diff --git a/docmd/implementation/chunkreader.md b/docmd/implementation/chunkreader.md
index 4021c57..06184d1 100644
--- a/docmd/implementation/chunkreader.md
+++ b/docmd/implementation/chunkreader.md
@@ -1,6 +1,24 @@
 # Chunk reader — implementation
 
-The `obiread` crate provides a streaming iterator that reads FASTA or FASTQ files in fixed-size blocks and yields self-contained chunks, each ending on a complete sequence record boundary. Chunks are consumed in parallel by downstream workers.
+`obiread` exposes two distinct sequence reading paths, each optimised for a different use case.
+
+## Two reading paths
+
+| Path | API | Output unit | Per-record identity | Use case |
+|------|-----|-------------|---------------------|----------|
+| **Record path** | `read_sequence_chunks` → `parse_chunk` | `SeqRecord` (id + raw sequence + normalised rope) | yes | `query` — must read complete records |
+| **Stream path** | `open_nuc_stream` | `NucPage` (flat normalised byte buffer) | no | `index`, `superkmer` — bulk throughput |
+
+The record path uses `Rope`-backed chunks and is described in detail below.
+The stream path (`NucStream` / `NucPage`) is described in the scatter section of [pipeline](pipeline.md).
+
+---
+
+## Record path: chunk reader
+
+The chunk reader reads FASTA or FASTQ files in fixed-size blocks and yields self-contained chunks, each ending on a complete sequence record boundary. `parse_chunk` then converts each chunk into a `Vec<SeqRecord>`, where each record carries its identifier, raw sequence bytes, and a normalised rope ready for superkmer building.
+
+This path is mandatory for `query`, where superkmers must be tracked back to their originating sequence (id, kmer offset) for output annotation.
 
 ## Output type: Rope
 
diff --git a/docmd/implementation/pipeline.md b/docmd/implementation/pipeline.md
index 2cfac4b..b95a075 100644
--- a/docmd/implementation/pipeline.md
+++ b/docmd/implementation/pipeline.md
@@ -19,7 +19,11 @@ The histogram gives:
 
 ## Phase 1 — Scatter
 
-Single streaming pass over raw input files (FASTA/FASTQ, gzip). FASTQ quality scores are ignored. For each read:
+Single streaming pass over raw input files (FASTA/FASTQ, gzip). FASTQ quality scores are ignored.
+
+Input files are read via `open_nuc_stream`, which opens and decompresses the file, auto-detects the format (FASTA / FASTQ / GenBank), and yields a sequence of `NucPage` buffers. Each `NucPage` is a flat 64 KB buffer of normalised bytes (`ACGT` + `\x00` separators), carrying a k−1 byte overlap from the preceding page so that no k-mer is lost at page boundaries. Per-record identity (sequence id, raw bytes) is not preserved; this is intentional — the scatter phase only needs normalised bases to produce superkmers.
+
+For each read fragment within a page:
 
 1. **Ambiguous base filter**: cut at any non-ACGT base; discard fragments shorter than k.
 2. **Entropy filter**: scan each fragment with a sliding window of size k. When the kmer $K_i = S[i \mathinner{..} i+k-1]$ ended by nucleotide $S[j]$ (with $j = i+k-1$) has entropy below threshold $\theta$, emit the current segment and start a new one (see algorithm below). $K_i$ belongs to neither segment, and no valid kmer is lost.
diff --git a/src/obikmer/src/cli.rs b/src/obikmer/src/cli.rs
index 82c3a38..8a84c03 100644
--- a/src/obikmer/src/cli.rs
+++ b/src/obikmer/src/cli.rs
@@ -1,9 +1,8 @@
-use std::io;
 use std::path::PathBuf;
 use std::sync::{Arc, Condvar, Mutex};
 
 use clap::Args;
-use obikrope::Rope;
+use obiread::NucPage;
 use obikseq::RoutableSuperKmer;
 
 // ── Shared arguments ──────────────────────────────────────────────────────────
@@ -45,9 +44,11 @@ pub struct CommonArgs {
     )]
     pub threads: usize,
 
-    /// Maximum number of input files open simultaneously
-    #[arg(long, default_value_t = 20)]
-    pub max_open_files: usize,
+    /// Maximum number of input files open simultaneously.
+    /// Defaults to threads/4 (minimum 1). Keep below the number of workers
+    /// to ensure CPU workers are always available for the transform stage.
+    #[arg(long)]
+    pub max_open_files: Option<usize>,
 }
 
 /// Smallest `b` such that `2^b >= n` (i.e. `n.next_power_of_two().ilog2()`).
@@ -86,6 +87,12 @@ impl CommonArgs {
         }
     }
 
+    pub fn effective_max_open(&self) -> usize {
+        self.max_open_files
+            .unwrap_or_else(|| (self.threads / 4).max(1))
+            .max(1)
+    }
+
     pub fn seqfile_paths(&self) -> obiread::PathIter {
         let paths: Vec<PathBuf> = if self.inputs.is_empty() {
             vec![PathBuf::from("-")]
@@ -144,13 +151,10 @@ pub struct PathWithSlot {
 
 pub enum PipelineData {
     Path(PathWithSlot),
-    RawChunk(Rope),
-    NormChunk(Rope),
+    NucPage(NucPage),
     Batch(Vec<RoutableSuperKmer>),
 }
 
-// SAFETY: Rope contains Cell<u8> which is !Sync, but pipeline ownership transfers
-// exclusively through channels — no item is ever shared across threads.
 unsafe impl Send for PipelineData {}
 unsafe impl Sync for PipelineData {}
 
@@ -171,18 +175,3 @@ pub fn throttle_paths(
     })
 }
 
-// ── I/O plumbing ──────────────────────────────────────────────────────────────
-
-pub fn open_chunks(path: PathBuf) -> io::Result<impl Iterator<Item = Rope>> {
-    let path_str = path
-        .to_str()
-        .ok_or_else(|| io::Error::new(io::ErrorKind::InvalidInput, "non-UTF-8 path"))?;
-    let iter = obiread::read_sequence_chunks(path_str)?;
-    Ok(iter.filter_map(|r| match r {
-        Ok(rope) => Some(rope),
-        Err(e) => {
-            eprintln!("chunk read error: {e}");
-            None
-        }
-    }))
-}
diff --git a/src/obikmer/src/cmd/index.rs b/src/obikmer/src/cmd/index.rs
index 13268d8..1a796f9 100644
--- a/src/obikmer/src/cmd/index.rs
+++ b/src/obikmer/src/cmd/index.rs
@@ -231,7 +231,7 @@ pub fn run(args: IndexArgs) {
         let theta     = args.common.theta;
         let n_workers = args.common.threads.max(1);
 
-        let max_open = args.common.max_open_files.max(1);
+        let max_open = args.common.effective_max_open();
         scatter(idx.partition_mut(), args.common.seqfile_paths(), k, level_max, theta, n_workers, max_open, &mut rep);
 
         idx.mark_scattered().unwrap_or_else(|e| {
diff --git a/src/obikmer/src/cmd/superkmer.rs b/src/obikmer/src/cmd/superkmer.rs
index a1c6398..1b0d7cd 100644
--- a/src/obikmer/src/cmd/superkmer.rs
+++ b/src/obikmer/src/cmd/superkmer.rs
@@ -4,7 +4,7 @@ use clap::Args;
 use obifastwrite::write_scatter;
 use obikseq::{RoutableSuperKmer, set_k, set_m};
 
-use crate::cli::{CommonArgs, PipelineData, PathWithSlot, open_chunks, partitions_to_bits, throttle_paths};
+use crate::cli::{CommonArgs, PipelineData, PathWithSlot, partitions_to_bits, throttle_paths};
 
 #[derive(Args)]
 pub struct SuperkmerArgs {
@@ -41,7 +41,7 @@ pub fn run(args: SuperkmerArgs) {
     let level_max = args.common.level_max;
     let partition_bits = partitions_to_bits(args.common.partitions);
     let n_workers = args.common.threads.max(1);
-    let max_open = args.common.max_open_files.max(1);
+    let max_open = args.common.effective_max_open();
 
     set_k(k);
     set_m(m);
@@ -50,9 +50,14 @@ pub fn run(args: SuperkmerArgs) {
 
     let pipe = obipipeline::make_pipe! {
         PipelineData : PathWithSlot => Vec<RoutableSuperKmer>,
-        ||? { |pw: PathWithSlot| open_chunks(pw.path) }                                : Path     => RawChunk,
-        |?  { move |rope| obiread::normalize_sequence_chunk(rope, k) }                 : RawChunk => NormChunk,
-        |   { move |rope| obiskbuilder::build_superkmers(rope, k, level_max, theta) }  : NormChunk => Batch,
+        ||? {
+            let k = k;
+            move |pw: PathWithSlot| {
+                let path_str = pw.path.to_str().unwrap_or("").to_owned();
+                obiread::open_nuc_stream(&path_str, k)
+            }
+        } : Path => NucPage,
+        | { move |page| obiskbuilder::build_superkmers_page(page, k, level_max, theta) } : NucPage => Batch,
     };
 
     let mut out = BufWriter::new(io::stdout());
diff --git a/src/obikmer/src/steps/scatter.rs b/src/obikmer/src/steps/scatter.rs
index 777e92b..80d789e 100644
--- a/src/obikmer/src/steps/scatter.rs
+++ b/src/obikmer/src/steps/scatter.rs
@@ -1,30 +1,37 @@
 use std::path::PathBuf;
-use std::sync::atomic::{AtomicU64, Ordering};
+use std::sync::atomic::{AtomicU32, AtomicU64, Ordering};
 use std::sync::Arc;
 use std::time::{Duration, Instant};
 
 use indicatif::{ProgressBar, ProgressStyle};
+use obiread::NucPage;
 use obikpartitionner::KmerPartition;
-use obikrope::Rope;
 use obisys::{Reporter, Stage};
 use tracing::info;
 
-use crate::cli::{PipelineData, PathWithSlot, open_chunks, throttle_paths};
+use crate::cli::{PipelineData, PathWithSlot, throttle_paths};
 
 // ── Iterator that keeps the slot guard alive until the file is exhausted ──────
 
-struct GuardedIter<I> {
-    inner:  I,
-    _guard: Box<dyn Send + 'static>,
+struct GuardedIter {
+    inner:       Box<dyn Iterator<Item = NucPage> + Send>,
+    _guard:      Box<dyn Send + 'static>,
+    flat_active: Arc<AtomicU32>,
 }
 
-impl<I: Iterator<Item = Rope>> Iterator for GuardedIter<I> {
-    type Item = Rope;
-    fn next(&mut self) -> Option<Rope> {
+impl Iterator for GuardedIter {
+    type Item = NucPage;
+    fn next(&mut self) -> Option<NucPage> {
         self.inner.next()
     }
 }
 
+impl Drop for GuardedIter {
+    fn drop(&mut self) {
+        self.flat_active.fetch_sub(1, Ordering::Relaxed);
+    }
+}
+
 // ── scatter ───────────────────────────────────────────────────────────────────
 
 /// Run scatter: normalise → build superkmers → route to partition → close.
@@ -44,23 +51,36 @@ pub fn scatter(
     // Throttle in the source thread — never in a worker — to prevent deadlock.
     let throttled = throttle_paths(path_source, max_open);
 
-    let file_count = Arc::new(AtomicU64::new(0));
+    let file_count      = Arc::new(AtomicU64::new(0));
+    let flat_active     = Arc::new(AtomicU32::new(0));
+    let transform_active = Arc::new(AtomicU32::new(0));
 
     let t = Stage::start("scatter");
     let pipe = obipipeline::make_pipe! {
         PipelineData : PathWithSlot => Vec<RoutableSuperKmer>,
         ||? {
-            let file_count = Arc::clone(&file_count);
+            let file_count   = Arc::clone(&file_count);
+            let flat_active  = Arc::clone(&flat_active);
+            let k = k;
             move |pw: PathWithSlot| {
                 let PathWithSlot { path, _guard } = pw;
                 let n = file_count.fetch_add(1, Ordering::Relaxed) + 1;
                 info!("indexing [{}]: {}", n, path.display());
-                // _guard travels into GuardedIter; released when all chunks are read
-                open_chunks(path).map(|iter| GuardedIter { inner: iter, _guard })
+                let path_str = path.to_str().unwrap_or("").to_owned();
+                flat_active.fetch_add(1, Ordering::Relaxed);
+                obiread::open_nuc_stream(&path_str, k)
+                    .map(|iter| GuardedIter { inner: iter, _guard, flat_active: Arc::clone(&flat_active) })
             }
-        } : Path => RawChunk,
-        |?  { move |rope| obiread::normalize_sequence_chunk(rope, k) }                 : RawChunk => NormChunk,
-        |   { move |rope| obiskbuilder::build_superkmers(rope, k, level_max, theta) }  : NormChunk => Batch,
+        } : Path => NucPage,
+        | {
+            let transform_active = Arc::clone(&transform_active);
+            move |page| {
+                transform_active.fetch_add(1, Ordering::Relaxed);
+                let result = obiskbuilder::build_superkmers_page(page, k, level_max, theta);
+                transform_active.fetch_sub(1, Ordering::Relaxed);
+                result
+            }
+        } : NucPage => Batch,
     };
 
     let pb = ProgressBar::new_spinner();
@@ -93,7 +113,9 @@ pub fn scatter(
                 (format!("{:.0} Mbp", bp / 1e6), format!("{:.0} Mbp/s", ema_rate / 1e6))
             };
             let n_files = file_count.load(Ordering::Relaxed);
-            pb.set_message(format!("{count_str}  {rate_str}  {n_files} files"));
+            let r = flat_active.load(Ordering::Relaxed);
+            let c = transform_active.load(Ordering::Relaxed);
+            pb.set_message(format!("{count_str}  {rate_str}  {n_files} files  [R:{r} C:{c}]"));
         }
         kp.write_batch(batch).unwrap_or_else(|e| {
             eprintln!("error: {e}");
diff --git a/src/obiread/src/lib.rs b/src/obiread/src/lib.rs
index 94f0b8e..2370bb4 100644
--- a/src/obiread/src/lib.rs
+++ b/src/obiread/src/lib.rs
@@ -10,19 +10,21 @@ mod fasta;
 mod fastq;
 mod mimetype;
 pub mod normalize;
+mod nucstream;
 mod path_iterator;
 pub mod peakreader;
 pub mod record;
-pub mod stream;
 pub mod xopen;
 
-pub use chunk::{SeqChunkIter, fasta_chunks, fastq_chunks,
-                read_fasta_chunks, read_fastq_chunks, read_sequence_chunks};
-pub use normalize::{normalize_fasta_chunk, normalize_fastq_chunk, normalize_sequence_chunk};
+pub use chunk::{
+    SeqChunkIter, fasta_chunks, fastq_chunks, read_fasta_chunks, read_fastq_chunks,
+    read_sequence_chunks,
+};
 pub use mimetype::MimeTypeGuesser;
+pub use normalize::{normalize_fasta_chunk, normalize_fastq_chunk, normalize_sequence_chunk};
+pub use nucstream::{NucPage, NucPageCursor, open_nuc_stream};
 pub use path_iterator::{PathIter, path_iter};
 pub use peakreader::PeekReader;
-pub use stream::NormalizedByteStream;
 pub use xopen::xopen;
 
 /// Default read block size: 1 MiB.
diff --git a/src/obiread/src/nucstream.rs b/src/obiread/src/nucstream.rs
new file mode 100644
index 0000000..eb252d6
--- /dev/null
+++ b/src/obiread/src/nucstream.rs
@@ -0,0 +1,732 @@
+use std::io::{self, Read};
+use std::mem::ManuallyDrop;
+use std::sync::{Arc, Mutex};
+
+use crate::mimetype::MimeTypeGuesser;
+use crate::xopen::open_raw;
+
+pub const MAX_K: usize = 31;
+const PAGE_SIZE: usize = 65536;
+// overlap (MAX_K - 1) + page data (PAGE_SIZE) + 1 byte for the end-of-page terminating 0
+const BUF_SIZE: usize = MAX_K + PAGE_SIZE;
+
+// ─── OverlapState ─────────────────────────────────────────────────────────────
+
+pub(crate) struct OverlapState {
+    data: [u8; MAX_K],
+    len: usize,
+    k: usize,
+}
+
+impl OverlapState {
+    pub(crate) fn new(k: usize) -> Self {
+        assert!(k > 0 && k <= MAX_K);
+        Self {
+            data: [0u8; MAX_K],
+            len: 0,
+            k,
+        }
+    }
+}
+
+// ─── NucParser trait ──────────────────────────────────────────────────────────
+
+// Transforms a raw page into a compacted nucleotide stream in-place.
+//
+// Buffer layout on each call:
+//   buf[0..overlap_len()]           — overlap bytes copied by write_overlap()
+//   buf[overlap_len()..overlap_len()+n] — raw bytes just read from the source
+//
+// Returns the number of output bytes in buf[0..returned].
+pub(crate) trait NucParser {
+    // required: format-specific
+    fn new(k: usize) -> Self
+    where
+        Self: Sized;
+    fn overlap_state(&self) -> &OverlapState;
+    fn overlap_state_mut(&mut self) -> &mut OverlapState;
+    fn is_in_seq(&self) -> bool;
+    fn parse_inplace(&mut self, buf: &mut [u8], n: usize) -> usize;
+
+    // provided: format-independent overlap management
+    fn overlap_len(&self) -> usize {
+        self.overlap_state().len
+    }
+
+    fn write_overlap(&self, buf: &mut [u8]) {
+        let ol = &self.overlap_state();
+        buf[..ol.len].copy_from_slice(&ol.data[..ol.len]);
+    }
+
+    // Called at end of parse_inplace: saves overlap state and returns adjusted j.
+    // seq_start is the j-position where the last sequence started in this call's output.
+    fn save_overlap(&mut self, buf: &mut [u8], j: usize, seq_start: usize) -> usize {
+        if !self.is_in_seq() {
+            self.overlap_state_mut().len = 0;
+            return j;
+        }
+        let seq_len = j - seq_start;
+        let k = self.overlap_state().k;
+        if seq_len >= k {
+            // Sequence long enough: save last k-1 nucleotides, terminate with 0.
+            let ol = k - 1;
+            self.overlap_state_mut().data[..ol].copy_from_slice(&buf[j - ol..j]);
+            self.overlap_state_mut().len = ol;
+            // SAFETY: j <= total - 1 < BUF_SIZE = buf.len()
+            // (total = overlap_len + n <= (MAX_K-1) + PAGE_SIZE = BUF_SIZE - 1)
+            unsafe {
+                *buf.get_unchecked_mut(j) = 0;
+            }
+            j + 1
+        } else if seq_len > 0 {
+            // Short sequence (< k): save whole fragment, strip from output.
+            self.overlap_state_mut().data[..seq_len].copy_from_slice(&buf[seq_start..j]);
+            self.overlap_state_mut().len = seq_len;
+            seq_start
+        } else {
+            self.overlap_state_mut().len = 0;
+            j
+        }
+    }
+}
+
+// ─── FASTA parser ─────────────────────────────────────────────────────────────
+
+#[derive(Clone, Copy)]
+enum FastaState {
+    OutSeq,
+    InTitle,
+    InSeq,
+    InAmbiguous,
+}
+
+pub(crate) struct FastaParser {
+    state: FastaState,
+    overlap: OverlapState,
+}
+
+impl NucParser for FastaParser {
+    fn new(k: usize) -> Self {
+        Self {
+            state: FastaState::OutSeq,
+            overlap: OverlapState::new(k),
+        }
+    }
+
+    #[inline]
+    fn overlap_state(&self) -> &OverlapState {
+        &self.overlap
+    }
+
+    #[inline]
+    fn overlap_state_mut(&mut self) -> &mut OverlapState {
+        &mut self.overlap
+    }
+
+    #[inline]
+    fn is_in_seq(&self) -> bool {
+        matches!(self.state, FastaState::InSeq)
+    }
+
+    fn parse_inplace(&mut self, buf: &mut [u8], n: usize) -> usize {
+        let total = self.overlap.len + n;
+        let mut i = 0; // read index
+        let mut j = 0; // write index (invariant: j <= i always)
+        // j-position where the current sequence started in this call's output;
+        // meaningful only when state is InSeq.
+        let mut seq_start: usize = 0;
+
+        while i < total {
+            // SAFETY: i < total <= BUF_SIZE = buf.len()
+            let byte = unsafe { *buf.get_unchecked(i) };
+
+            match self.state {
+                FastaState::OutSeq => {
+                    if byte == b'>' {
+                        self.state = FastaState::InTitle;
+                    }
+                    i += 1;
+                }
+                FastaState::InTitle => {
+                    if byte == b'\n' || byte == b'\r' {
+                        self.state = FastaState::InSeq;
+                        seq_start = j;
+                    }
+                    i += 1;
+                }
+                FastaState::InSeq => {
+                    if byte == b'\n' || byte == b'\r' {
+                        i += 1;
+                        continue;
+                    }
+                    let nuc = byte & 0xDF; // to uppercase
+                    if nuc == b'A' || nuc == b'C' || nuc == b'G' || nuc == b'T' {
+                        // SAFETY: j <= i < total <= BUF_SIZE = buf.len()
+                        unsafe {
+                            *buf.get_unchecked_mut(j) = nuc;
+                        }
+                        j += 1;
+                        i += 1;
+                    } else if byte == b'>' {
+                        if j > seq_start {
+                            unsafe {
+                                *buf.get_unchecked_mut(j) = 0;
+                            }
+                            j += 1;
+                        }
+                        self.state = FastaState::InTitle;
+                        i += 1;
+                    } else {
+                        // first ambiguous base: end current sequence if non-empty
+                        if j > seq_start {
+                            unsafe {
+                                *buf.get_unchecked_mut(j) = 0;
+                            }
+                            j += 1;
+                        }
+                        self.state = FastaState::InAmbiguous;
+                        i += 1;
+                    }
+                }
+                FastaState::InAmbiguous => {
+                    if byte == b'\n' || byte == b'\r' {
+                        i += 1;
+                        continue;
+                    }
+                    if byte == b'>' {
+                        self.state = FastaState::InTitle;
+                        i += 1;
+                        continue;
+                    }
+                    let nuc = byte & 0xDF;
+                    if nuc == b'A' || nuc == b'C' || nuc == b'G' || nuc == b'T' {
+                        seq_start = j;
+                        // SAFETY: j <= i < total <= BUF_SIZE = buf.len()
+                        unsafe {
+                            *buf.get_unchecked_mut(j) = nuc;
+                        }
+                        j += 1;
+                        self.state = FastaState::InSeq;
+                    }
+                    i += 1;
+                }
+            }
+        }
+
+        self.save_overlap(buf, j, seq_start)
+    }
+}
+
+// ─── FASTQ parser ─────────────────────────────────────────────────────────────
+
+#[derive(Clone, Copy)]
+enum FastqState {
+    OutSeq,
+    InTitle,
+    InSeq,
+    InAmbiguous,
+    InQualTitle,
+    InQual,
+}
+
+pub(crate) struct FastqParser {
+    state: FastqState,
+    overlap: OverlapState,
+}
+
+impl NucParser for FastqParser {
+    fn new(k: usize) -> Self {
+        Self {
+            state: FastqState::OutSeq,
+            overlap: OverlapState::new(k),
+        }
+    }
+
+    #[inline]
+    fn overlap_state(&self) -> &OverlapState {
+        &self.overlap
+    }
+
+    #[inline]
+    fn overlap_state_mut(&mut self) -> &mut OverlapState {
+        &mut self.overlap
+    }
+
+    #[inline]
+    fn is_in_seq(&self) -> bool {
+        matches!(self.state, FastqState::InSeq)
+    }
+
+    fn parse_inplace(&mut self, buf: &mut [u8], n: usize) -> usize {
+        let total = self.overlap.len + n;
+        let mut i = 0;
+        let mut j = 0;
+        let mut seq_start: usize = 0;
+
+        while i < total {
+            // SAFETY: i < total <= BUF_SIZE = buf.len()
+            let byte = unsafe { *buf.get_unchecked(i) };
+
+            match self.state {
+                FastqState::OutSeq => {
+                    if byte == b'@' {
+                        self.state = FastqState::InTitle;
+                    }
+                    i += 1;
+                }
+                FastqState::InTitle => {
+                    if byte == b'\n' || byte == b'\r' {
+                        self.state = FastqState::InSeq;
+                        seq_start = j;
+                    }
+                    i += 1;
+                }
+                FastqState::InSeq => {
+                    if byte == b'\n' || byte == b'\r' {
+                        if j > seq_start {
+                            unsafe {
+                                *buf.get_unchecked_mut(j) = 0;
+                            }
+                            j += 1;
+                        }
+                        self.state = FastqState::InQualTitle;
+                        i += 1;
+                        continue;
+                    }
+                    let nuc = byte & 0xDF;
+                    if nuc == b'A' || nuc == b'C' || nuc == b'G' || nuc == b'T' {
+                        // SAFETY: j <= i < total <= BUF_SIZE = buf.len()
+                        unsafe {
+                            *buf.get_unchecked_mut(j) = nuc;
+                        }
+                        j += 1;
+                    } else {
+                        if j > seq_start {
+                            unsafe {
+                                *buf.get_unchecked_mut(j) = 0;
+                            }
+                            j += 1;
+                        }
+                        self.state = FastqState::InAmbiguous;
+                    }
+                    i += 1;
+                }
+                FastqState::InAmbiguous => {
+                    if byte == b'\n' || byte == b'\r' {
+                        self.state = FastqState::InQualTitle;
+                        i += 1;
+                        continue;
+                    }
+                    let nuc = byte & 0xDF;
+                    if nuc == b'A' || nuc == b'C' || nuc == b'G' || nuc == b'T' {
+                        seq_start = j;
+                        // SAFETY: j <= i < total <= BUF_SIZE = buf.len()
+                        unsafe {
+                            *buf.get_unchecked_mut(j) = nuc;
+                        }
+                        j += 1;
+                        self.state = FastqState::InSeq;
+                    }
+                    i += 1;
+                }
+                FastqState::InQualTitle => {
+                    if byte == b'\n' || byte == b'\r' {
+                        self.state = FastqState::InQual;
+                    }
+                    i += 1;
+                }
+                FastqState::InQual => {
+                    if byte == b'\n' || byte == b'\r' {
+                        self.state = FastqState::OutSeq;
+                    }
+                    i += 1;
+                }
+            }
+        }
+
+        self.save_overlap(buf, j, seq_start)
+    }
+}
+
+// ─── GenBank parser ───────────────────────────────────────────────────────────
+
+const ORIGIN_TAIL: &[u8] = b"RIGIN";
+
+#[derive(Clone, Copy)]
+enum GenbankState {
+    OutSeq,
+    MatchOrigin,
+    SkipOriginLine,
+    InSeq,
+    InSlash,
+    InAmbiguous,
+}
+
+pub(crate) struct GenbankParser {
+    state: GenbankState,
+    overlap: OverlapState,
+    keyword_pos: usize,
+    at_line_start: bool,
+}
+
+impl NucParser for GenbankParser {
+    fn new(k: usize) -> Self {
+        Self {
+            state: GenbankState::OutSeq,
+            overlap: OverlapState::new(k),
+            keyword_pos: 0,
+            at_line_start: true,
+        }
+    }
+
+    #[inline]
+    fn overlap_state(&self) -> &OverlapState {
+        &self.overlap
+    }
+
+    #[inline]
+    fn overlap_state_mut(&mut self) -> &mut OverlapState {
+        &mut self.overlap
+    }
+
+    #[inline]
+    fn is_in_seq(&self) -> bool {
+        matches!(self.state, GenbankState::InSeq)
+    }
+
+    fn parse_inplace(&mut self, buf: &mut [u8], n: usize) -> usize {
+        let total = self.overlap.len + n;
+        let mut i = 0;
+        let mut j = 0;
+        let mut seq_start: usize = 0;
+
+        while i < total {
+            // SAFETY: i < total <= BUF_SIZE = buf.len()
+            let byte = unsafe { *buf.get_unchecked(i) };
+
+            match self.state {
+                GenbankState::OutSeq => {
+                    if byte == b'\n' || byte == b'\r' {
+                        self.at_line_start = true;
+                    } else if self.at_line_start && byte == b'O' {
+                        self.state = GenbankState::MatchOrigin;
+                        self.keyword_pos = 1;
+                        self.at_line_start = false;
+                    } else {
+                        self.at_line_start = false;
+                    }
+                    i += 1;
+                }
+                GenbankState::MatchOrigin => {
+                    if byte == b'\n' || byte == b'\r' {
+                        self.state = GenbankState::OutSeq;
+                        self.at_line_start = true;
+                    } else if byte == ORIGIN_TAIL[self.keyword_pos - 1] {
+                        self.keyword_pos += 1;
+                        if self.keyword_pos == 6 {
+                            self.state = GenbankState::SkipOriginLine;
+                        }
+                    } else {
+                        self.state = GenbankState::OutSeq;
+                        self.at_line_start = false;
+                    }
+                    i += 1;
+                }
+                GenbankState::SkipOriginLine => {
+                    if byte == b'\n' || byte == b'\r' {
+                        self.state = GenbankState::InSeq;
+                        seq_start = j;
+                    }
+                    i += 1;
+                }
+                GenbankState::InSeq => {
+                    if byte == b'\n' || byte == b'\r' {
+                        self.at_line_start = true;
+                        i += 1;
+                        continue;
+                    }
+                    if self.at_line_start && byte == b'/' {
+                        self.state = GenbankState::InSlash;
+                        self.at_line_start = false;
+                        i += 1;
+                        continue;
+                    }
+                    self.at_line_start = false;
+                    let nuc = byte & 0xDF;
+                    if nuc == b'A' || nuc == b'C' || nuc == b'G' || nuc == b'T' {
+                        // SAFETY: j <= i < total <= BUF_SIZE = buf.len()
+                        unsafe {
+                            *buf.get_unchecked_mut(j) = nuc;
+                        }
+                        j += 1;
+                    } else if byte.is_ascii_digit() || byte == b' ' {
+                        // position numbers and spacing between groups: skip
+                    } else {
+                        // ambiguous base: end current sequence if non-empty
+                        if j > seq_start {
+                            unsafe {
+                                *buf.get_unchecked_mut(j) = 0;
+                            }
+                            j += 1;
+                        }
+                        self.state = GenbankState::InAmbiguous;
+                    }
+                    i += 1;
+                }
+                GenbankState::InSlash => {
+                    if byte == b'/' {
+                        // confirmed "//": end of sequence record
+                        if j > seq_start {
+                            unsafe {
+                                *buf.get_unchecked_mut(j) = 0;
+                            }
+                            j += 1;
+                        }
+                        self.state = GenbankState::OutSeq;
+                        self.at_line_start = false;
+                    } else if byte == b'\n' || byte == b'\r' {
+                        // single '/' line: back to sequence
+                        self.state = GenbankState::InSeq;
+                        self.at_line_start = true;
+                    } else {
+                        // false positive: single '/' mid-line, resume sequence
+                        self.state = GenbankState::InSeq;
+                        self.at_line_start = false;
+                    }
+                    i += 1;
+                }
+                GenbankState::InAmbiguous => {
+                    if byte == b'\n' || byte == b'\r' {
+                        self.at_line_start = true;
+                        i += 1;
+                        continue;
+                    }
+                    if self.at_line_start && byte == b'/' {
+                        self.state = GenbankState::InSlash;
+                        self.at_line_start = false;
+                        i += 1;
+                        continue;
+                    }
+                    self.at_line_start = false;
+                    let nuc = byte & 0xDF;
+                    if nuc == b'A' || nuc == b'C' || nuc == b'G' || nuc == b'T' {
+                        seq_start = j;
+                        // SAFETY: j <= i < total <= BUF_SIZE = buf.len()
+                        unsafe {
+                            *buf.get_unchecked_mut(j) = nuc;
+                        }
+                        j += 1;
+                        self.state = GenbankState::InSeq;
+                    }
+                    // digits, spaces, other ambiguous codes: skip
+                    i += 1;
+                }
+            }
+        }
+
+        self.save_overlap(buf, j, seq_start)
+    }
+}
+
+// ─── NucPage ──────────────────────────────────────────────────────────────────
+
+/// Owned page of compacted nucleotides: uppercase A/C/G/T bytes separated by `0`
+/// at sequence boundaries. Automatically returns its buffer to the pool on drop.
+pub struct NucPage {
+    data: ManuallyDrop<Vec<u8>>,
+    len: usize,
+    pool: Arc<Mutex<Vec<Vec<u8>>>>,
+}
+
+impl std::ops::Deref for NucPage {
+    type Target = [u8];
+    fn deref(&self) -> &[u8] {
+        &self.data[..self.len]
+    }
+}
+
+impl Drop for NucPage {
+    fn drop(&mut self) {
+        // SAFETY: data is never accessed after this point
+        let buf = unsafe { ManuallyDrop::take(&mut self.data) };
+        self.pool.lock().unwrap().push(buf);
+    }
+}
+
+// ─── NucPageCursor ────────────────────────────────────────────────────────────
+
+/// A forward cursor over the normalised bytes of a [`NucPage`].
+///
+/// Provides the `next_byte` / `rewind` interface consumed by
+/// [`obiskbuilder::SuperKmerStreamIter`].
+pub struct NucPageCursor<'a> {
+    data: &'a [u8],
+    pos:  usize,
+}
+
+impl NucPageCursor<'_> {
+    /// Returns the next byte in the page, or `None` at end.
+    #[inline]
+    pub fn next_byte(&mut self) -> Option<u8> {
+        if self.pos < self.data.len() {
+            let b = self.data[self.pos];
+            self.pos += 1;
+            Some(b)
+        } else {
+            None
+        }
+    }
+
+    /// Steps the cursor back by `n` bytes.
+    ///
+    /// The caller guarantees that the last `n` bytes were all `ACGT`
+    /// (no `0x00` separators), so they are still in the page buffer.
+    #[inline]
+    pub fn rewind(&mut self, n: usize) {
+        self.pos -= n;
+    }
+
+    /// Total number of bytes in the underlying page.
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.data.len()
+    }
+
+    /// Returns `true` if the page contains no bytes.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.data.is_empty()
+    }
+}
+
+impl NucPage {
+    /// Creates a forward cursor positioned at the start of this page.
+    pub fn cursor(&self) -> NucPageCursor<'_> {
+        NucPageCursor { data: self, pos: 0 }
+    }
+}
+
+// ─── NucStream ────────────────────────────────────────────────────────────────
+
+pub(crate) struct NucStream<R: Read, P: NucParser> {
+    reader: R,
+    parser: P,
+    pool: Arc<Mutex<Vec<Vec<u8>>>>,
+    eof: bool,
+}
+
+impl<R: Read, P: NucParser> NucStream<R, P> {
+    pub(crate) fn new(reader: R, k: usize) -> Self {
+        Self {
+            reader,
+            parser: P::new(k),
+            pool: Arc::new(Mutex::new(Vec::new())),
+            eof: false,
+        }
+    }
+
+    pub(crate) fn read_page(&mut self) -> Option<NucPage> {
+        loop {
+            if self.eof {
+                return None;
+            }
+            // take a buffer from the pool, or allocate fresh if all are in-flight
+            let mut buf = self
+                .pool
+                .lock()
+                .unwrap()
+                .pop()
+                .unwrap_or_else(|| vec![0u8; BUF_SIZE]);
+
+            let ol = self.parser.overlap_len();
+            self.parser.write_overlap(&mut buf[..ol]);
+            let n = self.reader.read(&mut buf[ol..ol + PAGE_SIZE]).unwrap_or(0);
+            if n == 0 {
+                self.eof = true;
+                if ol == 0 {
+                    self.pool.lock().unwrap().push(buf);
+                    return None;
+                }
+            }
+            let out_len = self.parser.parse_inplace(&mut buf, n);
+            if out_len > 0 {
+                return Some(NucPage {
+                    data: ManuallyDrop::new(buf),
+                    len: out_len,
+                    pool: Arc::clone(&self.pool),
+                });
+            }
+            // empty page (all headers/ambiguous): return buf to pool and loop
+            self.pool.lock().unwrap().push(buf);
+        }
+    }
+}
+
+impl<R: Read, P: NucParser> Iterator for NucStream<R, P> {
+    type Item = NucPage;
+    fn next(&mut self) -> Option<NucPage> {
+        self.read_page()
+    }
+}
+
+// ─── FastaNucStream ───────────────────────────────────────────────────────────
+
+pub(crate) type FastaNucStream<R> = NucStream<R, FastaParser>;
+pub(crate) type FastqNucStream<R> = NucStream<R, FastqParser>;
+pub(crate) type GenbankNucStream<R> = NucStream<R, GenbankParser>;
+
+// ─── AnyNucStream ─────────────────────────────────────────────────────────────
+
+pub(crate) enum AnyNucStream<R: Read> {
+    Fasta(FastaNucStream<R>),
+    Fastq(FastqNucStream<R>),
+    Genbank(GenbankNucStream<R>),
+}
+
+impl<R: Read> Iterator for AnyNucStream<R> {
+    type Item = NucPage;
+    fn next(&mut self) -> Option<NucPage> {
+        match self {
+            AnyNucStream::Fasta(s)   => s.next(),
+            AnyNucStream::Fastq(s)   => s.next(),
+            AnyNucStream::Genbank(s) => s.next(),
+        }
+    }
+}
+
+fn dispatch<R: Read>(
+    mut guesser: MimeTypeGuesser<R>,
+    k: usize,
+) -> Option<AnyNucStream<MimeTypeGuesser<R>>> {
+    match guesser.mime_type() {
+        Some("text/fasta") => Some(AnyNucStream::Fasta(NucStream::new(guesser, k))),
+        Some("text/fastq") => Some(AnyNucStream::Fastq(NucStream::new(guesser, k))),
+        Some("text/gbff")  => Some(AnyNucStream::Genbank(NucStream::new(guesser, k))),
+        _                  => None,
+    }
+}
+
+/// Wraps an already-open reader in a nucleotide stream, detecting its format.
+/// Returns `None` if the format is not recognised.
+pub(crate) fn nuc_stream<R: Read>(
+    reader: R,
+    k: usize,
+) -> Option<AnyNucStream<MimeTypeGuesser<R>>> {
+    dispatch(MimeTypeGuesser::new(reader), k)
+}
+
+/// Opens a nucleotide stream from any source (file path, URL, or `-` for stdin),
+/// with transparent decompression and automatic format detection.
+///
+/// # Errors
+/// Returns an `io::Error` if the source cannot be opened, decompression fails,
+/// or the format is not recognised.
+pub fn open_nuc_stream(
+    source: &str,
+    k: usize,
+) -> io::Result<Box<dyn Iterator<Item = NucPage> + Send>> {
+    let reader = open_raw(source)?;
+    dispatch(MimeTypeGuesser::new(reader), k)
+        .map(|s| Box::new(s) as Box<dyn Iterator<Item = NucPage> + Send>)
+        .ok_or_else(|| io::Error::new(io::ErrorKind::InvalidData, "unknown sequence format"))
+}
diff --git a/src/obiread/src/stream.rs b/src/obiread/src/stream.rs
deleted file mode 100644
index e797ee2..0000000
--- a/src/obiread/src/stream.rs
+++ /dev/null
@@ -1,507 +0,0 @@
-//! Streaming byte normaliser for FASTA, FASTQ, and GBFF sequence files.
-//!
-//! [`NormalizedByteStream`] wraps any `Read` source in a 64 KiB circular ring
-//! buffer and emits a normalised byte stream:
-//!
-//! - Uppercase `ACGT` bytes for in-sequence positions.
-//! - `0x00` as a separator between independent sequence segments (record
-//!   boundary, non-ACGT base, or end of the GBFF `//` record).
-//! - `None` at true EOF.
-//!
-//! Unlike the Rope-based pipeline, the stream never accumulates a full
-//! record: it is safe for chromosomes of arbitrary size (e.g. GBFF files
-//! with 250 MiB ORIGIN sections).
-//!
-//! # Rewind invariant
-//!
-//! `rewind(n)` subtracts `n` from the read head. It is only called by
-//! [`obiskbuilder::SuperKmerStreamIter`] when the last `n` bytes were all
-//! ACGT (no format overhead in between), so the ring buffer always contains
-//! those bytes at positions `[head-n .. head]`.  The maximum rewind is
-//! `k ≤ 31`, well within the 32 KiB kept below the refill watermark.
-
-use std::io::{self, Read};
-
-// ── ring-buffer constants ─────────────────────────────────────────────────────
-
-const CAP: usize = 65536; // must be a power of two
-const MASK: usize = CAP - 1;
-const REFILL_THRESHOLD: usize = 32768; // refill when available drops below this
-const REFILL_CHUNK: usize = 32768;     // target bytes per refill call
-
-// ── format detection ──────────────────────────────────────────────────────────
-
-#[derive(Clone, Copy, PartialEq, Eq)]
-enum Format { Fasta, Fastq, Gbff }
-
-fn detect_format(buf: &[u8]) -> Format {
-    if buf.starts_with(b"LOCUS ") {
-        Format::Gbff
-    } else if buf.first() == Some(&b'>') {
-        Format::Fasta
-    } else {
-        Format::Fastq
-    }
-}
-
-// ── format state machines ─────────────────────────────────────────────────────
-
-#[derive(Clone, Copy)]
-enum FaState {
-    OutSeq, // reading header / between records
-    InSeq,  // reading sequence bases
-}
-
-#[derive(Clone, Copy)]
-enum FqState {
-    Header,      // reading @id line
-    Seq,         // reading sequence bases
-    Plus,        // reading +[id] separator line
-    Qual(usize), // skipping quality bytes; payload = remaining count
-}
-
-const ORIGIN: &[u8] = b"ORIGIN";
-
-#[derive(Clone, Copy)]
-enum GbState {
-    Pre(u8),   // pre-ORIGIN: n chars of "ORIGIN" matched at current line start
-    PreSkip,   // pre-ORIGIN: skip rest of non-ORIGIN line then → Pre(0)
-    LineStart, // ORIGIN data: at column 0 of a numbered data line
-    Num,       // ORIGIN data: skipping spaces + line-number digits
-    Seq,       // ORIGIN data: reading lowercase sequence bases
-    Slash,     // ORIGIN data: saw the first '/' of a potential '//' end marker
-}
-
-#[derive(Clone, Copy)]
-enum State {
-    Fa(FaState),
-    Fq(FqState),
-    Gb(GbState),
-}
-
-// ── NormalizedByteStream ──────────────────────────────────────────────────────
-
-/// A streaming normaliser over any `Read` source.
-///
-/// Call [`next_byte`](NormalizedByteStream::next_byte) to consume one normalised
-/// byte at a time.  Call [`rewind`](NormalizedByteStream::rewind) to step back
-/// up to 31 bytes (used by the superkmer builder when a minimizer or length
-/// boundary is crossed).
-pub struct NormalizedByteStream<R: Read> {
-    reader:  R,
-    buf:     Box<[u8; CAP]>,
-    head:    usize, // absolute read position; index = head & MASK
-    write:   usize, // absolute write position; index = write & MASK
-    eof:     bool,
-    state:   State,
-    seq_len: usize, // FASTQ only: non-newline chars seen in current sequence line
-}
-
-impl<R: Read> NormalizedByteStream<R> {
-    /// Wrap `reader` and detect its format from the first bytes.
-    pub fn new(mut reader: R) -> io::Result<Self> {
-        let mut buf = Box::new([0u8; CAP]);
-
-        // Initial read — enough to detect the format.
-        let n = reader.read(&mut buf[..REFILL_CHUNK])?;
-        let fmt = detect_format(&buf[..n]);
-        let state = match fmt {
-            Format::Fasta => State::Fa(FaState::OutSeq),
-            Format::Fastq => State::Fq(FqState::Header),
-            Format::Gbff  => State::Gb(GbState::Pre(0)),
-        };
-
-        Ok(Self {
-            reader,
-            buf,
-            head:    0,
-            write:   n,
-            eof:     n == 0,
-            state,
-            seq_len: 0,
-        })
-    }
-
-    /// Step the read head back by `n` bytes.
-    ///
-    /// # Safety
-    /// The caller guarantees that the last `n` bytes returned by
-    /// [`next_byte`] were all uppercase ACGT (no `0x00` separators), so those
-    /// bytes are still present in the ring buffer.
-    #[inline]
-    pub fn rewind(&mut self, n: usize) {
-        self.head -= n;
-    }
-
-    #[inline]
-    fn available(&self) -> usize {
-        self.write - self.head
-    }
-
-    fn try_refill(&mut self) -> io::Result<()> {
-        if self.eof || self.available() >= REFILL_THRESHOLD {
-            return Ok(());
-        }
-
-        // First segment: write_idx → end of buffer (contiguous, no wrap).
-        let write_idx = self.write & MASK;
-        let free      = CAP - self.available();
-        let to_end    = CAP - write_idx;
-        let chunk1    = free.min(to_end).min(REFILL_CHUNK);
-
-        if chunk1 > 0 {
-            let n = self.reader.read(&mut self.buf[write_idx..write_idx + chunk1])?;
-            self.write += n;
-            if n == 0 {
-                self.eof = true;
-                return Ok(());
-            }
-        }
-
-        // Second segment: if still below threshold, read into the wrapped region.
-        if self.available() < REFILL_THRESHOLD && !self.eof {
-            let write_idx2 = self.write & MASK;
-            let free2      = CAP - self.available();
-            let to_end2    = CAP - write_idx2;
-            let chunk2     = free2.min(to_end2).min(REFILL_CHUNK);
-            if chunk2 > 0 {
-                let n = self.reader.read(&mut self.buf[write_idx2..write_idx2 + chunk2])?;
-                self.write += n;
-                if n == 0 {
-                    self.eof = true;
-                }
-            }
-        }
-
-        Ok(())
-    }
-
-    /// Return the next normalised byte, or `None` at EOF.
-    ///
-    /// Emits uppercase `A`, `C`, `G`, `T`, or `0x00` (segment separator).
-    pub fn next_byte(&mut self) -> Option<u8> {
-        loop {
-            // Proactive refill: keep ≥ REFILL_THRESHOLD bytes in the buffer
-            // so rewinds are always safe.
-            if !self.eof && self.available() < REFILL_THRESHOLD {
-                let _ = self.try_refill();
-            }
-
-            if self.available() == 0 {
-                return None;
-            }
-
-            let b = self.buf[self.head & MASK];
-            self.head += 1;
-
-            // Copy state to avoid borrow conflicts while mutating self.state.
-            let state = self.state;
-            if let Some(out) = self.process(state, b) {
-                return Some(out);
-            }
-        }
-    }
-
-    // ── per-format byte processors ────────────────────────────────────────────
-
-    #[inline]
-    fn process(&mut self, state: State, b: u8) -> Option<u8> {
-        match state {
-            State::Fa(fa) => self.fasta(fa, b),
-            State::Fq(fq) => self.fastq(fq, b),
-            State::Gb(gb) => self.gbff(gb, b),
-        }
-    }
-
-    fn fasta(&mut self, fa: FaState, b: u8) -> Option<u8> {
-        match fa {
-            FaState::OutSeq => {
-                if b == b'\n' {
-                    self.state = State::Fa(FaState::InSeq);
-                }
-                None // skip header bytes
-            }
-            FaState::InSeq => {
-                if b == b'>' {
-                    self.state = State::Fa(FaState::OutSeq);
-                    Some(0x00) // record boundary
-                } else if b == b'\n' || b == b'\r' {
-                    None
-                } else {
-                    Some(normalize_nuc(b))
-                }
-            }
-        }
-    }
-
-    fn fastq(&mut self, fq: FqState, b: u8) -> Option<u8> {
-        match fq {
-            FqState::Header => {
-                if b == b'\n' {
-                    self.state = State::Fq(FqState::Seq);
-                    self.seq_len = 0;
-                }
-                None
-            }
-            FqState::Seq => {
-                if b == b'\n' {
-                    self.state = State::Fq(FqState::Plus);
-                    None
-                } else if b == b'\r' {
-                    None
-                } else {
-                    self.seq_len += 1; // count all chars, including non-ACGT
-                    Some(normalize_nuc(b))
-                }
-            }
-            FqState::Plus => {
-                if b == b'\n' {
-                    self.state = State::Fq(FqState::Qual(self.seq_len));
-                }
-                None
-            }
-            FqState::Qual(rem) => {
-                if b == b'\n' {
-                    if rem == 0 {
-                        // Quality line ended after all bases consumed → new record.
-                        self.state = State::Fq(FqState::Header);
-                        Some(0x00) // record boundary
-                    } else {
-                        // Newline inside multi-line quality — keep counting.
-                        None
-                    }
-                } else if b == b'\r' {
-                    None
-                } else if rem > 0 {
-                    self.state = State::Fq(FqState::Qual(rem - 1));
-                    None
-                } else {
-                    // rem == 0 but non-newline: shouldn't happen in valid FASTQ.
-                    None
-                }
-            }
-        }
-    }
-
-    fn gbff(&mut self, gb: GbState, b: u8) -> Option<u8> {
-        match gb {
-            GbState::Pre(n) => {
-                let n = n as usize;
-                if n < 6 {
-                    if b == ORIGIN[n] {
-                        self.state = State::Gb(GbState::Pre((n + 1) as u8));
-                    } else if b == b'\n' {
-                        self.state = State::Gb(GbState::Pre(0));
-                    } else {
-                        self.state = State::Gb(GbState::PreSkip);
-                    }
-                } else {
-                    // All 6 chars of "ORIGIN" matched; skip the rest of the header line.
-                    if b == b'\n' {
-                        self.state = State::Gb(GbState::LineStart);
-                    }
-                    // Non-'\n': stay in Pre(6) implicitly (state unchanged).
-                }
-                None
-            }
-            GbState::PreSkip => {
-                if b == b'\n' {
-                    self.state = State::Gb(GbState::Pre(0));
-                }
-                None
-            }
-            GbState::LineStart => {
-                if b == b'/' {
-                    self.state = State::Gb(GbState::Slash);
-                } else if b != b'\n' {
-                    // Space or digit: start of a numbered sequence line.
-                    self.state = State::Gb(GbState::Num);
-                }
-                None
-            }
-            GbState::Num => {
-                if b == b'\n' {
-                    self.state = State::Gb(GbState::LineStart);
-                    None
-                } else if b.is_ascii_digit() || b == b' ' {
-                    None // still in the number prefix
-                } else {
-                    // First letter: transition to Seq and process this byte.
-                    self.state = State::Gb(GbState::Seq);
-                    Some(normalize_nuc(b))
-                }
-            }
-            GbState::Seq => {
-                if b == b'\n' {
-                    self.state = State::Gb(GbState::LineStart);
-                    None
-                } else if b == b' ' {
-                    None // inter-group space
-                } else {
-                    Some(normalize_nuc(b))
-                }
-            }
-            GbState::Slash => {
-                if b == b'/' {
-                    // End of GBFF record ("//").
-                    self.state = State::Gb(GbState::Pre(0)); // ready for next record
-                    Some(0x00)
-                } else {
-                    // Stray '/' — shouldn't happen in valid GBFF; recover.
-                    self.state = State::Gb(GbState::LineStart);
-                    None
-                }
-            }
-        }
-    }
-}
-
-// ── nucleotide normalisation ──────────────────────────────────────────────────
-
-#[inline]
-fn normalize_nuc(b: u8) -> u8 {
-    match b | 0x20 {
-        b'a' => b'A',
-        b'c' => b'C',
-        b'g' => b'G',
-        b't' => b'T',
-        _ => 0x00,
-    }
-}
-
-// ── tests ─────────────────────────────────────────────────────────────────────
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    fn stream(data: &[u8]) -> Vec<u8> {
-        let mut s = NormalizedByteStream::new(data).unwrap();
-        let mut out = Vec::new();
-        while let Some(b) = s.next_byte() {
-            out.push(b);
-        }
-        out
-    }
-
-    // ── FASTA ─────────────────────────────────────────────────────────────────
-
-    #[test]
-    fn fasta_single_record() {
-        assert_eq!(stream(b">s1\nACGTACGT\n"), b"ACGTACGT");
-    }
-
-    #[test]
-    fn fasta_two_records_separated() {
-        let out = stream(b">s1\nACGT\n>s2\nTTTT\n");
-        assert_eq!(out, b"ACGT\x00TTTT");
-    }
-
-    #[test]
-    fn fasta_multiline_concatenated() {
-        assert_eq!(stream(b">s1\nACGT\nACGT\n"), b"ACGTACGT");
-    }
-
-    #[test]
-    fn fasta_lowercase_uppercased() {
-        assert_eq!(stream(b">s1\nacgt\n"), b"ACGT");
-    }
-
-    #[test]
-    fn fasta_non_acgt_becomes_null() {
-        let out = stream(b">s1\nACGTNACGT\n");
-        assert_eq!(out, b"ACGT\x00ACGT");
-    }
-
-    // ── FASTQ ─────────────────────────────────────────────────────────────────
-
-    fn make_fastq(records: &[&[u8]]) -> Vec<u8> {
-        let mut buf = Vec::new();
-        for seq in records {
-            buf.extend_from_slice(b"@hdr\n");
-            buf.extend_from_slice(seq);
-            buf.push(b'\n');
-            buf.extend_from_slice(b"+\n");
-            buf.extend_from_slice(&vec![b'I'; seq.len()]);
-            buf.push(b'\n');
-        }
-        buf
-    }
-
-    #[test]
-    fn fastq_single_record() {
-        // Trailing \x00 is emitted on the newline that ends the quality line.
-        assert_eq!(stream(&make_fastq(&[b"ACGTACGT"])), b"ACGTACGT\x00");
-    }
-
-    #[test]
-    fn fastq_two_records_separated() {
-        let out = stream(&make_fastq(&[b"ACGT", b"TTTT"]));
-        assert_eq!(out, b"ACGT\x00TTTT\x00");
-    }
-
-    #[test]
-    fn fastq_lowercase_uppercased() {
-        assert_eq!(stream(&make_fastq(&[b"acgt"])), b"ACGT\x00");
-    }
-
-    #[test]
-    fn fastq_non_acgt_becomes_null() {
-        let out = stream(&make_fastq(&[b"ACGTNACGT"]));
-        assert_eq!(out, b"ACGT\x00ACGT\x00");
-    }
-
-    // ── GBFF ──────────────────────────────────────────────────────────────────
-
-    fn make_gbff(seqs: &[&[u8]]) -> Vec<u8> {
-        let mut buf = Vec::new();
-        for seq in seqs {
-            buf.extend_from_slice(b"LOCUS       NC_000001\nFEATURES\nORIGIN\n");
-            // write numbered lines of 60 bases each
-            let mut pos = 1usize;
-            for chunk in seq.chunks(60) {
-                let groups: Vec<&[u8]> = chunk.chunks(10).collect();
-                let line = groups.join(&b' ');
-                buf.extend_from_slice(format!("{:9} ", pos).as_bytes());
-                buf.extend_from_slice(&line);
-                buf.push(b'\n');
-                pos += chunk.len();
-            }
-            buf.extend_from_slice(b"//\n");
-        }
-        buf
-    }
-
-    #[test]
-    fn gbff_single_record() {
-        let seq = b"acgtacgtacgt";
-        let out = stream(&make_gbff(&[seq]));
-        assert_eq!(out, b"ACGTACGTACGT\x00");
-    }
-
-    #[test]
-    fn gbff_two_records_separated() {
-        let out = stream(&make_gbff(&[b"acgtacgt", b"tttttttt"]));
-        assert_eq!(out, b"ACGTACGT\x00TTTTTTTT\x00");
-    }
-
-    #[test]
-    fn gbff_non_acgt_becomes_null() {
-        let out = stream(&make_gbff(&[b"acgtnacgt"]));
-        assert_eq!(out, b"ACGT\x00ACGT\x00");
-    }
-
-    // ── rewind ────────────────────────────────────────────────────────────────
-
-    #[test]
-    fn rewind_replays_bytes() {
-        let mut s = NormalizedByteStream::new(b">s\nACGT\n" as &[u8]).unwrap();
-        assert_eq!(s.next_byte(), Some(b'A'));
-        assert_eq!(s.next_byte(), Some(b'C'));
-        s.rewind(1);
-        assert_eq!(s.next_byte(), Some(b'C'));
-        assert_eq!(s.next_byte(), Some(b'G'));
-        assert_eq!(s.next_byte(), Some(b'T'));
-        assert_eq!(s.next_byte(), None);
-    }
-}
diff --git a/src/obiread/src/xopen.rs b/src/obiread/src/xopen.rs
index f5dbbf0..f86585c 100644
--- a/src/obiread/src/xopen.rs
+++ b/src/obiread/src/xopen.rs
@@ -20,13 +20,13 @@ use std::io::{self, Read};
 
 /// Open any source for reading, with transparent decompression.
 ///
-/// Returns a `Box<dyn Read>` that yields uncompressed bytes regardless of
-/// whether the underlying source is plain text, gzip, bzip2, xz or zstd.
+/// Returns a `Box<dyn Read + Send>` that yields uncompressed bytes regardless
+/// of whether the underlying source is plain text, gzip, bzip2, xz or zstd.
 ///
 /// # Errors
 /// Returns an `io::Error` if the file cannot be opened, the URL cannot be
 /// fetched, or the compression header is malformed.
-pub fn xopen(source: &str) -> io::Result<MimeTypeGuesser<Box<dyn Read + Send>>> {
+pub(crate) fn open_raw(source: &str) -> io::Result<Box<dyn Read + Send>> {
     let raw: Box<dyn Read + Send> = match source {
         "-" => Box::new(io::stdin()),
         s if s.starts_with("http://") || s.starts_with("https://") => http_reader(s)?,
@@ -35,8 +35,15 @@ pub fn xopen(source: &str) -> io::Result<MimeTypeGuesser<Box<dyn Read + Send>>>
             Box::new(File::open(expanded.as_ref())?)
         }
     };
-    let decompressed = decompress(raw)?;
-    Ok(MimeTypeGuesser::new(decompressed))
+    decompress(raw)
+}
+
+/// Open any source for reading, with transparent decompression and MIME detection.
+///
+/// Wraps [`open_raw`] in a [`MimeTypeGuesser`] so callers can inspect the
+/// format before consuming the stream.
+pub fn xopen(source: &str) -> io::Result<MimeTypeGuesser<Box<dyn Read + Send>>> {
+    Ok(MimeTypeGuesser::new(open_raw(source)?))
 }
 
 // ── internal helpers ──────────────────────────────────────────────────────────
diff --git a/src/obiskbuilder/src/lib.rs b/src/obiskbuilder/src/lib.rs
index a5409e1..6dc2de1 100644
--- a/src/obiskbuilder/src/lib.rs
+++ b/src/obiskbuilder/src/lib.rs
@@ -17,9 +17,21 @@ pub use iter::SuperKmerIter;
 pub use scratch::SuperKmerScratch;
 pub use stream_iter::SuperKmerStreamIter;
 
+use obiread::NucPage;
 use obikrope::Rope;
 use obikseq::RoutableSuperKmer;
 
+/// Collect all super-kmers from a normalised [`NucPage`].
+pub fn build_superkmers_page(
+    page: NucPage,
+    k: usize,
+    level_max: usize,
+    theta: f64,
+) -> Vec<RoutableSuperKmer> {
+    let cursor = page.cursor();
+    SuperKmerStreamIter::new(cursor, k, level_max, theta).collect()
+}
+
 /// Collect all super-kmers from a normalised rope chunk.
 pub fn build_superkmers(
     rope: Rope,
diff --git a/src/obiskbuilder/src/stream_iter.rs b/src/obiskbuilder/src/stream_iter.rs
index 25f41c5..dabb72b 100644
--- a/src/obiskbuilder/src/stream_iter.rs
+++ b/src/obiskbuilder/src/stream_iter.rs
@@ -1,35 +1,23 @@
-//! Streaming superkmer iterator that does not require a fully-buffered record.
-//!
-//! [`SuperKmerStreamIter`] wraps a [`NormalizedByteStream`] and yields
-//! [`RoutableSuperKmer`] values one by one, exactly as [`SuperKmerIter`] does
-//! over a [`Rope`], but without accumulating the whole input in memory first.
-//!
-//! This makes it suitable for large GBFF chromosomes (250 MiB ORIGIN sections)
-//! or any other source where buffering the full record would exhaust memory.
-//!
-//! The cut conditions and superkmer semantics are identical to [`SuperKmerIter`]:
-//!
-//! | Condition              | stream rewind |
-//! |------------------------|---------------|
-//! | entropy(kmer) ≤ θ      | k−1           |
-//! | minimizer changed      | k             |
-//! | super-kmer length = 256| k             |
-//!
-//! [`SuperKmerIter`]: crate::iter::SuperKmerIter
-//! [`Rope`]: obikrope::Rope
+//! Streaming superkmer iterator over a [`NucPageCursor`].
 
-use std::io::Read;
-
-use obiread::stream::NormalizedByteStream;
+use obiread::NucPageCursor;
 use obikseq::RoutableSuperKmer;
 use obikseq::kmer::Minimizer;
 
 use crate::rolling_stat::RollingStat;
 use crate::scratch::SuperKmerScratch;
 
-/// Streaming iterator over [`RoutableSuperKmer`] values.
-pub struct SuperKmerStreamIter<R: Read> {
-    stream:       NormalizedByteStream<R>,
+/// Streaming iterator over [`RoutableSuperKmer`] values from a [`NucPageCursor`].
+///
+/// Cut conditions (checked in order per nucleotide, once the k-mer window is full):
+///
+/// | Condition              | cursor rewind |
+/// |------------------------|---------------|
+/// | entropy(kmer) ≤ θ      | k−1           |
+/// | minimizer changed      | k             |
+/// | super-kmer length = 256| k             |
+pub struct SuperKmerStreamIter<'a> {
+    cursor:       NucPageCursor<'a>,
     k:            usize,
     theta:        f64,
     scratch:      SuperKmerScratch,
@@ -38,23 +26,18 @@ pub struct SuperKmerStreamIter<R: Read> {
     prev_min_pos: usize,
 }
 
-impl<R: Read> SuperKmerStreamIter<R> {
-    /// Build a streaming superkmer iterator from any `Read` source.
-    ///
-    /// - `reader`: raw bytes (FASTA, FASTQ, or GBFF; format auto-detected)
-    /// - `k`: k-mer size (must be odd, 11 ≤ k ≤ 31)
-    /// - `level_max`: maximum sub-word size for entropy (1–6)
-    /// - `theta`: entropy threshold; k-mers with score ≤ theta are rejected
-    pub fn new(reader: R, k: usize, level_max: usize, theta: f64) -> std::io::Result<Self> {
-        Ok(Self {
-            stream:       NormalizedByteStream::new(reader)?,
+impl<'a> SuperKmerStreamIter<'a> {
+    /// Build an iterator from a [`NucPageCursor`] over normalised sequence data.
+    pub fn new(cursor: NucPageCursor<'a>, k: usize, level_max: usize, theta: f64) -> Self {
+        Self {
+            cursor,
             k,
             theta,
             scratch:      SuperKmerScratch::new(),
             stat:         RollingStat::new(level_max),
             prev_min:     None,
             prev_min_pos: 0,
-        })
+        }
     }
 
     fn reset(&mut self) {
@@ -73,12 +56,12 @@ impl<R: Read> SuperKmerStreamIter<R> {
     }
 }
 
-impl<R: Read> Iterator for SuperKmerStreamIter<R> {
+impl Iterator for SuperKmerStreamIter<'_> {
     type Item = RoutableSuperKmer;
 
     fn next(&mut self) -> Option<RoutableSuperKmer> {
         loop {
-            let byte = match self.stream.next_byte() {
+            let byte = match self.cursor.next_byte() {
                 None => {
                     return self.try_emit();
                 }
@@ -103,7 +86,7 @@ impl<R: Read> Iterator for SuperKmerStreamIter<R> {
             // ── 1. Entropy check ─────────────────────────────────────────────
             if self.stat.normalized_entropy().unwrap_or(1.0) < self.theta {
                 let result = self.try_emit();
-                self.stream.rewind(self.k - 1);
+                self.cursor.rewind(self.k - 1);
                 self.reset();
                 if result.is_some() {
                     return result;
@@ -114,11 +97,11 @@ impl<R: Read> Iterator for SuperKmerStreamIter<R> {
             let min     = self.stat.canonical_minimizer().unwrap();
             let min_pos = self.stat.minimizer_position().unwrap_or(0);
 
-            // ── 2. Minimizer change check ─────────────────────────────────────
+            // ── 2. Minimizer change ───────────────────────────────────────────
             if let Some(prev) = self.prev_min {
                 if min != prev {
                     let result = self.try_emit();
-                    self.stream.rewind(self.k);
+                    self.cursor.rewind(self.k);
                     self.reset();
                     if result.is_some() {
                         return result;
@@ -127,10 +110,10 @@ impl<R: Read> Iterator for SuperKmerStreamIter<R> {
                 }
             }
 
-            // ── 3. Super-kmer length check ────────────────────────────────────
+            // ── 3. Super-kmer length cap ──────────────────────────────────────
             if self.scratch.len() == 256 {
                 let result = self.try_emit();
-                self.stream.rewind(self.k);
+                self.cursor.rewind(self.k);
                 self.reset();
                 if result.is_some() {
                     return result;