.gitignore

@@ -9,6 +9,7 @@ data-stress
 ./**/*.json
 *.bin
 *.log
+*.csv
 Betula_exilis--IGA-24-33
 benchmark/genomes
 benchmark/simulated_data

docmd/theory/evolutionary_distances.md

@@ -65,6 +65,50 @@ reverse-complement (`m -> k-1-m = m`, base complemented). A transition maps to
 a transition, a transversion to a transversion — the transition/transversion
 split is well-defined in canonical space.
 
+### Definitions: family, and the canonical form of a family
+
+**Family.** The family of a k-mer `x` is the set of (up to) 4 k-mers sharing
+`x`'s `2m` flanking bases, differing only at the central base `m`. Membership
+is a property of the flank pattern, not of `x` itself: any of the 4 possible
+central substitutions belongs to the same family.
+
+**`central_canonical_neighbors()`** (`obikseq`, `CanonicalKmerOf::central_canonical_neighbors`)
+generates all 4 members from any one of them (observed or not), each
+independently canonicalised (`.canonical()`, i.e. `min(kmer, revcomp(kmer))`).
+This independent canonicalisation is necessary because a central substitution
+can flip which orientation is lexicographically smaller — two members of the
+same family can end up canonicalised in *different* orientations. Despite
+that, the **set** of 4 resulting canonical k-mers is invariant: calling
+`central_canonical_neighbors()` on any member of a family — present in the
+index or not — yields the same 4 values. This is relied upon throughout the
+rest of this document.
+
+**Canonical form of a family.** Because orientation can differ member to
+member, "which of the 4 is the reference" cannot be defined relative to
+*whichever member happened to be visited first*, nor relative to the
+minorant (see below) — both are data-dependent (they depend on what is
+actually observed), so using either as the reference would make the
+reference itself vary depending on what happens to be present in a given
+index. Instead: **the canonical form of a family is, by definition, the
+member whose own central base — read in its own already-canonical
+orientation — is `A`.** This is well-defined for every family, computed
+purely from the flank pattern, whether or not that specific member (or any
+member at all) is actually observed anywhere in the index. Concretely: call
+`central_canonical_neighbors()` on any member (observed or not) to get the
+family's 4 canonical forms; the one among them whose own centre nucleotide is
+`A` is the family's canonical form. The other 3 (`C`, `G`, `T`) are labelled
+relative to *that* fixed reference, not relative to the calling member's own
+orientation.
+
+**Consequence for the minorant.** With this fixed A-referenced labelling,
+`minorant` (the smallest raw encoding among the family's *observed* members,
+introduced further below) becomes directly computable rather than needing to
+be tracked as extra state: regenerate the family's 4 canonical forms from
+any member's own k-mer (cheap, no lookup), compare the raw encodings of
+whichever are marked present, and take the smallest. No separate stored bit
+is required — see Step 2b below, where this replaces the earlier
+minorant-bit design.
+
 ## Locus eligibility: raw definition vs. paralogy filter
 
 For each k-mer `x` observed in genome A (source, one MPHF slot; the 3
@@ -482,31 +526,58 @@ here.
 
 ### Step 2b — sibling-count / minorant annex (consolidated plan)
 
-Scope: only the precursor annex (sibling count 0-3 per slot, minorant
-decided on demand) — not the SNP tally itself, whose Step 2 sweep remains
-unresolved above. This piece is simpler than the sweep, because it writes to
-an independent per-slot value, not a shared cross-k-mer accumulator, so it
-needs no dedup/ownership logic at all at this stage.
+Scope: only the precursor annex — not the SNP tally itself, whose Step 2
+sweep remains unresolved above. This piece is simpler than the sweep,
+because it writes to an independent per-slot value, not a shared
+cross-k-mer accumulator, so it needs no dedup/ownership logic at all at this
+stage.
+
+**Revised annex encoding — 4-bit presence mask, not 3-bit (minorant +
+count).** Superseded after settling the "canonical form of a family"
+definition above. The 3-bit design (1 minorant bit + 2-bit sibling count,
+§ below, kept for the historical record) had two problems: it discards
+*which* variants are present (only how many), so any future consumer
+(the SNP sweep, or a stats pass — see below) that needs to know which bases
+exist still has to regenerate and blindly re-query all 3 candidates; and
+the minorant bit's meaning was tied to whichever member was visited, not to
+a fixed reference. Storing instead a **4-bit mask** — one bit per base
+(A/C/G/T), set iff that member of the family (labelled relative to the
+family's fixed canonical form, i.e. the member with `A` at the centre — see
+above) is observed anywhere in the index — fixes both:
+- **Sibling count is derived, not stored**: `siblings = popcount(mask) - 1`.
+- **Minorant is derived, not stored**: regenerate the family's 4 canonical
+  forms from the slot's own k-mer (cheap, no lookup — see above), compare
+  the raw encodings of whichever bits are set in the mask, take the
+  smallest.
+- **A future consumer knows exactly which variants to (re-)query** —
+  `popcount(mask) - 1` lookups instead of always 3, and it knows *which*
+  3 (or fewer) to issue, not just how many hits to expect.
+- The all-zero value (no base present at all) is still logically
+  unreachable as a real result — the slot's *own* base is always present in
+  its own family — so it remains available as a free "not yet computed"
+  sentinel, exactly as before.
 
 1. **Primitive.** Reuse `central_canonical_neighbors()` from Step 0
-   unchanged — the 3 canonicalised central-substitution variants of a k-mer.
-2. **New annex type** (`obicompactvec`, alongside `bitmatrix.rs`): a 3-bit-
-   per-slot packed array, one per partition — same on-disk shape family as
-   `PersistentBitMatrix`'s `Packed` variant, but simpler (no per-genome
-   columns, a single derived read-only value per slot). 3 bits, not 2:
-   revised to also store minorant status alongside sibling count, since it
-   comes for free from the same lookups (point 3 below) — 5 real states
+   unchanged — the 3 canonicalised central-substitution variants of a k-mer
+   (plus the identity, i.e. all 4 members of the family — see "Definitions"
+   above).
+2. **New annex type** (`obicompactvec`, alongside `bitmatrix.rs`): a 4-bit-
+   per-slot packed array (the presence mask above), one per partition — same
+   on-disk shape family as `PersistentBitMatrix`'s `Packed` variant, but
+   simpler (no per-genome columns, a single derived read-only value per
+   slot).
+   <details><summary>Superseded 3-bit design (historical)</summary>
+   3 bits, storing minorant status alongside sibling count directly, since
+   it came for free from the same lookups (point 3 below) — 5 real states
    (not-minorant; minorant with 0/1/2/3 siblings) fit in 3 bits (8 states,
-   3 unused). This lets the future SNP sweep discard a non-minorant slot
-   **instantly**, with no lookup at all, instead of having to regenerate and
-   look up its siblings just to rediscover it isn't the designated writer —
-   moving that cost into this one-time, cached pass instead of repeating it
-   on every future sweep. The otherwise-unreachable combination
-   "not-minorant + 0 siblings" (impossible: 0 siblings always implies
-   minorant, see below) doubles as a free **"not yet computed" sentinel** —
-   annex files for all partitions/layers can be pre-initialised to this
-   value before the computation pass runs, distinguishing genuinely-computed
-   0-sibling slots from not-yet-processed ones with no extra storage.
+   3 unused). This let the future SNP sweep discard a non-minorant slot
+   instantly, with no lookup at all. The otherwise-unreachable combination
+   "not-minorant + 0 siblings" (0 siblings always implies minorant) doubled
+   as the "not yet computed" sentinel. Replaced by the 4-bit mask above,
+   which subsumes this benefit (minorant still derivable, now for free at
+   read time rather than stored) while also fixing the "which variant"
+   blindness.
+   </details>
 3. **Computation pass** (`obikindex`, new `siblings.rs`): **one
    `obipipeline` run per layer, iterated sequentially over the index's
    layers** — settled after two false starts, worth recording both.

src/obicompactvec/src/lib.rs

@@ -19,7 +19,7 @@ pub use builder::PersistentCompactIntVecBuilder;
 pub use colgroup::{ColGroup, FilterMask, MatrixGroupOps, eval_filter_mask};
 pub use intmatrix::{PersistentCompactIntMatrix, PersistentCompactIntMatrixBuilder, pack_compact_int_matrix};
 pub use layer_meta::LayerMeta;
-pub use siblingannex::{SiblingAnnex, SiblingAnnexBuilder, SiblingInfo};
+pub use siblingannex::{FamilyMask, SiblingAnnex, SiblingAnnexBuilder};
 pub use reader::{PersistentCompactIntVec, Iter as CompactIntVecIter};
 pub use tempbitvec::{TempBitVec, TempBitVecBuilder};
 pub use tempintvec::{TempCompactIntVec, TempCompactIntVecBuilder};

src/obicompactvec/src/siblingannex.rs

@@ -1,27 +1,33 @@
-//! Sibling-count / minorant annex: a compact, read-only-after-build, per-slot
+//! Family presence-mask annex: a compact, read-only-after-build, per-slot
 //! derived value used by the central-position SNP distance estimator (see
-//! `docmd/theory/evolutionary_distances.md`, "Step 2b").
+//! `docmd/theory/evolutionary_distances.md`, "Step 2b" and "Definitions:
+//! family, and the canonical form of a family").
 //!
-//! One byte is stored per MPHF slot of a partition/layer, encoding two
-//! independent facts about the slot's k-mer's "family" (the up-to-4 k-mers
-//! sharing the same flanks, differing only at the central base), both
-//! properties of the whole current multi-genome index, not of any one
-//! genome:
+//! One byte is stored per MPHF slot of a partition/layer, its low 4 bits
+//! encoding a **presence mask** for the slot's k-mer's "family" (the up to 4
+//! k-mers sharing the same flanks, differing only at the central base):
+//! bit `b` (`b` = 0..3, in the fixed A/C/G/T = 0/1/2/3 encoding already used
+//! for a single nucleotide) is set iff the family member whose *own* central
+//! base — in its own canonical orientation — is `b`, is observed anywhere in
+//! the current multi-genome index. This is a property of the whole index,
+//! not of any one genome.
 //!
-//! - bit 0: `minorant` — is this k-mer's canonical encoding the smallest
-//!   among the family members actually observed in the index?
-//! - bits 1-2: `siblings` — how many *other* family members (0-3) are
-//!   observed anywhere in the index.
+//! Both facts the earlier (superseded) 3-bit design stored explicitly are
+//! derived from the mask instead, not stored:
+//! - sibling count = `popcount(mask) - 1`;
+//! - minorant = regenerate the family's 4 canonical forms from the slot's
+//!   own k-mer (`CanonicalKmerOf::central_canonical_neighbors`, cheap, no
+//!   lookup), compare the raw encodings of whichever are set in the mask,
+//!   take the smallest — see `obikindex::siblings`.
 //!
-//! Byte value 0 (`minorant = false`, `siblings = 0`) is logically
-//! unreachable as a real result (0 siblings always implies minorant — see
-//! the design doc) and is reused as the "not yet computed" sentinel: annex
-//! files are pre-initialised to all-zero, and a real value is only ever
-//! written once, by the computation pass.
+//! Mask value 0 is logically unreachable as a real result (a slot's own base
+//! is always present in its own family) and is reused as the "not yet
+//! computed" sentinel: annex files are pre-initialised to all-zero, and a
+//! real value is only ever written once, by the computation pass.
 //!
-//! Deliberately simpler than a true 3-bit pack (1 byte/slot instead of 3
-//! bits/slot): correctness and simplicity first: for a first implementation.
-//! Packing to 3 bits/slot is a pure storage-density follow-up, not a
+//! Deliberately simpler than a true 4-bit pack (1 byte/slot instead of 4
+//! bits/slot): correctness and simplicity first, for a first implementation.
+//! Packing to 4 bits/slot is a pure storage-density follow-up, not a
 //! behavioural change, left for later.
 
 use std::fs::{File, OpenOptions};
@@ -35,30 +41,70 @@ const MAGIC: [u8; 4] = *b"PSIB";
 // Header: magic(4) + _pad(4) + n(8) = 16 bytes. Data (1 byte/slot) follows.
 const HEADER_SIZE: usize = 16;
 
-/// Decoded value of one slot's annex entry.
+/// A family presence mask: bit `b` set iff the member whose own canonical
+/// central base is `b` (0=A, 1=C, 2=G, 3=T) is observed in the index.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct SiblingInfo {
-    pub minorant: bool,
-    pub siblings: u8, // 0..=3
+pub struct FamilyMask(u8);
+
+impl FamilyMask {
+    /// The empty mask — never a valid *computed* result (a slot's own base
+    /// is always present in its own family) — used only to build up a mask
+    /// via repeated [`with`](Self::with) calls before storing it.
+    pub const EMPTY: FamilyMask = FamilyMask(0);
+
+    /// Set bit `base` (0=A, 1=C, 2=G, 3=T).
+    #[inline]
+    pub fn with(self, base: u8) -> Self {
+        debug_assert!(base < 4, "base out of range: {base}");
+        FamilyMask(self.0 | (1 << base))
+    }
+
+    /// Is the member with central base `base` (0..3) present?
+    #[inline]
+    pub fn has(self, base: u8) -> bool {
+        debug_assert!(base < 4, "base out of range: {base}");
+        self.0 & (1 << base) != 0
+    }
+
+    /// Number of family members observed anywhere in the index (1..=4).
+    #[inline]
+    pub fn family_size(self) -> u32 {
+        self.0.count_ones()
+    }
+
+    /// Number of *other* members observed (0..=3) — `family_size() - 1`.
+    #[inline]
+    pub fn siblings(self) -> u32 {
+        self.family_size() - 1
+    }
+
+    /// Raw bitmask (bit `b` = base `b` present) — for callers that build up
+    /// a mask via their own bit operations (e.g. concurrently, via an
+    /// `AtomicU8`) and only need the `FamilyMask` wrapper at the end.
+    #[inline]
+    pub fn bits(self) -> u8 {
+        self.0
+    }
+
+    /// Construct from a raw bitmask (only the low 4 bits are kept).
+    #[inline]
+    pub fn from_bits(bits: u8) -> Self {
+        FamilyMask(bits & 0b1111)
     }
 
-impl SiblingInfo {
     #[inline]
     fn encode(self) -> u8 {
-        (self.siblings << 1) | (self.minorant as u8)
+        self.0
     }
 
     #[inline]
     fn decode(byte: u8) -> Option<Self> {
         if byte == 0 {
-            // The unreachable "not minorant + 0 siblings" combination —
-            // reserved as the "not yet computed" sentinel.
+            // Unreachable for a real result — reserved as the "not yet
+            // computed" sentinel.
             return None;
         }
-        Some(SiblingInfo {
-            minorant: byte & 1 != 0,
-            siblings: (byte >> 1) & 0b11,
-        })
+        Some(FamilyMask(byte & 0b1111))
     }
 }
 
@@ -91,8 +137,8 @@ impl SiblingAnnex {
     pub fn is_empty(&self) -> bool { self.n == 0 }
 
     /// `None` means the slot has not (yet) been computed — see module docs.
-    pub fn get(&self, slot: usize) -> Option<SiblingInfo> {
-        SiblingInfo::decode(self.mmap[HEADER_SIZE + slot])
+    pub fn get(&self, slot: usize) -> Option<FamilyMask> {
+        FamilyMask::decode(self.mmap[HEADER_SIZE + slot])
     }
 }
 
@@ -124,17 +170,17 @@ impl SiblingAnnexBuilder {
     pub fn len(&self) -> usize { self.n }
     pub fn is_empty(&self) -> bool { self.n == 0 }
 
-    pub fn get(&self, slot: usize) -> Option<SiblingInfo> {
-        SiblingInfo::decode(self.mmap[HEADER_SIZE + slot])
+    pub fn get(&self, slot: usize) -> Option<FamilyMask> {
+        FamilyMask::decode(self.mmap[HEADER_SIZE + slot])
     }
 
-    pub fn set(&mut self, slot: usize, info: SiblingInfo) {
+    pub fn set(&mut self, slot: usize, mask: FamilyMask) {
         // Redundant concurrent writes from independent recomputation paths
         // converge to the same encoded byte for a given slot, so a plain
         // store here is safe even without external synchronisation, as long
         // as the byte write itself is atomic (true for a single aligned
         // byte on every platform this project targets).
-        self.mmap[HEADER_SIZE + slot] = info.encode();
+        self.mmap[HEADER_SIZE + slot] = mask.encode();
     }
 
     pub fn close(self) -> io::Result<()> { self.mmap.flush() }
@@ -163,36 +209,37 @@ mod tests {
     }
 
     #[test]
-    fn roundtrip_all_valid_states() {
+    fn roundtrip_all_valid_masks() {
         let dir = tempdir().unwrap();
         let path = dir.path().join("test.psib");
-        let mut builder = SiblingAnnexBuilder::new(5, &path).unwrap();
+        let mut builder = SiblingAnnexBuilder::new(4, &path).unwrap();
 
-        let cases = [
-            SiblingInfo { minorant: true, siblings: 0 },
-            SiblingInfo { minorant: true, siblings: 1 },
-            SiblingInfo { minorant: true, siblings: 2 },
-            SiblingInfo { minorant: true, siblings: 3 },
-            SiblingInfo { minorant: false, siblings: 2 },
+        let masks = [
+            FamilyMask::EMPTY.with(0),                   // just A: family size 1
+            FamilyMask::EMPTY.with(0).with(3),            // A + T: size 2
+            FamilyMask::EMPTY.with(1).with(2).with(3),    // C+G+T: size 3
+            FamilyMask::EMPTY.with(0).with(1).with(2).with(3), // all 4
         ];
-        for (slot, info) in cases.iter().enumerate() {
-            builder.set(slot, *info);
+        for (slot, mask) in masks.iter().enumerate() {
+            builder.set(slot, *mask);
         }
         let annex = builder.finish().unwrap();
-        for (slot, info) in cases.iter().enumerate() {
-            assert_eq!(annex.get(slot), Some(*info));
+        for (slot, mask) in masks.iter().enumerate() {
+            assert_eq!(annex.get(slot), Some(*mask));
         }
+        assert_eq!(annex.get(0).unwrap().siblings(), 0);
+        assert_eq!(annex.get(1).unwrap().siblings(), 1);
+        assert_eq!(annex.get(2).unwrap().siblings(), 2);
+        assert_eq!(annex.get(3).unwrap().siblings(), 3);
+        assert_eq!(annex.get(3).unwrap().family_size(), 4);
     }
 
     #[test]
-    fn not_minorant_zero_siblings_is_unreachable_via_set_and_decodes_as_sentinel() {
-        // Documented invariant, not enforced by the type: callers must never
-        // construct this combination. If they do, it is indistinguishable
-        // from "not computed" — exercised here to pin the behaviour down.
-        let dir = tempdir().unwrap();
-        let path = dir.path().join("test.psib");
-        let mut builder = SiblingAnnexBuilder::new(1, &path).unwrap();
-        builder.set(0, SiblingInfo { minorant: false, siblings: 0 });
-        assert_eq!(builder.get(0), None);
+    fn has_reflects_individual_bits() {
+        let mask = FamilyMask::EMPTY.with(0).with(2);
+        assert!(mask.has(0));
+        assert!(!mask.has(1));
+        assert!(mask.has(2));
+        assert!(!mask.has(3));
     }
 }

src/obikindex/src/siblings.rs

@@ -1,39 +1,49 @@
-//! Sibling-count / minorant annex construction.
+//! Family presence-mask annex construction.
 //!
-//! See `docmd/theory/evolutionary_distances.md`, "Step 2b — sibling-count /
-//! minorant annex", for the full design discussion this implements.
+//! See `docmd/theory/evolutionary_distances.md`, "Definitions: family, and
+//! the canonical form of a family" and "Step 2b", for the full design
+//! discussion this implements.
 //!
 //! For each distinct k-mer of each layer of the (already built/merged)
-//! index, computes two facts about its "family" (the up to 4 k-mers sharing
-//! its flanks, differing only at the central base — well-defined for odd
-//! k), both properties of the whole current multi-genome index rather than
-//! of any one genome:
-//! - how many *other* family members (0-3) are observed anywhere in the
-//!   index;
-//! - whether this k-mer is the "minorant" of its family — the smallest
-//!   canonical encoding among the members actually observed.
+//! index, computes a 4-bit presence mask for its "family" (the up to 4
+//! k-mers sharing its flanks, differing only at the central base —
+//! well-defined for odd k): bit `b` set iff the family member whose own
+//! canonical central base is `b` (0=A, 1=C, 2=G, 3=T) is observed anywhere
+//! in the current multi-genome index — a property of the whole index, not
+//! of any one genome. Sibling count and minorant are *derived* from the
+//! mask by callers, not stored (see `FamilyMask` and
+//! [`sibling_annex_stats`](KmerIndex::sibling_annex_stats) below).
 //!
-//! Per layer, the computation runs as an `obipipeline` pipeline with several
-//! elementary stages (a `Flat` stage generating each k-mer's 3 central
-//! variants, a `Transform` stage looking each variant up in its destination
-//! partition), so the scheduler's shared worker pool interleaves this work
-//! across many in-flight k-mers/variants rather than processing everything
-//! on a single thread — see `docmd/theory/evolutionary_distances.md`, Step
-//! 2b, "Mechanism", for why elementary stages were chosen deliberately over
-//! a few coarse ones.
+//! Per layer, an `obipipeline` `Flat` stage (throttled — see
+//! `obipipeline::throttle`) generates each k-mer's 3 central variants,
+//! interleaved across many in-flight k-mers by the scheduler's shared
+//! worker pool rather than processed on a single thread. The actual
+//! cross-partition lookup, though, reuses
+//! `KmerPartition::query_partition_with` — the same partition-batching
+//! mechanism `obikmer query` already uses (open a partition's files once,
+//! answer a whole batch of queries against it) — rather than a per-item
+//! pipeline stage: an earlier per-item design reopened/re-mmap'd every
+//! target partition's files on every single lookup, which was fine at
+//! toy scale but manifested as ~90% system time against a real index.
+//! See `docmd/theory/evolutionary_distances.md`, Step 2b, "Mechanism".
 
 use std::path::Path;
+use std::sync::atomic::{AtomicU8, Ordering};
 use std::sync::Arc;
 
+use rayon::prelude::*;
+
 use obicompactvec::{
-    PersistentBitMatrix, PersistentCompactIntMatrix, SiblingAnnex, SiblingAnnexBuilder, SiblingInfo,
+    FamilyMask, PersistentBitMatrix, PersistentCompactIntMatrix, SiblingAnnex, SiblingAnnexBuilder,
 };
 use obikpartitionner::KmerPartition;
+use obipipeline::ThrottleGuard;
 use obikseq::{CanonicalKmer, Minimizer};
 use obilayeredmap::{MphfLayer, OLMError};
 use obilayeredmap::meta::PartitionMeta;
 use obiskbuilder::rolling_stat::RollingStat;
 use obiskio::UnitigFileReader;
+use obisys::progress_bar;
 
 use crate::error::{OKIError, OKIResult};
 use crate::index::KmerIndex;
@@ -48,6 +58,26 @@ fn olm_to_ok(e: OLMError) -> OKIError {
     }
 }
 
+/// Central-position base of a canonical k-mer, in the fixed 0=A/1=C/2=G/3=T
+/// encoding — the mask's bit index. `k` must be odd (project invariant).
+#[inline]
+fn central_base(kmer: CanonicalKmer, k: usize) -> u8 {
+    kmer.nucleotide((k - 1) / 2)
+}
+
+/// Is `kmer` the minorant of its family, given the family's presence mask?
+/// Regenerates the family's 4 canonical forms from `kmer` itself (cheap, no
+/// lookup — see the design doc's "Definitions" section for why this is
+/// always safe: the set of 4 forms is invariant regardless of which member
+/// you start from), and compares the raw encodings of whichever are marked
+/// present in `mask`.
+fn is_minorant(kmer: CanonicalKmer, mask: FamilyMask, k: usize) -> bool {
+    kmer.central_canonical_neighbors().into_iter().all(|other| {
+        other == kmer || !mask.has(central_base(other, k)) || kmer.raw() <= other.raw()
+    })
+}
+
+
 /// Minimiser of a single, isolated canonical k-mer (not part of a streamed
 /// sequence). `RollingStat` computes minimisers incrementally along a
 /// sequence; this feeds one k-mer's bases through a fresh instance to get
@@ -74,79 +104,144 @@ fn partition_of(kmer: CanonicalKmer, n_partitions: usize) -> usize {
     (lone_kmer_minimizer(kmer).seq_hash() & mask) as usize
 }
 
-/// Running reconciliation state for one source k-mer, initialised to the
-/// trivial "no siblings observed yet" state and folded incrementally (in any
-/// order — commutative) as query answers come back.
-#[derive(Clone, Copy)]
-struct RunningState {
-    minorant: bool,
-    siblings: u8,
-}
-
-impl Default for RunningState {
-    fn default() -> Self {
-        RunningState { minorant: true, siblings: 0 }
-    }
-}
-
 // ── obipipeline data types ─────────────────────────────────────────────────
 
 /// One distinct k-mer of the layer currently being processed, at its local
-/// MPHF slot — the pipeline's source item.
-#[derive(Clone, Copy)]
+/// MPHF slot — the pipeline's source item. Carries a throttle slot (shared,
+/// `Arc`-wrapped so it can be cloned into each of the (up to 3) items this
+/// one fans out into via the `Flat` stage) that is only released once every
+/// one of those descendants has been fully processed — see the module docs'
+/// "Throttling" note for why this is required, not optional, once a `Flat`
+/// stage is in the pipeline.
 struct SourceItem {
     slot: usize,
     kmer: CanonicalKmer,
+    _permit: Arc<ThrottleGuard>,
 }
 
 /// One of a source k-mer's (up to 3) central-substitution variants, already
-/// routed to its destination partition and carrying, decided here (both
-/// encodings are already in hand — no need to wait for the lookup answer),
-/// whether this specific variant would outrank the source as minorant.
-#[derive(Clone, Copy)]
+/// routed to its destination partition and carrying its own central base
+/// (0=A/1=C/2=G/3=T) — the mask bit it will set on a hit. Carries a clone of
+/// the source item's throttle permit.
 struct VariantQuery {
     source_slot: usize,
     dest_partition: usize,
     variant: CanonicalKmer,
-    smaller: bool,
+    base: u8,
+    _permit: Arc<ThrottleGuard>,
 }
 
-/// Outcome of looking a [`VariantQuery`] up in its destination partition.
-#[derive(Clone, Copy)]
-struct AnswerMsg {
-    source_slot: usize,
-    hit: bool,
-    smaller: bool,
-}
-
-#[derive(Clone, Copy)]
 enum SibData {
     Item(SourceItem),
     Query(VariantQuery),
-    Answer(AnswerMsg),
 }
 
-/// Existence-only lookup of `variant` in partition `dest_partition`: tries
-/// each of the partition's layers in turn, stopping at the first hit — the
-/// same "try every layer's MPHF" shape as `QueryLayer::find_slot` (private
-/// to `obikpartitionner`), reimplemented here directly against the public
-/// `MphfLayer::open`/`find` since only existence is needed, not a column
-/// fetch.
-fn lookup_exists(partition: &KmerPartition, dest_partition: usize, variant: CanonicalKmer) -> bool {
-    let index_dir = partition.part_dir(dest_partition).join(INDEX_SUBDIR);
-    if !index_dir.exists() {
-        return false;
+/// Every partition's already-open MPHF layers, built **once** for the whole
+/// `build_sibling_annex` run and shared (read-only) across every lookup, in
+/// every source layer, for the rest of the run — not reopened/re-mmap'd per
+/// query, nor per source layer.
+///
+/// Confirmed necessary by sampling a real run: routing lookups through
+/// `KmerPartition::query_partition_with` (the same batching `obikmer query`
+/// uses) still reopens+re-mmaps every target partition's files on every
+/// call, and it is called once per destination partition **per source
+/// layer** — for an index with many layers this repeats the same
+/// `MphfLayer::open`/`Evidence::open`/`PersistentBitMatrix::open` work over
+/// and over. Parallelising those calls (see the gather step below) spread
+/// the redundant work across more cores but did not reduce it: sampling
+/// showed Rayon workers spending their time inside repeated `open()`
+/// syscalls, not computation. This cache amortises that cost to once per
+/// partition for the entire run, regardless of how many source layers or
+/// lookups follow.
+/// A cached layer's opened presence/count matrix, alongside its `MphfLayer`.
+enum Mat {
+    Count(PersistentCompactIntMatrix),
+    Presence(PersistentBitMatrix),
 }
-    let Ok(meta) = PartitionMeta::load(&index_dir) else { return false };
+
+impl Mat {
+    fn n_cols(&self) -> usize {
+        match self {
+            Mat::Count(m) => m.n_cols(),
+            Mat::Presence(m) => m.n_cols(),
+        }
+    }
+    fn carries(&self, g: usize, slot: usize) -> bool {
+        match self {
+            Mat::Count(m) => m.col_view(g).get(slot) != 0,
+            Mat::Presence(m) => m.get(g, slot) != 0,
+        }
+    }
+}
+
+struct PartitionCache {
+    /// `layers[partition][layer]` = that partition's opened MPHF layers,
+    /// paired 1:1 with `mats[partition][layer]`; empty if the partition
+    /// directory doesn't exist. Used by both [`KmerIndex::build_sibling_annex`]
+    /// (`layers` only) and [`KmerIndex::sibling_annex_stats`] (both).
+    layers: Vec<Vec<MphfLayer>>,
+    mats: Vec<Vec<Mat>>,
+}
+
+impl PartitionCache {
+    fn build(partition: &KmerPartition, n_parts: usize, with_counts: bool) -> OKIResult<Self> {
+        let pb = progress_bar("open_partitions", n_parts as u64, "partitions");
+        let built: Vec<(Vec<MphfLayer>, Vec<Mat>)> = (0..n_parts)
+            .into_par_iter()
+            .map(|part| -> OKIResult<(Vec<MphfLayer>, Vec<Mat>)> {
+                let index_dir = partition.part_dir(part).join(INDEX_SUBDIR);
+                if !index_dir.exists() {
+                    pb.inc(1);
+                    return Ok((Vec::new(), Vec::new()));
+                }
+                let meta = PartitionMeta::load(&index_dir).map_err(olm_to_ok)?;
+                let mut layers = Vec::with_capacity(meta.n_layers);
+                let mut mats = Vec::with_capacity(meta.n_layers);
                 for l in 0..meta.n_layers {
                     let layer_dir = index_dir.join(format!("layer_{l}"));
-        if let Ok(mphf) = MphfLayer::open(&layer_dir, &meta.mode) {
-            if mphf.find(variant).is_some() {
-                return true;
+                    let Ok(mphf) = MphfLayer::open(&layer_dir, &meta.mode) else { continue };
+                    let use_counts = with_counts && layer_dir.join("counts").exists();
+                    let mat = if use_counts {
+                        PersistentCompactIntMatrix::open(&layer_dir).ok().map(Mat::Count)
+                    } else {
+                        PersistentBitMatrix::open(&layer_dir).ok().map(Mat::Presence)
+                    };
+                    let Some(mat) = mat else { continue };
+                    layers.push(mphf);
+                    mats.push(mat);
+                }
+                pb.inc(1);
+                Ok((layers, mats))
+            })
+            .collect::<OKIResult<Vec<_>>>()?;
+        pb.finish_and_clear();
+        let (layers, mats) = built.into_iter().unzip();
+        Ok(Self { layers, mats })
+    }
+
+    /// Existence-only lookup of `variant` in partition `dest_partition`:
+    /// tries each of the partition's already-open layers in turn, stopping
+    /// at the first hit.
+    fn find(&self, dest_partition: usize, variant: CanonicalKmer) -> bool {
+        self.layers
+            .get(dest_partition)
+            .is_some_and(|layers| layers.iter().any(|mphf| mphf.find(variant).is_some()))
+    }
+
+    /// Per-genome presence vector for `variant` in partition `dest_partition`
+    /// (`true` iff that genome carries it), `None` on a miss. Same shape as
+    /// `find`, but also reads the cached matrix instead of just the MPHF.
+    fn find_presence(&self, dest_partition: usize, variant: CanonicalKmer, n_genomes: usize) -> Option<Vec<bool>> {
+        let layers = self.layers.get(dest_partition)?;
+        let mats = self.mats.get(dest_partition)?;
+        for (mphf, mat) in layers.iter().zip(mats.iter()) {
+            if let Some(slot) = mphf.find(variant) {
+                let n_cols = mat.n_cols().min(n_genomes);
+                return Some((0..n_cols).map(|g| mat.carries(g, slot)).collect());
             }
         }
+        None
     }
-    false
 }
 
 impl KmerIndex {
@@ -169,42 +264,50 @@ impl KmerIndex {
         let n_parts = self.n_partitions();
         let n_bits = n_parts.trailing_zeros() as usize;
 
-        // A fresh, owned `KmerPartition` handle (read-only use only — no
-        // writers opened), wrapped in `Arc` so pipeline stage closures
-        // (which must be `'static`, running in spawned threads) can share
-        // it without borrowing `self`.
-        let partition = Arc::new(
-            KmerPartition::open_with_config(
+        let partition = KmerPartition::open_with_config(
             &self.root_path,
             self.kmer_size(),
             self.minimizer_size(),
             n_bits,
         )
-            .map_err(OKIError::Partition)?,
-        );
+        .map_err(OKIError::Partition)?;
 
+        tracing::info!("opening {n_parts} partition(s) for the sibling-annex sweep");
+        let cache = Arc::new(PartitionCache::build(&partition, n_parts, self.meta.config.with_counts)?);
+
+        let pb = progress_bar("sibling_annex", n_parts as u64, "partitions");
+        let mut total_slots: u64 = 0;
         for part in 0..n_parts {
             let index_dir = self.partition().part_dir(part).join(INDEX_SUBDIR);
             if !index_dir.exists() {
+                pb.inc(1);
                 continue;
             }
             let meta = PartitionMeta::load(&index_dir).map_err(olm_to_ok)?;
 
+            let mut part_slots: u64 = 0;
             for l in 0..meta.n_layers {
                 let layer_dir = index_dir.join(format!("layer_{l}"));
-                self.build_layer_sibling_annex(&layer_dir, n_parts, &partition)?;
+                part_slots += self.build_layer_sibling_annex(&layer_dir, n_parts, &cache)?;
             }
+            total_slots += part_slots;
+            pb.inc(1);
+            pb.set_message(format!("partition {part}: {part_slots} kmers ({total_slots} total)"));
         }
+        pb.finish_and_clear();
+        tracing::info!("sibling annex built — {total_slots} kmers across {n_parts} partitions");
 
         Ok(())
     }
 
+    /// Returns the number of distinct k-mers (annex slots) processed, for
+    /// progress reporting.
     fn build_layer_sibling_annex(
         &self,
         layer_dir: &Path,
         n_parts: usize,
-        partition: &Arc<KmerPartition>,
-    ) -> OKIResult<()> {
+        cache: &Arc<PartitionCache>,
+    ) -> OKIResult<u64> {
         let index_dir = layer_dir.parent().expect("layer_dir has a parent index dir");
         let meta = PartitionMeta::load(index_dir).map_err(olm_to_ok)?;
         let mphf = MphfLayer::open(layer_dir, &meta.mode).map_err(olm_to_ok)?;
@@ -220,22 +323,61 @@ impl KmerIndex {
             }
         }
 
-        let sources: Vec<SourceItem> = slot_kmer
-            .iter()
-            .enumerate()
-            .filter_map(|(slot, maybe_kmer)| maybe_kmer.map(|kmer| SourceItem { slot, kmer }))
-            .collect();
+        let k = self.kmer_size();
 
-        // ── obipipeline: Flat (generate variants) -> Transform (lookup) ─────
+        // ── Reconciliation state, initialised with each slot's own base —
+        // that member is trivially present, no lookup needed. Built before
+        // the pipeline runs, from the same enumeration, since `sources`
+        // below is consumed as a throttled iterator, not collected.
+        // `AtomicU8`, not `FamilyMask`, because the gather phase below
+        // parallelises across destination partitions (independent
+        // `query_partition_with` calls, safe to run concurrently) and their
+        // `Found` hits can land on arbitrary, possibly-shared slots — a
+        // lock-free `fetch_or` avoids needing any synchronisation beyond
+        // that. ─────────────────────────────────────────────────────────
+        let mask: Vec<AtomicU8> = (0..n_slots).map(|_| AtomicU8::new(0)).collect();
+        for (slot, kmer) in slot_kmer.iter().enumerate().filter_map(|(s, k)| k.map(|k| (s, k))) {
+            mask[slot].fetch_or(1 << central_base(kmer, k), Ordering::Relaxed);
+        }
+
+        // ── obipipeline: Flat stage generates variants only — the actual
+        // cross-partition lookup reuses `KmerPartition::query_partition_with`
+        // (the same batching mechanism `obikmer query` already uses: open a
+        // partition's files once, answer a whole batch of queries against
+        // it) instead of one lookup per pipeline item. A per-item lookup
+        // (tried first) reopened/re-mmap'd every target partition's files on
+        // every single variant — fine at the scale of a handful of test
+        // k-mers, but with billions of lookups against a real index this
+        // manifested as ~90% system time, observed in practice. ───────────
         let n_workers = std::thread::available_parallelism().map(|n| n.get()).unwrap_or(4);
         let capacity = 256;
 
-        let partition_for_lookup = Arc::clone(partition);
+        // Throttling is not optional once a `Flat` stage is in the pipeline
+        // (see `obipipeline::throttle`'s docs): without it, every worker can
+        // become a simultaneous `Flat` producer, saturate the shared output
+        // channel, and deadlock against the scheduler's own dispatch loop —
+        // also observed in practice. The permit acquired here for a source
+        // k-mer is held (via the `Arc`-shared guard carried through
+        // `SourceItem` -> `VariantQuery`) until every one of its (up to 3)
+        // descendants has been read out of the pipeline by the accumulation
+        // loop below, not just until the `Flat` stage itself returns.
+        let sources: Vec<(usize, CanonicalKmer)> = slot_kmer
+            .iter()
+            .enumerate()
+            .filter_map(|(slot, maybe_kmer)| maybe_kmer.map(|kmer| (slot, kmer)))
+            .collect();
+        let throttled = obipipeline::throttle(sources.into_iter(), n_workers).map(|t| SourceItem {
+            slot: t.item.0,
+            kmer: t.item.1,
+            _permit: Arc::new(t.guard),
+        });
+
         let pipe = obipipeline::make_pipe! {
-            SibData : SourceItem => AnswerMsg,
+            SibData : SourceItem => VariantQuery,
             || {
                 move |item: SourceItem| -> Vec<VariantQuery> {
                     let kmer = item.kmer;
+                    let permit = item._permit;
                     kmer.central_canonical_neighbors()
                         .into_iter()
                         .filter(|variant| *variant != kmer)
@@ -243,88 +385,112 @@ impl KmerIndex {
                             source_slot: item.slot,
                             dest_partition: partition_of(variant, n_parts),
                             variant,
-                            smaller: variant.raw() < kmer.raw(),
+                            base: central_base(variant, k),
+                            _permit: Arc::clone(&permit),
                         })
                         .collect::<Vec<_>>()
                 }
             } : Item => Query,
-            | {
-                let partition_for_lookup = Arc::clone(&partition_for_lookup);
-                move |vq: VariantQuery| -> AnswerMsg {
-                    let hit = lookup_exists(&partition_for_lookup, vq.dest_partition, vq.variant);
-                    AnswerMsg { source_slot: vq.source_slot, hit, smaller: vq.smaller }
-                }
-            } : Query => Answer,
         };
 
-        // ── Reconciliation (the pipeline's sink): commutative fold of every
-        // answer into its origin k-mer's running state, in whatever order
-        // the pipeline delivers them. ─────────────────────────────────────
-        let mut state = vec![RunningState::default(); n_slots];
-        for ans in pipe.apply(sources.into_iter(), n_workers, capacity) {
-            if ans.hit {
-                let st = &mut state[ans.source_slot];
-                st.siblings = (st.siblings + 1).min(3);
-                if ans.smaller {
-                    st.minorant = false;
+        // ── Group generated variants by destination partition. `cache`
+        // holds every partition already mmap'd (no more `open()` cost), but
+        // `mmap` pages are still loaded on demand and can be evicted — a
+        // lookup is not free just because the file isn't reopened. Grouping
+        // keeps one partition's pages hot while its whole batch is resolved,
+        // instead of faulting pages in and out as lookups jump between
+        // partitions in whatever order the `Flat` stage happens to produce
+        // them. The throttle permit drops here, once accumulated. ─────────
+        let mut outgoing: Vec<Vec<(CanonicalKmer, usize, u8)>> = (0..n_parts).map(|_| Vec::new()).collect();
+        for vq in pipe.apply(throttled, n_workers, capacity) {
+            outgoing[vq.dest_partition].push((vq.variant, vq.source_slot, vq.base));
         }
+
+        // ── Resolve each partition's batch against the cache in one
+        // contiguous pass; parallelised across partitions (independent,
+        // read-only) so this keeps using multiple cores without giving up
+        // the per-partition locality above. ─────────────────────────────
+        outgoing.par_iter().enumerate().filter(|(_, q)| !q.is_empty()).for_each(|(dest, queries)| {
+            for &(variant, source_slot, base) in queries {
+                if cache.find(dest, variant) {
+                    mask[source_slot].fetch_or(1 << base, Ordering::Relaxed);
                 }
             }
+        });
 
         // ── Write the layer's annex file ─────────────────────────────────────
         let annex_path = layer_dir.join(ANNEX_FILE_NAME);
         let mut builder = SiblingAnnexBuilder::new(n_slots, &annex_path)?;
-        for (slot, st) in state.iter().enumerate() {
+        for (slot, m) in mask.iter().enumerate() {
             if slot_kmer[slot].is_none() {
                 continue; // unused MPHF slot, if any — leave at the sentinel
             }
-            builder.set(slot, SiblingInfo { minorant: st.minorant, siblings: st.siblings });
+            builder.set(slot, FamilyMask::from_bits(m.load(Ordering::Relaxed)));
         }
         builder.close()?;
 
-        Ok(())
+        Ok(n_slots as u64)
     }
 }
 
-/// Distribution of sibling counts (0-3), read back from an already-built
+/// Distribution of family sizes (1-4), read back from an already-built
 /// annex (see [`KmerIndex::build_sibling_annex`]) plus the index's
 /// presence/count data — a separate, occasional diagnostic pass, not fused
 /// into construction.
+///
+/// Every count here is **per family, not per slot**: a family with `F`
+/// members occupies `F` annex slots (one per observed member), all sharing
+/// the same mask. Counting every slot would count each family up to 4
+/// times over; only the minorant's slot is tallied (minorant is derived on
+/// the fly — see `is_minorant` — not stored, but cheap: no lookup, pure
+/// bit arithmetic on already-in-hand data).
 #[derive(Debug, Clone, Default)]
 pub struct SiblingAnnexStats {
-    /// `counts[s]` = number of k-mers (slots) with exactly `s` siblings,
-    /// counted once each regardless of how many genomes carry them.
+    /// `counts[s]` = number of *families* of size `s + 1` (index 0 = size 1,
+    /// i.e. 0 siblings, ... index 3 = size 4, i.e. 3 siblings).
     pub counts: [u64; 4],
-    /// Of those, how many are minorant.
-    pub minorant_counts: [u64; 4],
-    /// `per_genome[g][s]` = number of k-mers with exactly `s` siblings that
-    /// genome `g` (index into `KmerIndex::meta().genomes`) carries.
+    /// `per_genome[g][s]` = number of families of size `s + 1` for which
+    /// genome `g` (index into `KmerIndex::meta().genomes`) carries at least
+    /// one member.
     pub per_genome: Vec<[u64; 4]>,
 }
 
 impl KmerIndex {
-    /// Tally the sibling-count distribution of an already-built annex
-    /// (globally, and per genome). Errors if [`build_sibling_annex`] has not
-    /// been run on this index first.
+    /// Tally the family-size distribution of an already-built annex
+    /// (globally, and per genome), counting each family once (at its
+    /// minorant slot). Errors if [`build_sibling_annex`] has not been run on
+    /// this index first.
     ///
     /// [`build_sibling_annex`]: Self::build_sibling_annex
     pub fn sibling_annex_stats(&self) -> OKIResult<SiblingAnnexStats> {
         let n_parts = self.n_partitions();
         let n_genomes = self.meta.genomes.len();
         let with_counts = self.meta.config.with_counts;
+        let k = self.kmer_size();
+        let n_bits = n_parts.trailing_zeros() as usize;
 
-        let mut stats = SiblingAnnexStats {
-            per_genome: vec![[0u64; 4]; n_genomes],
-            ..Default::default()
-        };
+        // Same whole-run cache as `build_sibling_annex` — see its docs for
+        // why re-opening per lookup (or per call to a batching helper) is
+        // not good enough on a real index.
+        let partition = KmerPartition::open_with_config(
+            &self.root_path,
+            self.kmer_size(),
+            self.minimizer_size(),
+            n_bits,
+        )
+        .map_err(OKIError::Partition)?;
+        let cache = PartitionCache::build(&partition, n_parts, with_counts)?;
 
+        // Gather the (partition, layer) pairs to process — cheap metadata
+        // reads only, checking every annex file exists up front so a
+        // missing one is reported before any real work starts.
+        let mut layer_dirs = Vec::new();
         for part in 0..n_parts {
             let index_dir = self.partition().part_dir(part).join(INDEX_SUBDIR);
             if !index_dir.exists() {
                 continue;
             }
             let meta = PartitionMeta::load(&index_dir).map_err(olm_to_ok)?;
-
             for l in 0..meta.n_layers {
                 let layer_dir = index_dir.join(format!("layer_{l}"));
                 let annex_path = layer_dir.join(ANNEX_FILE_NAME);
@@ -334,45 +500,111 @@ impl KmerIndex {
                         annex_path.display()
                     )));
                 }
-                let annex = SiblingAnnex::open(&annex_path)?;
-                let use_counts = with_counts && layer_dir.join("counts").exists();
+                layer_dirs.push(layer_dir);
+            }
+        }
 
-                // Opened once per layer, outside the slot loop.
-                enum Mat {
-                    Count(PersistentCompactIntMatrix),
-                    Presence(PersistentBitMatrix),
-                }
-                let mat = if use_counts {
-                    Mat::Count(PersistentCompactIntMatrix::open(&layer_dir)?)
-                } else {
-                    Mat::Presence(PersistentBitMatrix::open(&layer_dir)?)
+        // One layer's worth of work, parallelised across layers with Rayon
+        // — independent, read-only, each producing its own partial tally
+        // merged at the end.
+        let pb = progress_bar("sibling_annex_stats", layer_dirs.len() as u64, "layers");
+        let partials: Vec<SiblingAnnexStats> = layer_dirs
+            .par_iter()
+            .map(|layer_dir| -> OKIResult<SiblingAnnexStats> {
+                let mut stats = SiblingAnnexStats {
+                    per_genome: vec![[0u64; 4]; n_genomes],
+                    ..Default::default()
                 };
-                let n_cols = match &mat {
-                    Mat::Count(m) => m.n_cols(),
-                    Mat::Presence(m) => m.n_cols(),
+
+                let index_dir = layer_dir.parent().expect("layer_dir has a parent index dir");
+                let meta = PartitionMeta::load(index_dir).map_err(olm_to_ok)?;
+                let annex = SiblingAnnex::open(&layer_dir.join(ANNEX_FILE_NAME))?;
+
+                // Need each slot's own k-mer to derive minorant — same
+                // enumeration as construction.
+                let mphf = MphfLayer::open(layer_dir, &meta.mode).map_err(olm_to_ok)?;
+                let mut slot_kmer: Vec<Option<CanonicalKmer>> = vec![None; annex.len()];
+                let reader = UnitigFileReader::open_sequential(&layer_dir.join("unitigs.bin"))
+                    .map_err(OKIError::Partition)?;
+                for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
+                    if let Some(slot) = mphf.find(kmer) {
+                        slot_kmer[slot] = Some(kmer);
                     }
-                .min(n_genomes);
+                }
+
+                let use_counts = with_counts && layer_dir.join("counts").exists();
+                let mat = if use_counts {
+                    Mat::Count(PersistentCompactIntMatrix::open(layer_dir)?)
+                } else {
+                    Mat::Presence(PersistentBitMatrix::open(layer_dir)?)
+                };
+                let n_cols = mat.n_cols().min(n_genomes);
 
                 for slot in 0..annex.len() {
-                    let Some(info) = annex.get(slot) else { continue };
-                    let s = info.siblings as usize;
-                    stats.counts[s] += 1;
-                    if info.minorant {
-                        stats.minorant_counts[s] += 1;
+                    let Some(mask) = annex.get(slot) else { continue };
+                    let Some(kmer) = slot_kmer[slot] else { continue };
+                    if !is_minorant(kmer, mask, k) {
+                        continue; // this family is tallied at its minorant's slot only
                     }
+                    let s = mask.siblings() as usize;
+                    stats.counts[s] += 1;
+
+                    // "Genome g represents this family" means g carries
+                    // *any* of its members, not just the minorant's own —
+                    // start from the minorant's own presence (already
+                    // open, no lookup) and OR in every other present
+                    // member's presence vector, resolved against the
+                    // whole-run cache (no I/O) — exactly `mask.siblings()`
+                    // of them, the mask tells us precisely which to fetch.
+                    let mut carries = vec![false; n_cols];
                     for g in 0..n_cols {
-                        let carried = match &mat {
-                            Mat::Count(m) => m.col_view(g).get(slot) != 0,
-                            Mat::Presence(m) => m.get(g, slot) != 0,
-                        };
+                        carries[g] = mat.carries(g, slot);
+                    }
+                    for other in kmer.central_canonical_neighbors() {
+                        if other == kmer {
+                            continue;
+                        }
+                        let base = central_base(other, k);
+                        if !mask.has(base) {
+                            continue;
+                        }
+                        let dest = partition_of(other, n_parts);
+                        if let Some(other_presence) = cache.find_presence(dest, other, n_genomes) {
+                            for (g, &present) in other_presence.iter().enumerate() {
+                                if present {
+                                    carries[g] = true;
+                                }
+                            }
+                        }
+                    }
+
+                    for (g, &carried) in carries.iter().enumerate() {
                         if carried {
                             stats.per_genome[g][s] += 1;
                         }
                     }
                 }
-            }
-        }
 
+                pb.inc(1);
+                Ok(stats)
+            })
+            .collect::<OKIResult<Vec<_>>>()?;
+        pb.finish_and_clear();
+
+        let mut stats = SiblingAnnexStats {
+            per_genome: vec![[0u64; 4]; n_genomes],
+            ..Default::default()
+        };
+        for part in partials {
+            for s in 0..4 {
+                stats.counts[s] += part.counts[s];
+            }
+            for g in 0..n_genomes {
+                for s in 0..4 {
+                    stats.per_genome[g][s] += part.per_genome[g][s];
+                }
+            }
+        }
         Ok(stats)
     }
 }
@@ -442,7 +674,7 @@ mod tests {
 
     /// Read back the annex entry for a given canonical k-mer from the merged
     /// index's (single) partition/layer, asserting it was found at all.
-    fn annex_info_for(idx: &KmerIndex, kmer: CanonicalKmer) -> SiblingInfo {
+    fn annex_info_for(idx: &KmerIndex, kmer: CanonicalKmer) -> FamilyMask {
         let index_dir = idx.partition().part_dir(0).join(INDEX_SUBDIR);
         let meta = PartitionMeta::load(&index_dir).unwrap();
         for l in 0..meta.n_layers {
@@ -474,24 +706,33 @@ mod tests {
     fn sibling_annex_one_sibling_each() {
         // k=11, centre = index 5 (0-based). Two genomes, each exactly one
         // k-mer, sharing every base except the centre:
-        //   g1 = "AACCGCTTAAG" (centre 'C')
-        //   g2 = "AACCGGTTAAG" (centre 'G')
+        //   g1 = "AACCGCTTAAG" (centre 'C', base index 1)
+        //   g2 = "AACCGGTTAAG" (centre 'G', base index 2)
         // Hand-verified: both stay forward-oriented under canonicalisation
         // (each is lexicographically smaller than its own reverse
         // complement, since both start with "AA"), and raw(g1) < raw(g2)
         // (only differing base: C=0b01 < G=0b10 at the centre) — so g1 is
-        // the minorant, g2 is not, and each is the other's one sibling.
+        // the minorant, g2 is not. The mask is a family-wide value: both
+        // slots must read back the *same* mask (bits 1 and 2 set).
         let dir = tempdir().unwrap();
         let g1 = build_single_genome_index(dir.path(), "g1", b"AACCGCTTAAG");
         let g2 = build_single_genome_index(dir.path(), "g2", b"AACCGGTTAAG");
         let merged = merge_two(dir.path(), &g1, &g2);
         merged.build_sibling_annex().expect("build_sibling_annex");
 
-        let a = annex_info_for(&merged, canonical(b"AACCGCTTAAG"));
-        assert_eq!(a, SiblingInfo { minorant: true, siblings: 1 }, "AACCGCTTAAG");
+        let g1_kmer = canonical(b"AACCGCTTAAG");
+        let g2_kmer = canonical(b"AACCGGTTAAG");
+        let expected_mask = FamilyMask::EMPTY.with(1).with(2);
 
-        let b = annex_info_for(&merged, canonical(b"AACCGGTTAAG"));
-        assert_eq!(b, SiblingInfo { minorant: false, siblings: 1 }, "AACCGGTTAAG");
+        let a = annex_info_for(&merged, g1_kmer);
+        assert_eq!(a, expected_mask, "AACCGCTTAAG");
+        assert_eq!(a.siblings(), 1);
+        assert!(is_minorant(g1_kmer, a, K), "g1 should be the minorant");
+
+        let b = annex_info_for(&merged, g2_kmer);
+        assert_eq!(b, expected_mask, "AACCGGTTAAG");
+        assert_eq!(b.siblings(), 1);
+        assert!(!is_minorant(g2_kmer, b, K), "g2 should not be the minorant");
     }
 
     #[test]
@@ -504,7 +745,36 @@ mod tests {
         let merged = merge_two(dir.path(), &g1, &g2);
         merged.build_sibling_annex().expect("build_sibling_annex");
 
-        let info = annex_info_for(&merged, canonical(b"GATTACAGATC"));
-        assert_eq!(info, SiblingInfo { minorant: true, siblings: 0 }, "GATTACAGATC");
+        let kmer = canonical(b"GATTACAGATC");
+        let mask = annex_info_for(&merged, kmer);
+        assert_eq!(mask.siblings(), 0, "GATTACAGATC");
+        assert_eq!(mask.family_size(), 1);
+        assert!(is_minorant(kmer, mask, K));
+    }
+
+    #[test]
+    fn sibling_annex_stats_counts_each_family_once_and_per_genome() {
+        // Reuses the one-sibling-each fixture: a single family of size 2
+        // (g1's centre-C form + g2's centre-G form), each genome carrying
+        // exactly one of the two members. Stats must report exactly one
+        // family of size 2 (`counts[1] == 1`, since index 1 = size 2), not
+        // two (which naively summing both slots would give), and both
+        // genomes represented at size 2, neither at any other size.
+        let dir = tempdir().unwrap();
+        let g1 = build_single_genome_index(dir.path(), "g1", b"AACCGCTTAAG");
+        let g2 = build_single_genome_index(dir.path(), "g2", b"AACCGGTTAAG");
+        let merged = merge_two(dir.path(), &g1, &g2);
+        merged.build_sibling_annex().expect("build_sibling_annex");
+
+        let stats = merged.sibling_annex_stats().expect("sibling_annex_stats");
+
+        assert_eq!(stats.counts, [0, 1, 0, 0], "one family of size 2, counted once");
+        assert_eq!(stats.per_genome.len(), 2);
+        for g in 0..2 {
+            assert_eq!(
+                stats.per_genome[g], [0, 1, 0, 0],
+                "genome {g} should represent exactly one size-2 family"
+            );
+        }
     }
 }

src/obikmer/src/cmd/distance.rs

@@ -78,8 +78,7 @@ pub struct DistanceArgs {
     pub sibling_stats: bool,
 
     /// Output prefix: <prefix>_dist.csv, <prefix>_shared.csv,
-    /// <prefix>_siblings.csv, <prefix>_siblings_per_genome.csv,
-    /// <prefix>_nj.nwk, <prefix>_upgma.nwk.
+    /// <prefix>_siblings.csv, <prefix>_nj.nwk, <prefix>_upgma.nwk.
     /// If omitted, the distance matrix is written to stdout.
     #[arg(short, long)]
     pub output: Option<PathBuf>,
@@ -114,6 +113,17 @@ pub fn run(args: DistanceArgs) {
         write_sibling_stats_csv(&stats, &labels, &args.output);
     }
 
+    // `--sibling-annex`/`--sibling-stats` are their own operation, not a
+    // modifier on top of a distance-metric computation — a metric was
+    // never requested by asking for either of them, so there is nothing
+    // for the rest of this function to compute. Not a historical accident
+    // to keep: stop here rather than always also running a Jaccard (or
+    // whichever `--metric` defaults to) pass and printing an unrequested
+    // matrix.
+    if args.sibling_annex || args.sibling_stats {
+        return;
+    }
+
     info!(
         "computing {:?} distances for {} genome(s)",
         args.metric, n
@@ -225,43 +235,38 @@ pub fn run(args: DistanceArgs) {
     }
 }
 
-// ── Sibling-count distribution → CSV ────────────────────────────────────────
+// ── Family-size distribution → CSV ──────────────────────────────────────────
+//
+// Each row is a family (the up-to-4 k-mers sharing flanks, differing only at
+// the centre), counted once — at its minorant — regardless of how many of
+// its members are observed. Family size 1..4 (not "sibling count" 0..3):
+// see `docmd/theory/evolutionary_distances.md`, "Definitions".
 
 fn write_sibling_stats_csv(stats: &SiblingAnnexStats, labels: &[String], output: &Option<PathBuf>) {
-    // Global histogram: one row per sibling count (0-3).
-    let global_path = output.as_ref()
+    // One row per genome (4 columns, family size 1-4: number of families of
+    // that size for which the genome carries at least one member), plus a
+    // `global` row — the actual deduplicated family-size histogram
+    // (`stats.counts`), NOT a sum of the per-genome columns (a family shared
+    // by several genomes would otherwise be counted once per genome it
+    // appears in, inflating the total beyond the real family count).
+    let path = output.as_ref()
         .map(|p| format!("{}_siblings.csv", p.display()))
         .unwrap_or_else(|| "siblings.csv".into());
-    let mut f = BufWriter::new(std::fs::File::create(&global_path).unwrap_or_else(|e| {
-        eprintln!("error creating {global_path}: {e}");
+    let mut f = BufWriter::new(std::fs::File::create(&path).unwrap_or_else(|e| {
+        eprintln!("error creating {path}: {e}");
         std::process::exit(1);
     }));
-    writeln!(f, "siblings,slots,minorant_slots").unwrap();
-    for s in 0..4 {
-        writeln!(f, "{s},{},{}", stats.counts[s], stats.minorant_counts[s]).unwrap();
-    }
-    let total: u64 = stats.counts.iter().sum();
-    info!("sibling-count distribution → {global_path} (total {total} slot(s))");
-
-    // Per-genome breakdown: one row per genome, 4 columns (0-3), + a total row.
-    let per_genome_path = output.as_ref()
-        .map(|p| format!("{}_siblings_per_genome.csv", p.display()))
-        .unwrap_or_else(|| "siblings_per_genome.csv".into());
-    let mut f = BufWriter::new(std::fs::File::create(&per_genome_path).unwrap_or_else(|e| {
-        eprintln!("error creating {per_genome_path}: {e}");
-        std::process::exit(1);
-    }));
-    writeln!(f, "genome,0,1,2,3").unwrap();
-    let mut column_totals = [0u64; 4];
+    writeln!(f, "genome,1,2,3,4").unwrap();
     for (label, counts) in labels.iter().zip(stats.per_genome.iter()) {
         writeln!(f, "{label},{},{},{},{}", counts[0], counts[1], counts[2], counts[3]).unwrap();
-        for s in 0..4 { column_totals[s] += counts[s]; }
     }
     writeln!(
-        f, "total,{},{},{},{}",
-        column_totals[0], column_totals[1], column_totals[2], column_totals[3],
+        f, "global,{},{},{},{}",
+        stats.counts[0], stats.counts[1], stats.counts[2], stats.counts[3],
     ).unwrap();
-    info!("per-genome sibling-count distribution → {per_genome_path}");
+    let total: u64 = stats.counts.iter().sum();
+    info!("family-size distribution → {path} (total {total} famil{})",
+        if total == 1 { "y" } else { "ies" });
 }
 
 // ── UPGMA Newick from kodama dendrogram ───────────────────────────────────────