+
+/// Collect the set of canonical k-mers emitted by SuperKmerIter over a rope.
+fn iter_canonical_kmers(rope: &Rope) -> std::collections::HashSet<Vec<u8>> {
+    SuperKmerIter::new(rope, K, 1, 0.0)
+        .flat_map(|rsk| {
+            rsk.superkmer()
+                .iter_canonical_kmers()
+                .map(|km| km.to_ascii())
+                .collect::<Vec<_>>()
+        })
+        .collect()
+}
+
+#[test]
+fn coverage_single_segment() {
+    setup();
+    let seq = b"ACGTACGTACGTACGTACGT";
+    let rope = make_rope(&[seq.as_ref(), b"\x00"].concat());
+    let direct = direct_canonical_kmers(seq);
+    let from_iter = iter_canonical_kmers(&rope);
+    let missing: Vec<_> = direct.difference(&from_iter).collect();
+    assert!(
+        missing.is_empty(),
+        "k-mers perdus dans segment unique : {missing:?}"
+    );
+}
+
+#[test]
+fn coverage_two_segments() {
+    setup();
+    let seg1 = b"ACGTACGTACGTACGTACGT";
+    let seg2 = b"TGCATGCATGCATGCATGCA";
+    let rope = make_rope(&[seg1.as_ref(), b"\x00", seg2.as_ref(), b"\x00"].concat());
+    let mut direct = direct_canonical_kmers(seg1);
+    direct.extend(direct_canonical_kmers(seg2));
+    let from_iter = iter_canonical_kmers(&rope);
+    let missing: Vec<_> = direct.difference(&from_iter).collect();
+    assert!(
+        missing.is_empty(),
+        "k-mers perdus dans deux segments : {missing:?}"
+    );
+}
+
+#[test]
+fn coverage_minimizer_boundary() {
+    setup();
+    // sequence assez longue pour forcer plusieurs changements de minimiseur
+    let seq: Vec<u8> = (0..80).map(|i| b"ACGT"[i % 4]).collect();
+    let rope = make_rope(&[seq.as_slice(), b"\x00"].concat());
+    let direct = direct_canonical_kmers(&seq);
+    let from_iter = iter_canonical_kmers(&rope);
+    let missing: Vec<_> = direct.difference(&from_iter).collect();
+    assert!(
+        missing.is_empty(),
+        "k-mers perdus à la frontière de minimiseur : {missing:?}"
+    );
+}
+
+#[test]
+fn single_segment_one_superkmer() {
+    setup();
+    let out = run_nofilter(b"ACGTACGTACGTACGTACGT\x00", K);
+    assert!(!out.is_empty());
+    let total: Vec<u8> = out.into_iter().flatten().collect();
+    assert!(total.len() >= K);
+}
+
+#[test]
+fn segment_shorter_than_k_emits_nothing() {
+    setup();
+    let out = run_nofilter(b"ACGTACGT\x00", K);
+    assert_eq!(out, Vec::<Vec<u8>>::new());
+}
+
+#[test]
+fn empty_input_emits_nothing() {
+    setup();
+    let out = run_nofilter(b"", K);
+    assert_eq!(out, Vec::<Vec<u8>>::new());
+}
+
+#[test]
+fn two_segments_both_emitted() {
+    setup();
+    let out = run_nofilter(b"ACGTACGTACGTACGT\x00TGCATGCATGCATGCA\x00", K);
+    assert!(!out.is_empty());
+}
+
+#[test]
+fn low_complexity_kmer_is_rejected() {
+    setup();
+    let out_pass = run_nofilter(b"AAAAAAAAAAAACGTACGTACGT\x00", K);
+    assert!(!out_pass.is_empty());
+
+    let rope = make_rope(b"AAAAAAAAAAAAAAAAAAAA\x00");
+    let out_reject: Vec<Vec<u8>> = SuperKmerIter::new(&rope, K, 6, 0.9)
+        .map(|rsk| rsk.superkmer().to_ascii())
+        .collect();
+    assert!(out_reject.is_empty());
+}
+
+#[test]
+fn multi_slice_rope() {
+    setup();
+    let data = b"ACGTACGTACGTACGTACGT\x00";
+    let mid = data.len() / 2;
+    let mut rope = Rope::new(None);
+    rope.push(data[..mid].to_vec());
+    rope.push(data[mid..].to_vec());
+    let out: Vec<Vec<u8>> = SuperKmerIter::new(&rope, K, 1, 0.0)
+        .map(|rsk| rsk.superkmer().to_ascii())
+        .collect();
+    assert!(!out.is_empty());
+}
+
+#[test]
+fn yields_minimizer_value() {
+    setup();
+    let rope = make_rope(b"ACGTACGTACGTACGTACGT\x00");
+    let results: Vec<RoutableSuperKmer> = SuperKmerIter::new(&rope, K, 1, 0.0).collect();
+    assert!(!results.is_empty());
+}
diff --git a/src/obiskbuilder/src/tests/kmer_entropy.rs b/src/obiskbuilder/src/tests/kmer_entropy.rs
new file mode 100644
index 0000000..1456c3a
--- /dev/null
+++ b/src/obiskbuilder/src/tests/kmer_entropy.rs
@@ -0,0 +1,54 @@
+use super::*;
+use obikseq::Sequence;
+use obikseq::kmer::Kmer;
+
+const K: usize = 21;
+const M: usize = 9; // RollingStat also tracks the minimizer window internally
+const LEVEL_MAX: usize = 6;
+
+fn kmer_from_ascii(seq: &[u8]) -> CanonicalKmer {
+    obikseq::set_k(K);
+    obikseq::set_m(M);
+    Kmer::from_ascii(seq).expect("valid k-mer sequence").canonical()
+}
+
+#[test]
+fn homopolymer_scores_lower_than_diverse_sequence() {
+    let homopolymer = kmer_from_ascii(b"AAAAAAAAAAAAAAAAAAAAA"); // 21 bases
+    let diverse = kmer_from_ascii(b"CATTAGCGTACCTGATCAGGT"); // 21 bases, same as used elsewhere in this workspace's tests
+
+    let e_homopolymer = homopolymer.entropy(LEVEL_MAX);
+    let e_diverse = diverse.entropy(LEVEL_MAX);
+
+    assert!(
+        e_homopolymer < e_diverse,
+        "homopolymer ({e_homopolymer}) should score lower than a diverse sequence ({e_diverse})"
+    );
+    // A pure homopolymer is the most degenerate case representable — its
+    // score should sit near the bottom of the range, not just "somewhat lower".
+    assert!(e_homopolymer < 0.3, "homopolymer entropy unexpectedly high: {e_homopolymer}");
+}
+
+#[test]
+fn entropy_is_deterministic_for_the_same_kmer() {
+    let a = kmer_from_ascii(b"CATTAGCGTACCTGATCAGGT");
+    let b = kmer_from_ascii(b"CATTAGCGTACCTGATCAGGT");
+    assert_eq!(a.entropy(LEVEL_MAX), b.entropy(LEVEL_MAX));
+}
+
+#[test]
+fn entropy_is_within_zero_one_range() {
+    let mut repeat = "AT".repeat(K / 2 + 1);
+    repeat.truncate(K);
+
+    for seq in [
+        "AAAAAAAAAAAAAAAAAAAAA".to_string(),
+        repeat,
+        "CATTAGCGTACCTGATCAGGT".to_string(),
+    ] {
+        assert_eq!(seq.len(), K, "test sequence must be exactly K bases: {seq:?}");
+        let kmer = kmer_from_ascii(seq.as_bytes());
+        let e = kmer.entropy(LEVEL_MAX);
+        assert!((0.0..=1.0).contains(&e), "entropy {e} out of [0,1] for {seq:?}");
+    }
+}
-- 
2.54.0


From 912f788f7f1d8d288d49843234dc2dc3cbd5616e Mon Sep 17 00:00:00 2001
From: Eric Coissac <eric@coissac.eu>
Date: Wed, 8 Jul 2026 18:16:08 +0200
Subject: [PATCH 2/3] feat: extract k-mer entropy computation into new
 obikentropy crate

Extracts streaming entropy logic and sliding-window frequency tracking from obiskbuilder into a dedicated obikentropy crate. Introduces an EntropyTracker accumulator for O(1) per-base normalized Shannon entropy, replaces inline rolling statistics with delegated state management, and updates workspace dependencies across obikindex, obikpartitionner, and obiskbuilder. Adds criterion benchmarks to validate the refactored pipeline throughput.
---
 src/Cargo.lock                                |  10 +
 src/Cargo.toml                                |   2 +-
 src/obikentropy/Cargo.toml                    |  10 +
 src/{obiskbuilder => obikentropy}/build.rs    |   0
 src/obikentropy/src/kmer_entropy.rs           |  41 +++
 src/obikentropy/src/lib.rs                    |  17 ++
 src/obikentropy/src/ring.rs                   |  40 +++
 .../src/table.rs}                             |   4 +
 .../src/tests/kmer_entropy.rs                 |   2 -
 src/obikentropy/src/tracker.rs                | 280 ++++++++++++++++++
 src/obikpartitionner/Cargo.toml               |   1 +
 src/obikpartitionner/src/filter.rs            |   4 +-
 src/obiskbuilder/Cargo.toml                   |   6 +
 src/obiskbuilder/benches/superkmer_stream.rs  |  58 ++++
 src/obiskbuilder/src/kmer_entropy.rs          |  49 ---
 src/obiskbuilder/src/lib.rs                   |   3 -
 src/obiskbuilder/src/rolling_stat.rs          | 260 +---------------
 17 files changed, 482 insertions(+), 305 deletions(-)
 create mode 100644 src/obikentropy/Cargo.toml
 rename src/{obiskbuilder => obikentropy}/build.rs (100%)
 create mode 100644 src/obikentropy/src/kmer_entropy.rs
 create mode 100644 src/obikentropy/src/lib.rs
 create mode 100644 src/obikentropy/src/ring.rs
 rename src/{obiskbuilder/src/entropy_table.rs => obikentropy/src/table.rs} (93%)
 rename src/{obiskbuilder => obikentropy}/src/tests/kmer_entropy.rs (94%)
 create mode 100644 src/obikentropy/src/tracker.rs
 create mode 100644 src/obiskbuilder/benches/superkmer_stream.rs
 delete mode 100644 src/obiskbuilder/src/kmer_entropy.rs

diff --git a/src/Cargo.lock b/src/Cargo.lock
index 3e31a1e..f7c63e3 100644
--- a/src/Cargo.lock
+++ b/src/Cargo.lock
@@ -1682,6 +1682,13 @@ dependencies = [
  "xxhash-rust",
 ]
 
+[[package]]
+name = "obikentropy"
+version = "0.1.0"
+dependencies = [
+ "obikseq",
+]
+
 [[package]]
 name = "obikindex"
 version = "0.1.0"
@@ -1742,6 +1749,7 @@ dependencies = [
  "niffler 3.0.0",
  "obicompactvec",
  "obidebruinj",
+ "obikentropy",
  "obikrope",
  "obikseq",
  "obilayeredmap",
@@ -1824,7 +1832,9 @@ dependencies = [
 name = "obiskbuilder"
 version = "0.1.0"
 dependencies = [
+ "criterion2",
  "lazy_static",
+ "obikentropy",
  "obikrope",
  "obikseq",
  "obiread",
diff --git a/src/Cargo.toml b/src/Cargo.toml
index 141df02..5684237 100644
--- a/src/Cargo.toml
+++ b/src/Cargo.toml
@@ -1,5 +1,5 @@
 [workspace]
 resolver = "3"
-members = ["obikseq", "obiread", "obiskbuilder", "obifastwrite", "obikmer","obikrope","obipipeline", "obikpartitionner","obiskio","obidebruinj","obilayeredmap", "obicompactvec", "obisys", "obikindex", "obitaxonomy"]
+members = ["obikseq", "obiread", "obiskbuilder", "obifastwrite", "obikmer","obikrope","obipipeline", "obikpartitionner","obiskio","obidebruinj","obilayeredmap", "obicompactvec", "obisys", "obikindex", "obitaxonomy", "obikentropy"]
 [profile.release]
 debug = 1
diff --git a/src/obikentropy/Cargo.toml b/src/obikentropy/Cargo.toml
new file mode 100644
index 0000000..3d1cd69
--- /dev/null
+++ b/src/obikentropy/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "obikentropy"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+obikseq = { path = "../obikseq" }
+
+[dev-dependencies]
+obikseq = { path = "../obikseq", features = ["test-utils"] }
diff --git a/src/obiskbuilder/build.rs b/src/obikentropy/build.rs
similarity index 100%
rename from src/obiskbuilder/build.rs
rename to src/obikentropy/build.rs
diff --git a/src/obikentropy/src/kmer_entropy.rs b/src/obikentropy/src/kmer_entropy.rs
new file mode 100644
index 0000000..1989972
--- /dev/null
+++ b/src/obikentropy/src/kmer_entropy.rs
@@ -0,0 +1,41 @@
+//! Normalized entropy of an isolated, already-built k-mer (e.g. one
+//! reconstructed from an index's `unitigs.bin`, with no surrounding
+//! sequence) — drives the window through [`EntropyTracker`] one base at a
+//! time, exactly like the streaming path, so a `theta` threshold means the
+//! same thing whether applied during index construction or after the fact
+//! (e.g. `obikmer filter`).
+
+use obikseq::CanonicalKmer;
+
+use crate::tracker::{EntropyTracker, sub_word_canon};
+
+/// Extension trait: compute the normalized entropy of a single canonical
+/// k-mer, independent of any surrounding sequence.
+pub trait KmerEntropy {
+    /// Normalized entropy across sub-word orders `1..=level_max` (the
+    /// minimum is taken across orders). Lower means less complex; `theta`
+    /// in `index`/`filter` rejects k-mers with a score `< theta`.
+    fn entropy(&self, level_max: usize) -> f64;
+}
+
+impl KmerEntropy for CanonicalKmer {
+    fn entropy(&self, level_max: usize) -> f64 {
+        let raw = self.raw(); // left-aligned, 2 bits/base, MSB-first
+        let k = obikseq::params::k();
+        let mask = (!0u64) >> (64 - k * 2);
+
+        let mut tracker = EntropyTracker::new(k);
+        let mut rolling: u64 = 0;
+        for i in 0..k {
+            let shift = 64 - 2 * (i + 1);
+            let base = (raw >> shift) & 3;
+            rolling = ((rolling << 2) | base) & mask;
+            tracker.push(i + 1, sub_word_canon(rolling));
+        }
+        tracker.normalized_entropy(level_max)
+    }
+}
+
+#[cfg(test)]
+#[path = "tests/kmer_entropy.rs"]
+mod tests;
diff --git a/src/obikentropy/src/lib.rs b/src/obikentropy/src/lib.rs
new file mode 100644
index 0000000..113bdd3
--- /dev/null
+++ b/src/obikentropy/src/lib.rs
@@ -0,0 +1,17 @@
+//! Normalized k-mer entropy: formulas, tables, and a streaming tracker.
+//!
+//! This crate holds every piece of the entropy computation described in
+//! `docmd/theory/entropy.md`: the compile-time tables ([`table`], private),
+//! the incremental accumulator ([`EntropyTracker`]) that callers compose
+//! into their own streaming state, and the [`KmerEntropy`] convenience trait
+//! for scoring a single, already-built k-mer.
+
+#![deny(missing_docs)]
+
+mod kmer_entropy;
+mod ring;
+mod table;
+mod tracker;
+
+pub use kmer_entropy::KmerEntropy;
+pub use tracker::{EntropyTracker, SubWordCanon, sub_word_canon};
diff --git a/src/obikentropy/src/ring.rs b/src/obikentropy/src/ring.rs
new file mode 100644
index 0000000..b729beb
--- /dev/null
+++ b/src/obikentropy/src/ring.rs
@@ -0,0 +1,40 @@
+//! Stack-allocated ring buffer backing the sliding sub-word windows.
+
+/// Fixed-capacity ring buffer backed by a stack array.
+/// N must be a power of two; operations are branchless via `% N`.
+pub(crate) struct Ring<T: Copy + Default, const N: usize> {
+    buf: [T; N],
+    head: usize,
+    len: usize,
+}
+
+impl<T: Copy + Default, const N: usize> Ring<T, N> {
+    #[inline]
+    pub(crate) fn new() -> Self {
+        Self {
+            buf: [T::default(); N],
+            head: 0,
+            len: 0,
+        }
+    }
+
+    #[inline]
+    pub(crate) fn clear(&mut self) {
+        self.len = 0;
+        self.head = 0;
+    }
+
+    #[inline]
+    pub(crate) fn push_back(&mut self, val: T) {
+        self.buf[(self.head + self.len) % N] = val;
+        self.len += 1;
+    }
+
+    #[inline]
+    pub(crate) fn pop_front(&mut self) -> T {
+        let val = self.buf[self.head];
+        self.head = (self.head + 1) % N;
+        self.len -= 1;
+        val
+    }
+}
diff --git a/src/obiskbuilder/src/entropy_table.rs b/src/obikentropy/src/table.rs
similarity index 93%
rename from src/obiskbuilder/src/entropy_table.rs
rename to src/obikentropy/src/table.rs
index 9be69c3..289aa94 100644
--- a/src/obiskbuilder/src/entropy_table.rs
+++ b/src/obikentropy/src/table.rs
@@ -1,3 +1,7 @@
+//! Compile-time tables backing the normalized k-mer entropy formula:
+//! circular-canonical sub-word classes, their log-sizes, and the max-entropy
+//! correction for small samples. See `docmd/theory/entropy.md`.
+
 pub(crate) const NORMK1: [u64; 4] = build_normalized_kmer::<4>();
 pub(crate) const NORMK2: [u64; 16] = build_normalized_kmer::<16>();
 pub(crate) const NORMK3: [u64; 64] = build_normalized_kmer::<64>();
diff --git a/src/obiskbuilder/src/tests/kmer_entropy.rs b/src/obikentropy/src/tests/kmer_entropy.rs
similarity index 94%
rename from src/obiskbuilder/src/tests/kmer_entropy.rs
rename to src/obikentropy/src/tests/kmer_entropy.rs
index 1456c3a..c75522e 100644
--- a/src/obiskbuilder/src/tests/kmer_entropy.rs
+++ b/src/obikentropy/src/tests/kmer_entropy.rs
@@ -3,12 +3,10 @@ use obikseq::Sequence;
 use obikseq::kmer::Kmer;
 
 const K: usize = 21;
-const M: usize = 9; // RollingStat also tracks the minimizer window internally
 const LEVEL_MAX: usize = 6;
 
 fn kmer_from_ascii(seq: &[u8]) -> CanonicalKmer {
     obikseq::set_k(K);
-    obikseq::set_m(M);
     Kmer::from_ascii(seq).expect("valid k-mer sequence").canonical()
 }
 
diff --git a/src/obikentropy/src/tracker.rs b/src/obikentropy/src/tracker.rs
new file mode 100644
index 0000000..98686fa
--- /dev/null
+++ b/src/obikentropy/src/tracker.rs
@@ -0,0 +1,280 @@
+//! Incremental (streaming) normalized k-mer entropy.
+//!
+//! [`EntropyTracker`] maintains, over a sliding window of the last `k` bases,
+//! the per-sub-word-size frequency statistics needed to evaluate the
+//! corrected Shannon entropy described in `docmd/theory/entropy.md`, updated
+//! in O(1) per base rather than recomputed from scratch.
+//!
+//! It carries no notion of minimizers or superkmer segmentation — callers
+//! that need both (e.g. `obiskbuilder::RollingStat`) compose an
+//! `EntropyTracker` as a plain field alongside their own state, so the two
+//! concerns update in the same streaming pass without being conflated in one
+//! struct.
+
+use crate::ring::Ring;
+use crate::table::{WS_MAX, emax, entropy_norm_kmer, ln_class_size, log_nwords, n_log_n};
+
+/// Canonical sub-word values for word sizes 1..=6, computed by the caller
+/// from its own rolling k-mer bits (see [`EntropyTracker::push`]).
+pub type SubWordCanon = [u64; WS_MAX];
+
+/// Compute the six canonical sub-word values (word sizes 1..=6) for the
+/// current rolling right-aligned k-mer window.
+#[inline]
+pub fn sub_word_canon(rolling_kmer: u64) -> SubWordCanon {
+    [
+        entropy_norm_kmer::<false, 1>(rolling_kmer & 3),
+        entropy_norm_kmer::<false, 2>(rolling_kmer & 15),
+        entropy_norm_kmer::<false, 3>(rolling_kmer & 63),
+        entropy_norm_kmer::<false, 4>(rolling_kmer & 255),
+        entropy_norm_kmer::<false, 5>(rolling_kmer & 1023),
+        entropy_norm_kmer::<false, 6>(rolling_kmer & 4095),
+    ]
+}
+
+/// Incremental normalized-entropy accumulator over a sliding window of `k`
+/// bases. Composed as a plain field by callers that also need other
+/// per-base state (e.g. minimizer selection) in the same streaming pass.
+pub struct EntropyTracker {
+    k: usize,
+    steady: bool,
+
+    // Sliding-window queues over the last `k` canonical sub-words, one per
+    // word size — stack-allocated, capacity ≤ k ≤ 31.
+    k1q: Ring<u64, 32>,
+    k2q: Ring<u64, 32>,
+    k3q: Ring<u64, 32>,
+    k4q: Ring<u64, 32>,
+    k5q: Ring<u64, 32>,
+    k6q: Ring<u64, 32>,
+
+    // Frequency count arrays. Max count per cell ≤ k ≤ 31 → u8 is sufficient.
+    k1c: [u8; 4],
+    k2c: [u8; 16],
+    k3c: [u8; 64],
+    k4c: [u8; 256],
+    k5c: [u8; 1024],
+    k6c: [u8; 4096],
+
+    sum_f_log_f: [f64; WS_MAX + 1],
+    sum_f_log_s: [f64; WS_MAX + 1],
+}
+
+impl EntropyTracker {
+    /// New tracker for a window of `k` bases (1..=31).
+    pub fn new(k: usize) -> Self {
+        Self {
+            k,
+            steady: false,
+            k1q: Ring::new(),
+            k2q: Ring::new(),
+            k3q: Ring::new(),
+            k4q: Ring::new(),
+            k5q: Ring::new(),
+            k6q: Ring::new(),
+            k1c: [0; 4],
+            k2c: [0; 16],
+            k3c: [0; 64],
+            k4c: [0; 256],
+            k5c: [0; 1024],
+            k6c: [0; 4096],
+            sum_f_log_f: [0.0; WS_MAX + 1],
+            sum_f_log_s: [0.0; WS_MAX + 1],
+        }
+    }
+
+    /// Clear all accumulated state, ready to track a new window from
+    /// scratch (`k` is unchanged).
+    pub fn reset(&mut self) {
+        self.steady = false;
+
+        self.k1c.fill(0);
+        self.k2c.fill(0);
+        self.k3c.fill(0);
+        self.k4c.fill(0);
+        self.k5c.fill(0);
+        self.k6c.fill(0);
+
+        self.k1q.clear();
+        self.k2q.clear();
+        self.k3q.clear();
+        self.k4q.clear();
+        self.k5q.clear();
+        self.k6q.clear();
+
+        self.sum_f_log_f = [0.0; WS_MAX + 1];
+        self.sum_f_log_s = [0.0; WS_MAX + 1];
+    }
+
+    #[inline]
+    fn update_sums_decrement<const K: usize>(
+        sum_f_log_f: &mut [f64; WS_MAX + 1],
+        sum_f_log_s: &mut [f64; WS_MAX + 1],
+        canonical: u64,
+        f: usize,
+    ) {
+        sum_f_log_f[K] += n_log_n(f - 1) - n_log_n(f);
+        sum_f_log_s[K] -= ln_class_size::<false, K>(canonical);
+    }
+
+    #[inline]
+    fn update_sums_increment<const K: usize>(
+        sum_f_log_f: &mut [f64; WS_MAX + 1],
+        sum_f_log_s: &mut [f64; WS_MAX + 1],
+        canonical: u64,
+        g: usize,
+    ) {
+        sum_f_log_f[K] += n_log_n(g + 1) - n_log_n(g);
+        sum_f_log_s[K] += ln_class_size::<false, K>(canonical);
+    }
+
+    /// Advance the window by one base. `received` is the caller's running
+    /// count of bases pushed so far (1-based, i.e. after this base);
+    /// `canon` are the six canonical sub-word values for the current
+    /// rolling k-mer, from [`sub_word_canon`].
+    pub fn push(&mut self, received: usize, canon: SubWordCanon) {
+        let [canonical_k1, canonical_k2, canonical_k3, canonical_k4, canonical_k5, canonical_k6] =
+            canon;
+
+        if received > self.k {
+            let old1 = self.k1q.pop_front();
+            let f1 = self.k1c[old1 as usize] as usize;
+            Self::update_sums_decrement::<1>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, old1, f1);
+            self.k1c[old1 as usize] -= 1;
+
+            let old2 = self.k2q.pop_front();
+            let f2 = self.k2c[old2 as usize] as usize;
+            Self::update_sums_decrement::<2>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, old2, f2);
+            self.k2c[old2 as usize] -= 1;
+
+            let old3 = self.k3q.pop_front();
+            let f3 = self.k3c[old3 as usize] as usize;
+            Self::update_sums_decrement::<3>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, old3, f3);
+            self.k3c[old3 as usize] -= 1;
+
+            let old4 = self.k4q.pop_front();
+            let f4 = self.k4c[old4 as usize] as usize;
+            Self::update_sums_decrement::<4>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, old4, f4);
+            self.k4c[old4 as usize] -= 1;
+
+            let old5 = self.k5q.pop_front();
+            let f5 = self.k5c[old5 as usize] as usize;
+            Self::update_sums_decrement::<5>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, old5, f5);
+            self.k5c[old5 as usize] -= 1;
+
+            let old6 = self.k6q.pop_front();
+            let f6 = self.k6c[old6 as usize] as usize;
+            Self::update_sums_decrement::<6>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, old6, f6);
+            self.k6c[old6 as usize] -= 1;
+        }
+
+        if self.steady {
+            let g1 = self.k1c[canonical_k1 as usize] as usize;
+            Self::update_sums_increment::<1>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k1, g1);
+            self.k1c[canonical_k1 as usize] += 1;
+            self.k1q.push_back(canonical_k1);
+
+            let g2 = self.k2c[canonical_k2 as usize] as usize;
+            Self::update_sums_increment::<2>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k2, g2);
+            self.k2c[canonical_k2 as usize] += 1;
+            self.k2q.push_back(canonical_k2);
+
+            let g3 = self.k3c[canonical_k3 as usize] as usize;
+            Self::update_sums_increment::<3>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k3, g3);
+            self.k3c[canonical_k3 as usize] += 1;
+            self.k3q.push_back(canonical_k3);
+
+            let g4 = self.k4c[canonical_k4 as usize] as usize;
+            Self::update_sums_increment::<4>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k4, g4);
+            self.k4c[canonical_k4 as usize] += 1;
+            self.k4q.push_back(canonical_k4);
+
+            let g5 = self.k5c[canonical_k5 as usize] as usize;
+            Self::update_sums_increment::<5>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k5, g5);
+            self.k5c[canonical_k5 as usize] += 1;
+            self.k5q.push_back(canonical_k5);
+
+            let g6 = self.k6c[canonical_k6 as usize] as usize;
+            Self::update_sums_increment::<6>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k6, g6);
+            self.k6c[canonical_k6 as usize] += 1;
+            self.k6q.push_back(canonical_k6);
+        } else {
+            self.push_warmup_increments(received, canonical_k1, canonical_k2, canonical_k3, canonical_k4, canonical_k5, canonical_k6);
+        }
+    }
+
+    #[cold]
+    #[inline(never)]
+    fn push_warmup_increments(
+        &mut self,
+        received: usize,
+        canonical_k1: u64, canonical_k2: u64, canonical_k3: u64,
+        canonical_k4: u64, canonical_k5: u64, canonical_k6: u64,
+    ) {
+        let g1 = self.k1c[canonical_k1 as usize] as usize;
+        Self::update_sums_increment::<1>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k1, g1);
+        self.k1c[canonical_k1 as usize] += 1;
+        self.k1q.push_back(canonical_k1);
+
+        if received >= 2 {
+            let g2 = self.k2c[canonical_k2 as usize] as usize;
+            Self::update_sums_increment::<2>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k2, g2);
+            self.k2c[canonical_k2 as usize] += 1;
+            self.k2q.push_back(canonical_k2);
+
+            if received >= 3 {
+                let g3 = self.k3c[canonical_k3 as usize] as usize;
+                Self::update_sums_increment::<3>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k3, g3);
+                self.k3c[canonical_k3 as usize] += 1;
+                self.k3q.push_back(canonical_k3);
+
+                if received >= 4 {
+                    let g4 = self.k4c[canonical_k4 as usize] as usize;
+                    Self::update_sums_increment::<4>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k4, g4);
+                    self.k4c[canonical_k4 as usize] += 1;
+                    self.k4q.push_back(canonical_k4);
+
+                    if received >= 5 {
+                        let g5 = self.k5c[canonical_k5 as usize] as usize;
+                        Self::update_sums_increment::<5>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k5, g5);
+                        self.k5c[canonical_k5 as usize] += 1;
+                        self.k5q.push_back(canonical_k5);
+
+                        if received >= 6 {
+                            let g6 = self.k6c[canonical_k6 as usize] as usize;
+                            Self::update_sums_increment::<6>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k6, g6);
+                            self.k6c[canonical_k6 as usize] += 1;
+                            self.k6q.push_back(canonical_k6);
+                            self.steady = true;
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    /// Normalized entropy at sub-word size `order` (1..=6). The caller is
+    /// responsible for not calling this before the window is full (`k`
+    /// bases pushed) — an empty/partial window yields a meaningless value.
+    pub fn entropy(&self, order: usize) -> f64 {
+        let k = self.k;
+        let em = emax(k, order);
+        if em <= 0.0 {
+            return 1.0;
+        }
+        let nwords = k - order + 1;
+        let log_nw = log_nwords(k, order);
+        let nw_f = nwords as f64;
+        let h_corr = log_nw + (self.sum_f_log_s[order] - self.sum_f_log_f[order]) / nw_f;
+        (h_corr / em).max(0.0)
+    }
+
+    /// Minimum of [`Self::entropy`] over sub-word sizes `1..=order_max`, same
+    /// caller responsibility re: window readiness as `entropy`.
+    pub fn normalized_entropy(&self, order_max: usize) -> f64 {
+        let min_e = (1..=order_max)
+            .map(|ws| self.entropy(ws))
+            .fold(f64::MAX, f64::min);
+        if min_e == f64::MAX { 1.0 } else { min_e }
+    }
+}
diff --git a/src/obikpartitionner/Cargo.toml b/src/obikpartitionner/Cargo.toml
index a0c810e..918606f 100644
--- a/src/obikpartitionner/Cargo.toml
+++ b/src/obikpartitionner/Cargo.toml
@@ -13,6 +13,7 @@ obikrope  = { path = "../obikrope" }
 niffler = "3.0.0"
 remove_dir_all = "0.8"
 obikseq      = { path = "../obikseq" }
+obikentropy  = { path = "../obikentropy" }
 obiskbuilder = { path = "../obiskbuilder" }
 obiskio      = { path = "../obiskio" }
 obidebruinj  = { path = "../obidebruinj" }
diff --git a/src/obikpartitionner/src/filter.rs b/src/obikpartitionner/src/filter.rs
index db69aba..9f54a69 100644
--- a/src/obikpartitionner/src/filter.rs
+++ b/src/obikpartitionner/src/filter.rs
@@ -277,7 +277,7 @@ impl KmerFilter for GroupQuorumFilter {
 
 /// Reject k-mers with normalized entropy below `theta` — the same complexity
 /// metric `obikmer index`'s `--theta`/`--level-max` apply *during* superkmer
-/// construction (see [`obiskbuilder::KmerEntropy`]), applied here after the
+/// construction (see [`obikentropy::KmerEntropy`]), applied here after the
 /// fact, to k-mers already committed to a built index.
 ///
 /// Unlike every other filter in this module, this one needs the k-mer's own
@@ -292,7 +292,7 @@ pub struct MinComplexity {
 
 impl KmerFilter for MinComplexity {
     fn passes(&self, kmer: CanonicalKmer, _row: &[u32], _n_genomes: usize) -> bool {
-        use obiskbuilder::KmerEntropy;
+        use obikentropy::KmerEntropy;
         kmer.entropy(self.level_max) >= self.theta
     }
 }
diff --git a/src/obiskbuilder/Cargo.toml b/src/obiskbuilder/Cargo.toml
index 5e19801..1e5c0af 100644
--- a/src/obiskbuilder/Cargo.toml
+++ b/src/obiskbuilder/Cargo.toml
@@ -7,7 +7,13 @@ edition = "2024"
 obikseq = { path = "../obikseq" }
 obikrope = { path = "../obikrope" }
 obiread = { path = "../obiread" }
+obikentropy = { path = "../obikentropy" }
 lazy_static = "1.5.0"
 
 [dev-dependencies]
 obikseq = { path = "../obikseq", features = ["test-utils"] }
+criterion2 = { version = "3", features = ["cargo_bench_support"] }
+
+[[bench]]
+name = "superkmer_stream"
+harness = false
diff --git a/src/obiskbuilder/benches/superkmer_stream.rs b/src/obiskbuilder/benches/superkmer_stream.rs
new file mode 100644
index 0000000..776e6d3
--- /dev/null
+++ b/src/obiskbuilder/benches/superkmer_stream.rs
@@ -0,0 +1,58 @@
+//! Throughput of the streaming superkmer pipeline (`RollingStat`'s hot path:
+//! minimizer selection + entropy tracking fused in a single pass).
+//!
+//! Reference point for the `obikentropy` extraction: the entropy bookkeeping
+//! that used to live inline in `RollingStat` was pulled out into a composed
+//! `EntropyTracker`. This benchmark is run before and after that change to
+//! confirm no regression.
+
+use criterion::{Criterion, Throughput, criterion_group, criterion_main};
+use obikrope::Rope;
+use obiskbuilder::SuperKmerIter;
+
+const K: usize = 21;
+const M: usize = 9;
+const LEVEL_MAX: usize = 6;
+const THETA: f64 = 0.7;
+const SEQ_LEN: usize = 200_000;
+
+/// Deterministic pseudo-random ACGT sequence — high enough complexity that
+/// the entropy filter rarely rejects, so the bench stays on the steady-state
+/// path rather than repeatedly resetting.
+fn make_sequence(len: usize) -> Vec<u8> {
+    let mut state: u64 = 0x9E3779B97F4A7C15;
+    (0..len)
+        .map(|_| {
+            state ^= state << 13;
+            state ^= state >> 7;
+            state ^= state << 17;
+            b"ACGT"[(state % 4) as usize]
+        })
+        .collect()
+}
+
+fn make_rope(seq: &[u8]) -> Rope {
+    let mut rope = Rope::new(None);
+    rope.push(seq.to_vec());
+    rope
+}
+
+fn bench_build_superkmers(c: &mut Criterion) {
+    obikseq::set_k(K);
+    obikseq::set_m(M);
+
+    let seq = make_sequence(SEQ_LEN);
+    let rope = make_rope(&seq);
+
+    let mut group = c.benchmark_group("build_superkmers");
+    group.throughput(Throughput::Bytes(SEQ_LEN as u64));
+    group.bench_function("stream", |b| {
+        b.iter(|| {
+            SuperKmerIter::new(std::hint::black_box(&rope), K, LEVEL_MAX, THETA).count()
+        });
+    });
+    group.finish();
+}
+
+criterion_group!(benches, bench_build_superkmers);
+criterion_main!(benches);
diff --git a/src/obiskbuilder/src/kmer_entropy.rs b/src/obiskbuilder/src/kmer_entropy.rs
deleted file mode 100644
index f8fcd7f..0000000
--- a/src/obiskbuilder/src/kmer_entropy.rs
+++ /dev/null
@@ -1,49 +0,0 @@
-//! Normalized entropy of an isolated, already-built k-mer.
-//!
-//! [`SuperKmerIter`](crate::SuperKmerIter) uses [`RollingStat`] to reject
-//! low-complexity k-mers *during* superkmer construction, incrementally, over
-//! a streaming window. [`KmerEntropy`] exposes the same metric for a single
-//! k-mer taken in isolation (e.g. one already reconstructed from an index's
-//! `unitigs.bin`, with no surrounding sequence) — built on the identical
-//! `RollingStat` code path, not a re-derived formula, so a `theta` threshold
-//! chosen for index-build-time filtering means the same thing when applied
-//! after the fact (e.g. `obikmer filter`).
-
-use obikseq::CanonicalKmer;
-
-use crate::rolling_stat::RollingStat;
-
-/// Extension trait: compute the normalized entropy of a single canonical
-/// k-mer, independent of any surrounding sequence.
-pub trait KmerEntropy {
-    /// Normalized entropy across sub-word orders `1..=level_max` (the
-    /// minimum is taken across orders, same as
-    /// [`RollingStat::normalized_entropy`]). Lower means less complex;
-    /// `theta` in `index`/`filter` rejects k-mers with a score `< theta`.
-    ///
-    /// # Panics
-    ///
-    /// Requires both `obikseq::params::k()` *and* `params::m()` to already be
-    /// set (via `set_k`/`set_m`), even though the minimizer length plays no
-    /// conceptual role in an entropy score: `RollingStat` is a shared,
-    /// general-purpose struct (also used for minimizer selection during
-    /// superkmer decomposition) and unconditionally sizes an `m`-dependent
-    /// mask in its constructor. Callers must call `set_m` with a valid value
-    /// (e.g. the index's own stored `minimizer_size`) even when only the
-    /// entropy score is needed.
-    fn entropy(&self, level_max: usize) -> f64;
-}
-
-impl KmerEntropy for CanonicalKmer {
-    fn entropy(&self, level_max: usize) -> f64 {
-        let mut stat = RollingStat::new(level_max);
-        for &base in self.to_ascii().iter() {
-            stat.push(base);
-        }
-        stat.normalized_entropy().unwrap_or(1.0)
-    }
-}
-
-#[cfg(test)]
-#[path = "tests/kmer_entropy.rs"]
-mod tests;
diff --git a/src/obiskbuilder/src/lib.rs b/src/obiskbuilder/src/lib.rs
index 61d8cf1..2dd9c2f 100644
--- a/src/obiskbuilder/src/lib.rs
+++ b/src/obiskbuilder/src/lib.rs
@@ -6,16 +6,13 @@
 #![deny(missing_docs)]
 
 pub mod iter;
-pub mod kmer_entropy;
 pub mod stream_iter;
 mod scratch;
 
 pub(crate) mod encoding;
-pub(crate) mod entropy_table;
 pub(crate) mod rolling_stat;
 
 pub use iter::SuperKmerIter;
-pub use kmer_entropy::KmerEntropy;
 pub use scratch::SuperKmerScratch;
 pub use stream_iter::SuperKmerStreamIter;
 
diff --git a/src/obiskbuilder/src/rolling_stat.rs b/src/obiskbuilder/src/rolling_stat.rs
index 28d49a6..c824c99 100644
--- a/src/obiskbuilder/src/rolling_stat.rs
+++ b/src/obiskbuilder/src/rolling_stat.rs
@@ -1,8 +1,8 @@
+use obikentropy::{EntropyTracker, sub_word_canon};
 use obikseq::kmer::{Minimizer, hash_kmer};
 use obikseq::params;
 
 use crate::encoding::encode_nuc;
-use crate::entropy_table::{WS_MAX, emax, entropy_norm_kmer, ln_class_size, log_nwords, n_log_n};
 
 // ── Stack-allocated ring buffer ───────────────────────────────────────────────
 
@@ -83,33 +83,19 @@ pub struct RollingStat {
     entropy_max_k: usize,
     k: usize,
     m: usize,
-    steady: bool,
     rolling_k: u64,
     rolling_rck: u64,
     k_mask: u64,
     m_mask: u64,
     received: usize,
 
-    // Sliding-window queues — stack-allocated, capacity ≤ k ≤ 31.
-    k1q: Ring<u64, 32>,
-    k2q: Ring<u64, 32>,
-    k3q: Ring<u64, 32>,
-    k4q: Ring<u64, 32>,
-    k5q: Ring<u64, 32>,
-    k6q: Ring<u64, 32>,
+    // Minimizer selection state.
     minimier: Ring<MmerItem, 32>,
 
-    // Frequency count arrays.
-    // Max count per cell ≤ k ≤ 31 → u8 is sufficient.
-    k1c: [u8; 4],
-    k2c: [u8; 16],
-    k3c: [u8; 64],
-    k4c: [u8; 256],
-    k5c: [u8; 1024],
-    k6c: [u8; 4096],
-
-    sum_f_log_f: [f64; WS_MAX + 1],
-    sum_f_log_s: [f64; WS_MAX + 1],
+    // Entropy tracking, composed as a plain inline field so both concerns
+    // update in the same streaming pass without being conflated in one
+    // struct — see `obikentropy::EntropyTracker`.
+    entropy: EntropyTracker,
 }
 
 impl RollingStat {
@@ -120,27 +106,13 @@ impl RollingStat {
             entropy_max_k,
             k,
             m,
-            steady: false,
             rolling_k: 0,
             rolling_rck: 0,
             k_mask: (!0u64) >> (64 - k * 2),
             m_mask: (!0u64) >> (64 - m * 2),
             received: 0,
-            k1q: Ring::new(),
-            k2q: Ring::new(),
-            k3q: Ring::new(),
-            k4q: Ring::new(),
-            k5q: Ring::new(),
-            k6q: Ring::new(),
             minimier: Ring::new(),
-            k1c: [0; 4],
-            k2c: [0; 16],
-            k3c: [0; 64],
-            k4c: [0; 256],
-            k5c: [0; 1024],
-            k6c: [0; 4096],
-            sum_f_log_f: [0.0; WS_MAX + 1],
-            sum_f_log_s: [0.0; WS_MAX + 1],
+            entropy: EntropyTracker::new(k),
         }
     }
 
@@ -148,54 +120,9 @@ impl RollingStat {
         self.rolling_k = 0;
         self.rolling_rck = 0;
         self.received = 0;
-        self.steady = false;
 
-        //        for i in self.k1q.iter() { self.k1c[i as usize] = 0; }
-        //        for i in self.k2q.iter() { self.k2c[i as usize] = 0; }
-        //        for i in self.k3q.iter() { self.k3c[i as usize] = 0; }
-        //        for i in self.k4q.iter() { self.k4c[i as usize] = 0; }
-        //        for i in self.k5q.iter() { self.k5c[i as usize] = 0; }
-        //        for i in self.k6q.iter() { self.k6c[i as usize] = 0; }
-
-        self.k1c.fill(0);
-        self.k2c.fill(0);
-        self.k3c.fill(0);
-        self.k4c.fill(0);
-        self.k5c.fill(0);
-        self.k6c.fill(0);
-
-        self.k1q.clear();
-        self.k2q.clear();
-        self.k3q.clear();
-        self.k4q.clear();
-        self.k5q.clear();
-        self.k6q.clear();
         self.minimier.clear();
-
-        self.sum_f_log_f = [0.0; WS_MAX + 1];
-        self.sum_f_log_s = [0.0; WS_MAX + 1];
-    }
-
-    #[inline]
-    fn update_sums_decrement<const K: usize>(
-        sum_f_log_f: &mut [f64; WS_MAX + 1],
-        sum_f_log_s: &mut [f64; WS_MAX + 1],
-        canonical: u64,
-        f: usize,
-    ) {
-        sum_f_log_f[K] += n_log_n(f - 1) - n_log_n(f);
-        sum_f_log_s[K] -= ln_class_size::<false, K>(canonical);
-    }
-
-    #[inline]
-    fn update_sums_increment<const K: usize>(
-        sum_f_log_f: &mut [f64; WS_MAX + 1],
-        sum_f_log_s: &mut [f64; WS_MAX + 1],
-        canonical: u64,
-        g: usize,
-    ) {
-        sum_f_log_f[K] += n_log_n(g + 1) - n_log_n(g);
-        sum_f_log_s[K] += ln_class_size::<false, K>(canonical);
+        self.entropy.reset();
     }
 
     pub fn push(&mut self, nuc: u8) {
@@ -209,12 +136,7 @@ impl RollingStat {
         self.rolling_rck =
             ((self.rolling_rck >> 2) | ((cnuc as u64) << ((k - 1) * 2))) & self.k_mask;
 
-        let canonical_k1 = entropy_norm_kmer::<false, 1>(self.rolling_k & 3);
-        let canonical_k2 = entropy_norm_kmer::<false, 2>(self.rolling_k & 15);
-        let canonical_k3 = entropy_norm_kmer::<false, 3>(self.rolling_k & 63);
-        let canonical_k4 = entropy_norm_kmer::<false, 4>(self.rolling_k & 255);
-        let canonical_k5 = entropy_norm_kmer::<false, 5>(self.rolling_k & 1023);
-        let canonical_k6 = entropy_norm_kmer::<false, 6>(self.rolling_k & 4095);
+        let canon = sub_word_canon(self.rolling_k);
 
         self.received += 1;
 
@@ -248,153 +170,7 @@ impl RollingStat {
             }
         }
 
-        if self.received > k {
-            let old1 = self.k1q.pop_front();
-            let f1 = self.k1c[old1 as usize] as usize;
-            Self::update_sums_decrement::<1>(
-                &mut self.sum_f_log_f,
-                &mut self.sum_f_log_s,
-                old1,
-                f1,
-            );
-            self.k1c[old1 as usize] -= 1;
-
-            let old2 = self.k2q.pop_front();
-            let f2 = self.k2c[old2 as usize] as usize;
-            Self::update_sums_decrement::<2>(
-                &mut self.sum_f_log_f,
-                &mut self.sum_f_log_s,
-                old2,
-                f2,
-            );
-            self.k2c[old2 as usize] -= 1;
-
-            let old3 = self.k3q.pop_front();
-            let f3 = self.k3c[old3 as usize] as usize;
-            Self::update_sums_decrement::<3>(
-                &mut self.sum_f_log_f,
-                &mut self.sum_f_log_s,
-                old3,
-                f3,
-            );
-            self.k3c[old3 as usize] -= 1;
-
-            let old4 = self.k4q.pop_front();
-            let f4 = self.k4c[old4 as usize] as usize;
-            Self::update_sums_decrement::<4>(
-                &mut self.sum_f_log_f,
-                &mut self.sum_f_log_s,
-                old4,
-                f4,
-            );
-            self.k4c[old4 as usize] -= 1;
-
-            let old5 = self.k5q.pop_front();
-            let f5 = self.k5c[old5 as usize] as usize;
-            Self::update_sums_decrement::<5>(
-                &mut self.sum_f_log_f,
-                &mut self.sum_f_log_s,
-                old5,
-                f5,
-            );
-            self.k5c[old5 as usize] -= 1;
-
-            let old6 = self.k6q.pop_front();
-            let f6 = self.k6c[old6 as usize] as usize;
-            Self::update_sums_decrement::<6>(
-                &mut self.sum_f_log_f,
-                &mut self.sum_f_log_s,
-                old6,
-                f6,
-            );
-            self.k6c[old6 as usize] -= 1;
-        }
-
-        if self.steady {
-            let g1 = self.k1c[canonical_k1 as usize] as usize;
-            Self::update_sums_increment::<1>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k1, g1);
-            self.k1c[canonical_k1 as usize] += 1;
-            self.k1q.push_back(canonical_k1);
-
-            let g2 = self.k2c[canonical_k2 as usize] as usize;
-            Self::update_sums_increment::<2>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k2, g2);
-            self.k2c[canonical_k2 as usize] += 1;
-            self.k2q.push_back(canonical_k2);
-
-            let g3 = self.k3c[canonical_k3 as usize] as usize;
-            Self::update_sums_increment::<3>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k3, g3);
-            self.k3c[canonical_k3 as usize] += 1;
-            self.k3q.push_back(canonical_k3);
-
-            let g4 = self.k4c[canonical_k4 as usize] as usize;
-            Self::update_sums_increment::<4>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k4, g4);
-            self.k4c[canonical_k4 as usize] += 1;
-            self.k4q.push_back(canonical_k4);
-
-            let g5 = self.k5c[canonical_k5 as usize] as usize;
-            Self::update_sums_increment::<5>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k5, g5);
-            self.k5c[canonical_k5 as usize] += 1;
-            self.k5q.push_back(canonical_k5);
-
-            let g6 = self.k6c[canonical_k6 as usize] as usize;
-            Self::update_sums_increment::<6>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k6, g6);
-            self.k6c[canonical_k6 as usize] += 1;
-            self.k6q.push_back(canonical_k6);
-        } else {
-            self.push_warmup_increments(
-                canonical_k1, canonical_k2, canonical_k3,
-                canonical_k4, canonical_k5, canonical_k6,
-            );
-        }
-    }
-
-    #[cold]
-    #[inline(never)]
-    fn push_warmup_increments(
-        &mut self,
-        canonical_k1: u64, canonical_k2: u64, canonical_k3: u64,
-        canonical_k4: u64, canonical_k5: u64, canonical_k6: u64,
-    ) {
-        let g1 = self.k1c[canonical_k1 as usize] as usize;
-        Self::update_sums_increment::<1>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k1, g1);
-        self.k1c[canonical_k1 as usize] += 1;
-        self.k1q.push_back(canonical_k1);
-
-        if self.received >= 2 {
-            let g2 = self.k2c[canonical_k2 as usize] as usize;
-            Self::update_sums_increment::<2>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k2, g2);
-            self.k2c[canonical_k2 as usize] += 1;
-            self.k2q.push_back(canonical_k2);
-
-            if self.received >= 3 {
-                let g3 = self.k3c[canonical_k3 as usize] as usize;
-                Self::update_sums_increment::<3>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k3, g3);
-                self.k3c[canonical_k3 as usize] += 1;
-                self.k3q.push_back(canonical_k3);
-
-                if self.received >= 4 {
-                    let g4 = self.k4c[canonical_k4 as usize] as usize;
-                    Self::update_sums_increment::<4>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k4, g4);
-                    self.k4c[canonical_k4 as usize] += 1;
-                    self.k4q.push_back(canonical_k4);
-
-                    if self.received >= 5 {
-                        let g5 = self.k5c[canonical_k5 as usize] as usize;
-                        Self::update_sums_increment::<5>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k5, g5);
-                        self.k5c[canonical_k5 as usize] += 1;
-                        self.k5q.push_back(canonical_k5);
-
-                        if self.received >= 6 {
-                            let g6 = self.k6c[canonical_k6 as usize] as usize;
-                            Self::update_sums_increment::<6>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k6, g6);
-                            self.k6c[canonical_k6 as usize] += 1;
-                            self.k6q.push_back(canonical_k6);
-                            self.steady = true;
-                        }
-                    }
-                }
-            }
-        }
+        self.entropy.push(self.received, canon);
     }
 
     pub fn ready(&self) -> bool {
@@ -426,25 +202,13 @@ impl RollingStat {
         if !self.ready() {
             return None;
         }
-        let k = self.k;
-        let em = emax(k, order);
-        if em <= 0.0 {
-            return Some(1.0);
-        }
-        let nwords = k - order + 1;
-        let log_nw = log_nwords(k, order);
-        let nw_f = nwords as f64;
-        let h_corr = log_nw + (self.sum_f_log_s[order] - self.sum_f_log_f[order]) / nw_f;
-        Some((h_corr / em).max(0.0))
+        Some(self.entropy.entropy(order))
     }
 
     pub fn normalized_entropy(&self) -> Option<f64> {
         if !self.ready() {
             return None;
         }
-        let min_e = (1..=self.entropy_max_k)
-            .filter_map(|ws| self.entropy(ws))
-            .fold(f64::MAX, f64::min);
-        Some(if min_e == f64::MAX { 1.0 } else { min_e })
+        Some(self.entropy.normalized_entropy(self.entropy_max_k))
     }
 }
-- 
2.54.0


From fd2c23e7df12e71d18685be967f32c1210f347d9 Mon Sep 17 00:00:00 2001
From: Eric Coissac <eric@coissac.eu>
Date: Wed, 8 Jul 2026 19:26:33 +0200
Subject: [PATCH 3/3] refactor: remove equivalence class folding from entropy
 pipeline
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Removes circular-reverse complement machinery and explicit k-mer canonicalization across the entropy pipeline. Frequency tallying and Shannon entropy computation now operate directly on raw k-mer values, eliminating prior score inflation and alignment-dependent artifacts while preserving orientation invariance. Updates build scripts to generate normalized lookup tables for k-mer lengths 1–6, restricts the public API to `EntropyTracker`, and bumps crate versions. Documentation is updated to reflect the simplified raw-value approach and revised module structure.
---
 docmd/theory/entropy.md              |  48 ++++---
 docmd/theory/entropy.refs.md         |  10 +-
 src/Cargo.lock                       |   2 +-
 src/obikentropy/build.rs             |  63 +--------
 src/obikentropy/src/kmer_entropy.rs  |   4 +-
 src/obikentropy/src/lib.rs           |   2 +-
 src/obikentropy/src/table.rs         | 106 ++-------------
 src/obikentropy/src/tracker.rs       | 185 ++++++++++++---------------
 src/obikmer/Cargo.toml               |   2 +-
 src/obiskbuilder/src/rolling_stat.rs |   6 +-
 10 files changed, 142 insertions(+), 286 deletions(-)

diff --git a/docmd/theory/entropy.md b/docmd/theory/entropy.md
index 4cd159e..4811240 100644
--- a/docmd/theory/entropy.md
+++ b/docmd/theory/entropy.md
@@ -1,6 +1,6 @@
 # Kmer entropy filter
 
-Low-complexity kmers (polyA, polyT, tandem repeats) are detected and excluded during phase 1. The filter computes a **normalized Shannon entropy** over sub-words of multiple sizes, corrected for two sources of bias: the small number of observations within a single kmer, and the unequal sizes of circular equivalence classes.
+Low-complexity kmers (polyA, polyT, tandem repeats) are detected and excluded during phase 1. The filter computes a **normalized Shannon entropy** over sub-words of multiple sizes, corrected for one source of bias: the small number of observations within a single kmer relative to the number of possible sub-words.
 
 ## Sub-word frequencies
 
@@ -8,17 +8,15 @@ For a kmer of length k and a sub-word size ws (1 ≤ ws ≤ ws_max, typically ws
 
 $$w_i = \text{kmer}[i \mathinner{..} i+ws-1], \quad i = 0, \ldots, n_{\text{words}}-1$$
 
-Each sub-word is mapped to its **circular canonical form**: the lexicographic minimum among all cyclic rotations of the word **and all cyclic rotations of its reverse complement**. This extended equivalence relation ensures that entropy(K) = entropy(revcomp(K)) — the filter is strand-symmetric. Let $s_j$ be the size of equivalence class $j$ (number of distinct raw words mapping to canonical form $j$), and $f_j$ the count of canonical form $j$ among the $n_{\text{words}}$ sub-words ($\sum_j f_j = n_{\text{words}}$).
+Each sub-word is tallied under its own raw 2-bit-packed value — **no canonicalization**. Let $f_j$ be the count of raw word $j$ among the $n_{\text{words}}$ sub-words ($\sum_j f_j = n_{\text{words}}$), over the $4^{ws}$ possible raw words.
+
+An earlier version of this filter first folded each sub-word into a circular+reverse-complement equivalence class, then "unfolded" the observed class frequency back onto its members to correct for unequal class sizes. That machinery bought nothing it was claimed for — see *Why no equivalence classes* below — while measurably weakening detection of the very sequences the filter exists to catch, so it was removed.
 
 ## Corrected Shannon entropy
 
-The circular equivalence classes have unequal sizes: under a uniform distribution over all $4^{ws}$ raw words, class $j$ is visited with probability $s_j / 4^{ws}$, not $1/n_a$. Computing entropy directly over canonical classes therefore underestimates the entropy of a random sequence.
+$$H_{\text{corr}} = \log(n_{\text{words}}) - \frac{1}{n_{\text{words}}} \sum_j f_j \log f_j$$
 
-The correction "unfolds" each canonical class back to its member raw words, redistributing each observation of class $j$ equally among its $s_j$ members:
-
-$$H_{\text{corr}} = \log(n_{\text{words}}) - \frac{1}{n_{\text{words}}} \sum_j f_j \log f_j + \frac{1}{n_{\text{words}}} \sum_j f_j \log s_j$$
-
-The last term is the correction for unequal class sizes. For a uniformly random sequence ($f_j \approx n_{\text{words}} \cdot s_j / 4^{ws}$), this gives $H_{\text{corr}} \approx \log(4^{ws}) = 2 \cdot ws \cdot \log 2$, the maximum entropy over raw words.
+This is a plain Shannon entropy over the observed raw-word frequencies.
 
 ## Maximum entropy correction for small samples
 
@@ -42,27 +40,45 @@ $$\text{entropy}(kmer) = \min_{ws=1}^{ws_{\max}} \hat{H}(ws)$$
 
 A value near 0 indicates low complexity (e.g. AAAA…); near 1 indicates high complexity. A kmer is rejected if $\text{entropy}(kmer) < \theta$, where $\theta$ is a collection parameter (default 0.7). The minimum across word sizes ensures that any scale of repetition is detected independently: polyA is caught at ws=1, dinucleotide repeats at ws=2, etc.
 
+## Why no equivalence classes
+
+A prior design folded each sub-word into the canonical form of its circular-rotation + reverse-complement equivalence class before tallying, on the reasoning that (a) it guarantees $\text{entropy}(K) = \text{entropy}(\text{revcomp}(K))$, and (b) collapsing phase-shifted repeats (e.g. `ATG` ≡ `TGA` ≡ `GAT`) into one class better reflects that they are "the same" low-complexity pattern.
+
+Both properties already hold for the raw, unfolded entropy above, without any class machinery:
+
+- **Reverse complement**: for any K of length n, window $j$ of $\text{revcomp}(K)$ equals $\text{revcomp}$ of window $(n{-}ws{-}j)$ of K. This is a bijection between the window sets under which each window maps to its own revcomp — and revcomp is itself a bijection (involution) on the space of raw ws-mers. So the multiset of raw-word frequencies for $\text{revcomp}(K)$ is exactly a relabeling of the multiset for K, and Shannon entropy — a function of the frequency multiset alone — is exactly invariant. No folding required, for any K.
+- **Tandem repeats**: a period-p repeat sampled by a stride-1 sliding window naturally cycles through its own rotations as raw tokens (e.g. `ATGATGATG…` yields the raw words `ATG`, `TGA`, `GAT` in rotation as the window slides). The low diversity this represents (few distinct raw words out of $4^{ws}$ possible) is already visible in the raw frequency distribution — no folding needed to detect it.
+
+What the fold-then-unfold step actually did was credit each observed class with the frequency of equivalence-class members that were **never observed on the read strand**, inflating $H_{\text{corr}}$ for genuine repeats. Worked example: k=31, ws=3, kmer = `ATG` repeated ($n_{\text{words}}=29$, all 29 windows fall into one class of size 6 under the old scheme — 3 rotations × forward/revcomp):
+
+| | $H_{\text{corr}}$ | normalized |
+|---|---|---|
+| old (folded, class size 6) | $\log 6 \approx 1.79$ | $\approx 0.53$ |
+| current (raw, unfolded) | $\log 3 \approx 1.10$ | $\approx 0.33$ |
+
+The gap is not a rounding artifact: per sub-word order, the folded score for this same repeat swings from 0.53 (ws=3, aligned with the period) up to **1.03** (ws=5, misaligned with the period) — i.e. a period-3 repeat could score *above* the theoretical maximum for a random sequence, depending on which ws happens to divide the repeat's period. The raw formula stays flat at ≈0.33–0.40 across ws=2..6 regardless of alignment, which is the robustness the "minimum across ws" design was meant to provide in the first place.
+
 ## Interpretation as an effective number of classes
 
-$H_{\text{corr}}$ is a standard Shannon entropy over raw words (after unfolding the equivalence classes), so the classical perplexity interpretation holds directly: $N_{\text{eff}} = e^{H_{\text{corr}}}$ is the number of equiprobable classes that would yield the same entropy.
+$H_{\text{corr}}$ is a standard Shannon entropy over raw words, so the classical perplexity interpretation holds directly: $N_{\text{eff}} = e^{H_{\text{corr}}}$ is the number of equiprobable raw words that would yield the same entropy.
 
-For the normalised score $\hat{H}$, dividing by $H_{\text{max}}$ changes the logarithm base:
+For the normalised score $\hat{H}$, dividing by $H_{\max}$ changes the logarithm base:
 
-$$\hat{H} = \frac{\log N_{\text{eff}}}{\log N_{\text{max}}} = \log_{N_{\text{max}}} N_{\text{eff}} \quad \Longleftrightarrow \quad N_{\text{eff}} = N_{\text{max}}^{\,\hat{H}}$$
+$$\hat{H} = \frac{\log N_{\text{eff}}}{\log N_{\max}} = \log_{N_{\max}} N_{\text{eff}} \quad \Longleftrightarrow \quad N_{\text{eff}} = N_{\max}^{\,\hat{H}}$$
 
-The property is preserved: $\hat{H}$ is the logarithm (in base $N_{\text{max}}$) of the effective number of equi-represented classes.
+The property is preserved: $\hat{H}$ is the logarithm (in base $N_{\max}$) of the effective number of equi-represented raw words.
 
-In the large-sample limit ($n_{\text{words}} \gg 4^{ws}$), $N_{\text{max}} \approx 4^{ws}$, giving:
+In the large-sample limit ($n_{\text{words}} \gg 4^{ws}$), $N_{\max} \approx 4^{ws}$, giving:
 
 $$N_{\text{eff}} \approx 4^{ws \cdot \hat{H}}$$
 
-This has a clean interpretation: $ws \cdot \hat{H}$ is the **effective word length** (in bases) of a perfectly uniform distribution that would produce the same entropy. At $\hat{H} = 1$ the full space of $4^{ws}$ words is used; at $\hat{H} = 0.5$ with ws=2, only $4^1 = 4$ effective classes out of 16 are occupied.
+This has a clean interpretation: $ws \cdot \hat{H}$ is the **effective word length** (in bases) of a perfectly uniform distribution that would produce the same entropy. At $\hat{H} = 1$ the full space of $4^{ws}$ words is used; at $\hat{H} = 0.5$ with ws=2, only $4^1 = 4$ effective words out of 16 are occupied.
 
-In our actual regime, $n_{\text{words}}$ is small and $4^{ws}$ can exceed $n_{\text{words}}$, so $H_{\text{max}} < \log(4^{ws})$ due to the small-sample correction. The exact effective count is $N_{\text{max}}^{\hat{H}}$, not $4^{ws \cdot \hat{H}}$.
+In our actual regime, $n_{\text{words}}$ is small and $4^{ws}$ can exceed $n_{\text{words}}$, so $H_{\max} < \log(4^{ws})$ due to the small-sample correction. The exact effective count is $N_{\max}^{\hat{H}}$, not $4^{ws \cdot \hat{H}}$.
 
 ## Properties
 
 The entropy score is a function of the kmer sequence alone — it does not depend on the surrounding context or on the position within any genome. Two consequences:
 
-- **Orientation invariance**: $\text{entropy}(K) = \text{entropy}(\text{revcomp}(K))$, guaranteed by the strand-symmetric canonical form.
+- **Orientation invariance**: $\text{entropy}(K) = \text{entropy}(\text{revcomp}(K))$ — see *Why no equivalence classes* above for why this holds without any explicit strand-folding step.
 - **Context independence**: the same kmer is always rejected or always kept, regardless of which genome it occurs in, where in that genome it appears, or which strand is considered. The filter defines a fixed partition of the kmer space into low-complexity and valid kmers.
diff --git a/docmd/theory/entropy.refs.md b/docmd/theory/entropy.refs.md
index 79ca55c..7701468 100644
--- a/docmd/theory/entropy.refs.md
+++ b/docmd/theory/entropy.refs.md
@@ -3,10 +3,14 @@
 
 ## Code couvert
 
-- `obiskbuilder/src/entropy_table.rs` — filtre Shannon sur les kmers à basse complexité
-- `obiskbuilder/src/lib.rs` — application du filtre lors du scatter (phase 1)
+- `obikentropy/src/table.rs`, `obikentropy/src/tracker.rs` — formule d'entropie et tables de correction petits effectifs
+- `obikentropy/src/kmer_entropy.rs` — entropie d'un kmer isolé (`KmerEntropy`)
+- `obiskbuilder/src/rolling_stat.rs` — composition de `obikentropy::EntropyTracker` dans le suivi streaming (sélection de minimiseur + entropie)
+- `obiskbuilder/src/iter.rs`, `obiskbuilder/src/stream_iter.rs` — application du filtre lors du scatter (phase 1)
 
 ## Notes
 
-Document théorique stable. Vérifier que les paramètres `theta` et `level_max` dans le CLI
+Le repli en classes d'équivalence circulaires + brin inverse (décrit dans une version antérieure de ce document) a été supprimé : voir la section « Why no equivalence classes » de `entropy.md` pour la justification théorique et numérique.
+
+Vérifier que les paramètres `theta` et `level_max` dans le CLI
 (`obikmer/src/cli.rs` → `CommonArgs`) correspondent bien à ce qui est décrit.
diff --git a/src/Cargo.lock b/src/Cargo.lock
index f7c63e3..0dc087d 100644
--- a/src/Cargo.lock
+++ b/src/Cargo.lock
@@ -1711,7 +1711,7 @@ dependencies = [
 
 [[package]]
 name = "obikmer"
-version = "1.1.38"
+version = "1.1.39"
 dependencies = [
  "clap",
  "csv",
diff --git a/src/obikentropy/build.rs b/src/obikentropy/build.rs
index f422279..d2c33b9 100644
--- a/src/obikentropy/build.rs
+++ b/src/obikentropy/build.rs
@@ -4,57 +4,6 @@ use std::path::PathBuf;
 const K_MAX: usize = 32;
 const WS_MAX: usize = 6;
 
-fn normalize_circular(kmer: u64, ws: usize) -> u64 {
-    let mask = (1u64 << (ws * 2)) - 1;
-    let mut canonical = kmer & mask;
-    let mut current = canonical;
-    for _ in 0..ws - 1 {
-        let top = (current >> ((ws - 1) * 2)) & 3;
-        current = ((current << 2) | top) & mask;
-        if current < canonical {
-            canonical = current;
-        }
-    }
-    canonical
-}
-
-fn revcomp_raw(x: u64, k: usize) -> u64 {
-    let x = !x;
-    let x = x.swap_bytes();
-    let x = ((x >> 4) & 0x0F0F0F0F0F0F0F0F) | ((x & 0x0F0F0F0F0F0F0F0F) << 4);
-    let x = ((x >> 2) & 0x3333333333333333) | ((x & 0x3333333333333333) << 2);
-    x << (64 - 2 * k)
-}
-
-fn build_normalized_kmer(k: usize) -> Vec<u64> {
-    let n = 1usize << (k * 2);
-    let shift = 64 - k * 2;
-    let mut result = vec![0u64; n];
-    for i in 0..n {
-        let la = (i as u64) << shift;
-        let ra = i as u64;
-        let rc_ra = revcomp_raw(la, k) >> shift;
-        let circ = normalize_circular(ra, k);
-        let circ_rc = normalize_circular(rc_ra, k);
-        result[i] = if circ < circ_rc { circ } else { circ_rc };
-    }
-    result
-}
-
-fn build_ln_class(norm: &[u64]) -> Vec<f64> {
-    let n = norm.len();
-    let mut sizes = vec![0u32; n];
-    for &c in norm {
-        sizes[c as usize] += 1;
-    }
-    norm.iter()
-        .map(|&c| {
-            let s = sizes[c as usize];
-            if s > 0 { (s as f64).ln() } else { 0.0 }
-        })
-        .collect()
-}
-
 fn build_n_log_n() -> [f64; K_MAX + 1] {
     let mut t = [0.0f64; K_MAX + 1];
     for n in 1..=K_MAX {
@@ -63,6 +12,9 @@ fn build_n_log_n() -> [f64; K_MAX + 1] {
     t
 }
 
+/// Max achievable entropy over `4^ws` raw sub-words given only `nwords`
+/// observations (most-uniform integer partition), per
+/// `docmd/theory/entropy.md`.
 fn build_emax() -> [[f64; WS_MAX + 1]; K_MAX + 1] {
     let mut t = [[0.0f64; WS_MAX + 1]; K_MAX + 1];
     for k in 2..=K_MAX {
@@ -125,13 +77,6 @@ fn main() {
     let out_dir = PathBuf::from(std::env::var("OUT_DIR").unwrap());
     let mut out = String::new();
 
-    for k in 1..=6usize {
-        let n = 1usize << (k * 2);
-        let norm = build_normalized_kmer(k);
-        let ln_class = build_ln_class(&norm);
-        emit_f64_1d(&mut out, &format!("LN_CLASS{k}"), n, &ln_class);
-    }
-
     let n_log_n = build_n_log_n();
     emit_f64_1d(&mut out, "N_LOG_N", K_MAX + 1, &n_log_n);
 
@@ -141,5 +86,5 @@ fn main() {
     let log_nwords = build_log_nwords();
     emit_f64_2d(&mut out, "LOG_NWORDS", K_MAX + 1, WS_MAX + 1, &log_nwords);
 
-    fs::write(out_dir.join("ln_class_tables.rs"), out).unwrap();
+    fs::write(out_dir.join("entropy_tables.rs"), out).unwrap();
 }
diff --git a/src/obikentropy/src/kmer_entropy.rs b/src/obikentropy/src/kmer_entropy.rs
index 1989972..1efd64d 100644
--- a/src/obikentropy/src/kmer_entropy.rs
+++ b/src/obikentropy/src/kmer_entropy.rs
@@ -7,7 +7,7 @@
 
 use obikseq::CanonicalKmer;
 
-use crate::tracker::{EntropyTracker, sub_word_canon};
+use crate::tracker::EntropyTracker;
 
 /// Extension trait: compute the normalized entropy of a single canonical
 /// k-mer, independent of any surrounding sequence.
@@ -30,7 +30,7 @@ impl KmerEntropy for CanonicalKmer {
             let shift = 64 - 2 * (i + 1);
             let base = (raw >> shift) & 3;
             rolling = ((rolling << 2) | base) & mask;
-            tracker.push(i + 1, sub_word_canon(rolling));
+            tracker.push(i + 1, rolling);
         }
         tracker.normalized_entropy(level_max)
     }
diff --git a/src/obikentropy/src/lib.rs b/src/obikentropy/src/lib.rs
index 113bdd3..dc18360 100644
--- a/src/obikentropy/src/lib.rs
+++ b/src/obikentropy/src/lib.rs
@@ -14,4 +14,4 @@ mod table;
 mod tracker;
 
 pub use kmer_entropy::KmerEntropy;
-pub use tracker::{EntropyTracker, SubWordCanon, sub_word_canon};
+pub use tracker::EntropyTracker;
diff --git a/src/obikentropy/src/table.rs b/src/obikentropy/src/table.rs
index 289aa94..37ef4df 100644
--- a/src/obikentropy/src/table.rs
+++ b/src/obikentropy/src/table.rs
@@ -1,68 +1,16 @@
-//! Compile-time tables backing the normalized k-mer entropy formula:
-//! circular-canonical sub-word classes, their log-sizes, and the max-entropy
-//! correction for small samples. See `docmd/theory/entropy.md`.
+//! Compile-time tables backing the normalized k-mer entropy formula: the
+//! max-entropy correction for small samples. See `docmd/theory/entropy.md`.
+//!
+//! Entropy is computed directly on raw (non-canonicalized) sub-words — no
+//! equivalence-class folding. Empirically (see the discussion that produced
+//! this crate's history), folding sub-words into circular/revcomp classes
+//! before unfolding them back buys nothing for the invariances it was meant
+//! to guarantee (both hold for raw sub-word entropy already, by a direct
+//! bijection argument for revcomp and by the sliding window's own dynamics
+//! for tandem repeats), while it measurably *weakens* detection of the
+//! low-complexity sequences the filter exists to catch.
 
-pub(crate) const NORMK1: [u64; 4] = build_normalized_kmer::<4>();
-pub(crate) const NORMK2: [u64; 16] = build_normalized_kmer::<16>();
-pub(crate) const NORMK3: [u64; 64] = build_normalized_kmer::<64>();
-pub(crate) const NORMK4: [u64; 256] = build_normalized_kmer::<256>();
-pub(crate) const NORMK5: [u64; 1024] = build_normalized_kmer::<1024>();
-pub(crate) const NORMK6: [u64; 4096] = build_normalized_kmer::<4096>();
-
-include!(concat!(env!("OUT_DIR"), "/ln_class_tables.rs"));
-
-const fn normalize_circular(kmer: u64, ws: usize) -> u64 {
-    let mask = (1u64 << (ws * 2)) - 1;
-    let mut canonical = kmer & mask;
-    let mut current = canonical;
-    let mut i = 0;
-    while i < (ws - 1) {
-        let top = (current >> ((ws - 1) * 2)) & 3;
-        current = ((current << 2) | top) & mask;
-        if current < canonical {
-            canonical = current;
-        }
-        i += 1;
-    }
-    canonical
-}
-
-const fn build_normalized_kmer<const N: usize>() -> [u64; N] {
-    let mut result = [0u64; N];
-    let k = k_from_n::<N>();
-    let shift = 64 - k * 2;
-    let mut i = 0;
-    while i < N {
-        let la = (i as u64) << shift;
-        let ra = i as u64;
-        let rc_ra = revcomp_raw(la, k) >> shift;
-        let circ = normalize_circular(ra, k);
-        let circ_rc = normalize_circular(rc_ra, k);
-        result[i] = if circ < circ_rc { circ } else { circ_rc };
-        i += 1;
-    }
-    result
-}
-
-const fn revcomp_raw(x: u64, k: usize) -> u64 {
-    let x = !x;
-    let x = x.swap_bytes();
-    let x = ((x >> 4) & 0x0F0F0F0F0F0F0F0F) | ((x & 0x0F0F0F0F0F0F0F0F) << 4);
-    let x = ((x >> 2) & 0x3333333333333333) | ((x & 0x3333333333333333) << 2);
-    x << (64 - 2 * k)
-}
-
-const fn k_from_n<const N: usize>() -> usize {
-    match N {
-        4 => 1,
-        16 => 2,
-        64 => 3,
-        256 => 4,
-        1024 => 5,
-        4096 => 6,
-        _ => panic!("N must be a power of 4"),
-    }
-}
+include!(concat!(env!("OUT_DIR"), "/entropy_tables.rs"));
 
 pub(crate) const WS_MAX: usize = 6;
 
@@ -80,33 +28,3 @@ pub(crate) const fn emax(k: usize, ws: usize) -> f64 {
 pub(crate) const fn log_nwords(k: usize, ws: usize) -> f64 {
     LOG_NWORDS[k][ws]
 }
-
-#[inline(always)]
-pub(crate) const fn entropy_norm_kmer<const LEFT: bool, const K: usize>(kmer: u64) -> u64 {
-    const SHIFT: [usize; 7] = [0, 62, 60, 58, 56, 54, 52];
-    const NORM: [&[u64]; 7] = [&[], &NORMK1, &NORMK2, &NORMK3, &NORMK4, &NORMK5, &NORMK6];
-
-    let shift = SHIFT[K];
-    let ra = if LEFT { kmer >> shift } else { kmer };
-    let canonical_ra = NORM[K][ra as usize];
-    if LEFT {
-        canonical_ra << shift
-    } else {
-        canonical_ra
-    }
-}
-
-#[inline(always)]
-pub(crate) const fn ln_class_size<const LEFT: bool, const K: usize>(kmer: u64) -> f64 {
-    const SHIFT: [usize; 7] = [0, 62, 60, 58, 56, 54, 52];
-    let ra = if LEFT { kmer >> SHIFT[K] } else { kmer };
-    match K {
-        1 => LN_CLASS1[ra as usize],
-        2 => LN_CLASS2[ra as usize],
-        3 => LN_CLASS3[ra as usize],
-        4 => LN_CLASS4[ra as usize],
-        5 => LN_CLASS5[ra as usize],
-        6 => LN_CLASS6[ra as usize],
-        _ => panic!("k must be 1..=6"),
-    }
-}
diff --git a/src/obikentropy/src/tracker.rs b/src/obikentropy/src/tracker.rs
index 98686fa..eb71bab 100644
--- a/src/obikentropy/src/tracker.rs
+++ b/src/obikentropy/src/tracker.rs
@@ -1,9 +1,12 @@
 //! Incremental (streaming) normalized k-mer entropy.
 //!
 //! [`EntropyTracker`] maintains, over a sliding window of the last `k` bases,
-//! the per-sub-word-size frequency statistics needed to evaluate the
-//! corrected Shannon entropy described in `docmd/theory/entropy.md`, updated
-//! in O(1) per base rather than recomputed from scratch.
+//! the per-sub-word-size raw-word frequency statistics needed to evaluate
+//! the corrected Shannon entropy described in `docmd/theory/entropy.md`,
+//! updated in O(1) per base rather than recomputed from scratch. No
+//! canonicalization is applied — each sub-word is tallied under its own raw
+//! 2-bit-packed value; only the small-sample max-entropy correction departs
+//! from a textbook Shannon entropy.
 //!
 //! It carries no notion of minimizers or superkmer segmentation — callers
 //! that need both (e.g. `obiskbuilder::RollingStat`) compose an
@@ -12,25 +15,7 @@
 //! struct.
 
 use crate::ring::Ring;
-use crate::table::{WS_MAX, emax, entropy_norm_kmer, ln_class_size, log_nwords, n_log_n};
-
-/// Canonical sub-word values for word sizes 1..=6, computed by the caller
-/// from its own rolling k-mer bits (see [`EntropyTracker::push`]).
-pub type SubWordCanon = [u64; WS_MAX];
-
-/// Compute the six canonical sub-word values (word sizes 1..=6) for the
-/// current rolling right-aligned k-mer window.
-#[inline]
-pub fn sub_word_canon(rolling_kmer: u64) -> SubWordCanon {
-    [
-        entropy_norm_kmer::<false, 1>(rolling_kmer & 3),
-        entropy_norm_kmer::<false, 2>(rolling_kmer & 15),
-        entropy_norm_kmer::<false, 3>(rolling_kmer & 63),
-        entropy_norm_kmer::<false, 4>(rolling_kmer & 255),
-        entropy_norm_kmer::<false, 5>(rolling_kmer & 1023),
-        entropy_norm_kmer::<false, 6>(rolling_kmer & 4095),
-    ]
-}
+use crate::table::{WS_MAX, emax, log_nwords, n_log_n};
 
 /// Incremental normalized-entropy accumulator over a sliding window of `k`
 /// bases. Composed as a plain field by callers that also need other
@@ -39,8 +24,8 @@ pub struct EntropyTracker {
     k: usize,
     steady: bool,
 
-    // Sliding-window queues over the last `k` canonical sub-words, one per
-    // word size — stack-allocated, capacity ≤ k ≤ 31.
+    // Sliding-window queues over the last `k` raw sub-words, one per word
+    // size — stack-allocated, capacity ≤ k ≤ 31.
     k1q: Ring<u64, 32>,
     k2q: Ring<u64, 32>,
     k3q: Ring<u64, 32>,
@@ -48,7 +33,8 @@ pub struct EntropyTracker {
     k5q: Ring<u64, 32>,
     k6q: Ring<u64, 32>,
 
-    // Frequency count arrays. Max count per cell ≤ k ≤ 31 → u8 is sufficient.
+    // Frequency count arrays, indexed by the raw sub-word value (2 bits per
+    // base). Max count per cell ≤ k ≤ 31 → u8 is sufficient.
     k1c: [u8; 4],
     k2c: [u8; 16],
     k3c: [u8; 64],
@@ -57,7 +43,6 @@ pub struct EntropyTracker {
     k6c: [u8; 4096],
 
     sum_f_log_f: [f64; WS_MAX + 1],
-    sum_f_log_s: [f64; WS_MAX + 1],
 }
 
 impl EntropyTracker {
@@ -79,7 +64,6 @@ impl EntropyTracker {
             k5c: [0; 1024],
             k6c: [0; 4096],
             sum_f_log_f: [0.0; WS_MAX + 1],
-            sum_f_log_s: [0.0; WS_MAX + 1],
         }
     }
 
@@ -103,103 +87,94 @@ impl EntropyTracker {
         self.k6q.clear();
 
         self.sum_f_log_f = [0.0; WS_MAX + 1];
-        self.sum_f_log_s = [0.0; WS_MAX + 1];
     }
 
     #[inline]
-    fn update_sums_decrement<const K: usize>(
-        sum_f_log_f: &mut [f64; WS_MAX + 1],
-        sum_f_log_s: &mut [f64; WS_MAX + 1],
-        canonical: u64,
-        f: usize,
-    ) {
+    fn update_sums_decrement<const K: usize>(sum_f_log_f: &mut [f64; WS_MAX + 1], f: usize) {
         sum_f_log_f[K] += n_log_n(f - 1) - n_log_n(f);
-        sum_f_log_s[K] -= ln_class_size::<false, K>(canonical);
     }
 
     #[inline]
-    fn update_sums_increment<const K: usize>(
-        sum_f_log_f: &mut [f64; WS_MAX + 1],
-        sum_f_log_s: &mut [f64; WS_MAX + 1],
-        canonical: u64,
-        g: usize,
-    ) {
+    fn update_sums_increment<const K: usize>(sum_f_log_f: &mut [f64; WS_MAX + 1], g: usize) {
         sum_f_log_f[K] += n_log_n(g + 1) - n_log_n(g);
-        sum_f_log_s[K] += ln_class_size::<false, K>(canonical);
     }
 
     /// Advance the window by one base. `received` is the caller's running
     /// count of bases pushed so far (1-based, i.e. after this base);
-    /// `canon` are the six canonical sub-word values for the current
-    /// rolling k-mer, from [`sub_word_canon`].
-    pub fn push(&mut self, received: usize, canon: SubWordCanon) {
-        let [canonical_k1, canonical_k2, canonical_k3, canonical_k4, canonical_k5, canonical_k6] =
-            canon;
+    /// `rolling_kmer` is the current right-aligned, 2-bit-packed k-mer
+    /// window (same convention as `obiskbuilder::RollingStat::rolling_k`).
+    pub fn push(&mut self, received: usize, rolling_kmer: u64) {
+        let raw1 = rolling_kmer & 3;
+        let raw2 = rolling_kmer & 15;
+        let raw3 = rolling_kmer & 63;
+        let raw4 = rolling_kmer & 255;
+        let raw5 = rolling_kmer & 1023;
+        let raw6 = rolling_kmer & 4095;
 
         if received > self.k {
             let old1 = self.k1q.pop_front();
             let f1 = self.k1c[old1 as usize] as usize;
-            Self::update_sums_decrement::<1>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, old1, f1);
+            Self::update_sums_decrement::<1>(&mut self.sum_f_log_f, f1);
             self.k1c[old1 as usize] -= 1;
 
             let old2 = self.k2q.pop_front();
             let f2 = self.k2c[old2 as usize] as usize;
-            Self::update_sums_decrement::<2>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, old2, f2);
+            Self::update_sums_decrement::<2>(&mut self.sum_f_log_f, f2);
             self.k2c[old2 as usize] -= 1;
 
             let old3 = self.k3q.pop_front();
             let f3 = self.k3c[old3 as usize] as usize;
-            Self::update_sums_decrement::<3>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, old3, f3);
+            Self::update_sums_decrement::<3>(&mut self.sum_f_log_f, f3);
             self.k3c[old3 as usize] -= 1;
 
             let old4 = self.k4q.pop_front();
             let f4 = self.k4c[old4 as usize] as usize;
-            Self::update_sums_decrement::<4>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, old4, f4);
+            Self::update_sums_decrement::<4>(&mut self.sum_f_log_f, f4);
             self.k4c[old4 as usize] -= 1;
 
             let old5 = self.k5q.pop_front();
             let f5 = self.k5c[old5 as usize] as usize;
-            Self::update_sums_decrement::<5>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, old5, f5);
+            Self::update_sums_decrement::<5>(&mut self.sum_f_log_f, f5);
             self.k5c[old5 as usize] -= 1;
 
             let old6 = self.k6q.pop_front();
             let f6 = self.k6c[old6 as usize] as usize;
-            Self::update_sums_decrement::<6>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, old6, f6);
+            Self::update_sums_decrement::<6>(&mut self.sum_f_log_f, f6);
             self.k6c[old6 as usize] -= 1;
         }
 
         if self.steady {
-            let g1 = self.k1c[canonical_k1 as usize] as usize;
-            Self::update_sums_increment::<1>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k1, g1);
-            self.k1c[canonical_k1 as usize] += 1;
-            self.k1q.push_back(canonical_k1);
+            let g1 = self.k1c[raw1 as usize] as usize;
+            Self::update_sums_increment::<1>(&mut self.sum_f_log_f, g1);
+            self.k1c[raw1 as usize] += 1;
+            self.k1q.push_back(raw1);
 
-            let g2 = self.k2c[canonical_k2 as usize] as usize;
-            Self::update_sums_increment::<2>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k2, g2);
-            self.k2c[canonical_k2 as usize] += 1;
-            self.k2q.push_back(canonical_k2);
+            let g2 = self.k2c[raw2 as usize] as usize;
+            Self::update_sums_increment::<2>(&mut self.sum_f_log_f, g2);
+            self.k2c[raw2 as usize] += 1;
+            self.k2q.push_back(raw2);
 
-            let g3 = self.k3c[canonical_k3 as usize] as usize;
-            Self::update_sums_increment::<3>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k3, g3);
-            self.k3c[canonical_k3 as usize] += 1;
-            self.k3q.push_back(canonical_k3);
+            let g3 = self.k3c[raw3 as usize] as usize;
+            Self::update_sums_increment::<3>(&mut self.sum_f_log_f, g3);
+            self.k3c[raw3 as usize] += 1;
+            self.k3q.push_back(raw3);
 
-            let g4 = self.k4c[canonical_k4 as usize] as usize;
-            Self::update_sums_increment::<4>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k4, g4);
-            self.k4c[canonical_k4 as usize] += 1;
-            self.k4q.push_back(canonical_k4);
+            let g4 = self.k4c[raw4 as usize] as usize;
+            Self::update_sums_increment::<4>(&mut self.sum_f_log_f, g4);
+            self.k4c[raw4 as usize] += 1;
+            self.k4q.push_back(raw4);
 
-            let g5 = self.k5c[canonical_k5 as usize] as usize;
-            Self::update_sums_increment::<5>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k5, g5);
-            self.k5c[canonical_k5 as usize] += 1;
-            self.k5q.push_back(canonical_k5);
+            let g5 = self.k5c[raw5 as usize] as usize;
+            Self::update_sums_increment::<5>(&mut self.sum_f_log_f, g5);
+            self.k5c[raw5 as usize] += 1;
+            self.k5q.push_back(raw5);
 
-            let g6 = self.k6c[canonical_k6 as usize] as usize;
-            Self::update_sums_increment::<6>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k6, g6);
-            self.k6c[canonical_k6 as usize] += 1;
-            self.k6q.push_back(canonical_k6);
+            let g6 = self.k6c[raw6 as usize] as usize;
+            Self::update_sums_increment::<6>(&mut self.sum_f_log_f, g6);
+            self.k6c[raw6 as usize] += 1;
+            self.k6q.push_back(raw6);
         } else {
-            self.push_warmup_increments(received, canonical_k1, canonical_k2, canonical_k3, canonical_k4, canonical_k5, canonical_k6);
+            self.push_warmup_increments(received, raw1, raw2, raw3, raw4, raw5, raw6);
         }
     }
 
@@ -208,43 +183,43 @@ impl EntropyTracker {
     fn push_warmup_increments(
         &mut self,
         received: usize,
-        canonical_k1: u64, canonical_k2: u64, canonical_k3: u64,
-        canonical_k4: u64, canonical_k5: u64, canonical_k6: u64,
+        raw1: u64, raw2: u64, raw3: u64,
+        raw4: u64, raw5: u64, raw6: u64,
     ) {
-        let g1 = self.k1c[canonical_k1 as usize] as usize;
-        Self::update_sums_increment::<1>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k1, g1);
-        self.k1c[canonical_k1 as usize] += 1;
-        self.k1q.push_back(canonical_k1);
+        let g1 = self.k1c[raw1 as usize] as usize;
+        Self::update_sums_increment::<1>(&mut self.sum_f_log_f, g1);
+        self.k1c[raw1 as usize] += 1;
+        self.k1q.push_back(raw1);
 
         if received >= 2 {
-            let g2 = self.k2c[canonical_k2 as usize] as usize;
-            Self::update_sums_increment::<2>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k2, g2);
-            self.k2c[canonical_k2 as usize] += 1;
-            self.k2q.push_back(canonical_k2);
+            let g2 = self.k2c[raw2 as usize] as usize;
+            Self::update_sums_increment::<2>(&mut self.sum_f_log_f, g2);
+            self.k2c[raw2 as usize] += 1;
+            self.k2q.push_back(raw2);
 
             if received >= 3 {
-                let g3 = self.k3c[canonical_k3 as usize] as usize;
-                Self::update_sums_increment::<3>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k3, g3);
-                self.k3c[canonical_k3 as usize] += 1;
-                self.k3q.push_back(canonical_k3);
+                let g3 = self.k3c[raw3 as usize] as usize;
+                Self::update_sums_increment::<3>(&mut self.sum_f_log_f, g3);
+                self.k3c[raw3 as usize] += 1;
+                self.k3q.push_back(raw3);
 
                 if received >= 4 {
-                    let g4 = self.k4c[canonical_k4 as usize] as usize;
-                    Self::update_sums_increment::<4>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k4, g4);
-                    self.k4c[canonical_k4 as usize] += 1;
-                    self.k4q.push_back(canonical_k4);
+                    let g4 = self.k4c[raw4 as usize] as usize;
+                    Self::update_sums_increment::<4>(&mut self.sum_f_log_f, g4);
+                    self.k4c[raw4 as usize] += 1;
+                    self.k4q.push_back(raw4);
 
                     if received >= 5 {
-                        let g5 = self.k5c[canonical_k5 as usize] as usize;
-                        Self::update_sums_increment::<5>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k5, g5);
-                        self.k5c[canonical_k5 as usize] += 1;
-                        self.k5q.push_back(canonical_k5);
+                        let g5 = self.k5c[raw5 as usize] as usize;
+                        Self::update_sums_increment::<5>(&mut self.sum_f_log_f, g5);
+                        self.k5c[raw5 as usize] += 1;
+                        self.k5q.push_back(raw5);
 
                         if received >= 6 {
-                            let g6 = self.k6c[canonical_k6 as usize] as usize;
-                            Self::update_sums_increment::<6>(&mut self.sum_f_log_f, &mut self.sum_f_log_s, canonical_k6, g6);
-                            self.k6c[canonical_k6 as usize] += 1;
-                            self.k6q.push_back(canonical_k6);
+                            let g6 = self.k6c[raw6 as usize] as usize;
+                            Self::update_sums_increment::<6>(&mut self.sum_f_log_f, g6);
+                            self.k6c[raw6 as usize] += 1;
+                            self.k6q.push_back(raw6);
                             self.steady = true;
                         }
                     }
@@ -265,7 +240,7 @@ impl EntropyTracker {
         let nwords = k - order + 1;
         let log_nw = log_nwords(k, order);
         let nw_f = nwords as f64;
-        let h_corr = log_nw + (self.sum_f_log_s[order] - self.sum_f_log_f[order]) / nw_f;
+        let h_corr = log_nw - self.sum_f_log_f[order] / nw_f;
         (h_corr / em).max(0.0)
     }
 
diff --git a/src/obikmer/Cargo.toml b/src/obikmer/Cargo.toml
index d9ea7a6..9d00ffd 100644
--- a/src/obikmer/Cargo.toml
+++ b/src/obikmer/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "obikmer"
-version = "1.1.38"
+version = "1.1.39"
 edition = "2024"
 
 [[bin]]
diff --git a/src/obiskbuilder/src/rolling_stat.rs b/src/obiskbuilder/src/rolling_stat.rs
index c824c99..09b3f8d 100644
--- a/src/obiskbuilder/src/rolling_stat.rs
+++ b/src/obiskbuilder/src/rolling_stat.rs
@@ -1,4 +1,4 @@
-use obikentropy::{EntropyTracker, sub_word_canon};
+use obikentropy::EntropyTracker;
 use obikseq::kmer::{Minimizer, hash_kmer};
 use obikseq::params;
 
@@ -136,8 +136,6 @@ impl RollingStat {
         self.rolling_rck =
             ((self.rolling_rck >> 2) | ((cnuc as u64) << ((k - 1) * 2))) & self.k_mask;
 
-        let canon = sub_word_canon(self.rolling_k);
-
         self.received += 1;
 
         if self.received >= m {
@@ -170,7 +168,7 @@ impl RollingStat {
             }
         }
 
-        self.entropy.push(self.received, canon);
+        self.entropy.push(self.received, self.rolling_k);
     }
 
     pub fn ready(&self) -> bool {
-- 
2.54.0