♻️ refactor rope implementation to use obikrope

- rename `obirope` → `obikroper`
- replace legacy rope with new in-place, Cell-based implementation
  - add ForwardCursor/Backward Cursor & SeekMode support (no more BytesMut)
- update all dependents:
  - obiread: switch to Rope + cursors, remove tape.rs
    • chunk iterator yields `Rope` instead of Vec<Bytes>
  - obiskbuilder: use ForwardCursor over Rope
- remove bytes dependency from affected crates

src/Cargo.lock

@@ -595,6 +595,14 @@ dependencies = [
  "obiskbuilder",
 ]
 
+[[package]]
+name = "obikrope"
+version = "0.1.0"
+dependencies = [
+ "bytes",
+ "criterion2",
+]
+
 [[package]]
 name = "obikseq"
 version = "0.1.0"
@@ -607,8 +615,8 @@ dependencies = [
 name = "obiread"
 version = "0.1.0"
 dependencies = [
- "bytes",
  "niffler",
+ "obikrope",
  "ureq",
 ]
 
@@ -616,9 +624,8 @@ dependencies = [
 name = "obiskbuilder"
 version = "0.1.0"
 dependencies = [
- "bytes",
+ "obikrope",
  "obikseq",
- "obiread",
 ]
 
 [[package]]

src/Cargo.toml

@@ -1,3 +1,3 @@
 [workspace]
 resolver = "3"
-members = ["obikseq", "obiread", "obiskbuilder", "obifastwrite", "obikmer"]
+members = ["obikseq", "obiread", "obiskbuilder", "obifastwrite", "obikmer","obikrope"]

src/obikrope/Cargo.toml

@@ -6,6 +6,5 @@ edition = "2024"
 [dev-dependencies]
 criterion2 = { version = "3", features = ["cargo_bench_support"] }
 
-[[bench]]
-name = "superkmer"
-harness = false
+[dependencies]
+bytes = "1.11.1"

src/obikrope/src/cursor.rs

@@ -0,0 +1,310 @@
+use std::cell::Cell;
+
+use crate::{Rope, RopeError};
+
+#[derive(Clone, Copy)]
+pub enum SeekMode {
+    Absolute,
+    Relative,
+    RelativeToEnd,
+}
+
+// ── shared state ──────────────────────────────────────────────────────────────
+
+#[derive(Clone)]
+struct CursorState<'a> {
+    block_idx:   Cell<usize>,
+    block_start: Cell<usize>,
+    block_end:   Cell<usize>,
+    block:       Cell<&'a [Cell<u8>]>,
+    initialized: Cell<bool>,
+    current:     Cell<Option<usize>>,
+}
+
+impl<'a> CursorState<'a> {
+    fn new() -> Self {
+        Self {
+            block_idx:   Cell::new(0),
+            block_start: Cell::new(0),
+            block_end:   Cell::new(0),
+            block:       Cell::new(&[]),
+            initialized: Cell::new(false),
+            current:     Cell::new(None),
+        }
+    }
+
+    fn get(&self, rope: &'a Rope, i: usize) -> Option<u8> {
+        if !self.initialized.get() || i < self.block_start.get() || i >= self.block_end.get() {
+            let (bi, bs, be) = rope.lookup(i)?;
+            self.block_idx.set(bi);
+            self.block_start.set(bs);
+            self.block_end.set(be);
+            self.block.set(rope.get_block(bi)?);
+            self.initialized.set(true);
+        }
+        Some(self.block.get()[i - self.block_start.get()].get())
+    }
+
+    fn set(&self, rope: &'a Rope, i: usize, value: u8) -> Result<(), RopeError> {
+        if !self.initialized.get() || i < self.block_start.get() || i >= self.block_end.get() {
+            let (bi, bs, be) = rope.lookup(i).ok_or(
+                RopeError::OutOfBounds(format!("index out of bounds: i={} > {}", i, rope.len())),
+            )?;
+            self.block_idx.set(bi);
+            self.block_start.set(bs);
+            self.block_end.set(be);
+            self.block.set(rope.get_block(bi).ok_or(RopeError::BlockNotFound(format!(
+                "Cannot find block for index {}",
+                i
+            )))?);
+            self.initialized.set(true);
+        }
+        self.block.get()[i - self.block_start.get()].set(value);
+        Ok(())
+    }
+}
+
+// ── trait ─────────────────────────────────────────────────────────────────────
+
+pub trait RopeCursor {
+    fn get(&self, i: usize) -> Option<u8>;
+    fn set(&self, i: usize, value: u8) -> Result<(), RopeError>;
+    fn peek(&self) -> Option<u8>;
+    fn poke(&self, value: u8) -> Result<(), RopeError>;
+    fn tell(&self) -> Option<usize>;
+    fn seek(&self, pos: isize, mode: SeekMode) -> Result<usize, RopeError>;
+    fn rewind(&self, go_back_of: usize) -> Result<(), RopeError>;
+    fn len(&self) -> usize;
+    fn read_next(&self) -> Result<u8, RopeError>;
+}
+
+// ── ForwardCursor ─────────────────────────────────────────────────────────────
+
+#[derive(Clone)]
+pub struct ForwardCursor<'a> {
+    rope:  &'a Rope,
+    state: CursorState<'a>,
+}
+
+impl<'a> ForwardCursor<'a> {
+    pub fn new(rope: &'a Rope) -> Self {
+        Self { rope, state: CursorState::new() }
+    }
+
+    pub fn read_ahead(&self, ahead: usize) -> Result<u8, RopeError> {
+        let pos = self.state.current.get().ok_or(RopeError::CurrentNotSet)?;
+        self.state
+            .get(self.rope, pos + ahead)
+            .ok_or(RopeError::OutOfBounds(format!(
+                "index out of bounds: i={} + {} > {}",
+                pos, ahead, self.rope.len()
+            )))
+    }
+
+    pub fn write(&self, value: u8) -> Result<(), RopeError> {
+        let pos = self.state.current.get().unwrap_or(0);
+        self.state.set(self.rope, pos, value)?;
+        self.state.current.set(Some(pos + 1));
+        Ok(())
+    }
+
+    pub fn iter(&self) -> ForwardIter<'a, '_> {
+        ForwardIter { cursor: self }
+    }
+}
+
+impl<'a> RopeCursor for ForwardCursor<'a> {
+    fn get(&self, i: usize) -> Option<u8> {
+        self.state.get(self.rope, i)
+    }
+
+    fn set(&self, i: usize, value: u8) -> Result<(), RopeError> {
+        self.state.set(self.rope, i, value)
+    }
+
+    fn peek(&self) -> Option<u8> {
+        self.state.get(self.rope, self.state.current.get()?)
+    }
+
+    fn poke(&self, value: u8) -> Result<(), RopeError> {
+        let pos = self.state.current.get().ok_or(RopeError::CurrentNotSet)?;
+        self.state.set(self.rope, pos, value)
+    }
+
+    fn tell(&self) -> Option<usize> {
+        self.state.current.get()
+    }
+
+    fn len(&self) -> usize {
+        self.rope.len()
+    }
+
+    fn read_next(&self) -> Result<u8, RopeError> {
+        let next_pos = match self.state.current.get() {
+            Some(i) => i + 1,
+            None    => 0,
+        };
+        let value = self.state
+            .get(self.rope, next_pos)
+            .ok_or(RopeError::OutOfBounds(format!(
+                "index out of bounds: i={} > {}",
+                next_pos, self.rope.len()
+            )))?;
+        self.state.current.set(Some(next_pos));
+        Ok(value)
+    }
+
+    fn seek(&self, pos: isize, mode: SeekMode) -> Result<usize, RopeError> {
+        let pos = match mode {
+            SeekMode::Absolute    => pos,
+            SeekMode::Relative    => self.state.current.get().ok_or(RopeError::CurrentNotSet)? as isize + pos,
+            SeekMode::RelativeToEnd => self.rope.len() as isize - pos,
+        };
+        if pos < 0 {
+            return Err(RopeError::OutOfBounds(format!("index out of bounds: i={} < 0", pos)));
+        }
+        self.state.current.set(Some(pos as usize));
+        Ok(pos as usize)
+    }
+
+    fn rewind(&self, go_back_of: usize) -> Result<(), RopeError> {
+        self.seek(-(go_back_of as isize), SeekMode::Relative)?;
+        Ok(())
+    }
+}
+
+impl Iterator for ForwardCursor<'_> {
+    type Item = u8;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.read_next().ok()
+    }
+}
+
+pub struct ForwardIter<'a, 'b> {
+    cursor: &'b ForwardCursor<'a>,
+}
+
+impl Iterator for ForwardIter<'_, '_> {
+    type Item = u8;
+    fn next(&mut self) -> Option<u8> {
+        self.cursor.read_next().ok()
+    }
+}
+
+// ── BackwardCursor ────────────────────────────────────────────────────────────
+
+#[derive(Clone)]
+pub struct BackwardCursor<'a> {
+    rope:  &'a Rope,
+    state: CursorState<'a>,
+}
+
+impl<'a> BackwardCursor<'a> {
+    pub fn new(rope: &'a Rope) -> Self {
+        Self { rope, state: CursorState::new() }
+    }
+
+    pub fn read_behind(&self, behind: usize) -> Result<u8, RopeError> {
+        let pos = self.state.current.get().ok_or(RopeError::CurrentNotSet)?;
+        let target = pos
+            .checked_add(behind)
+            .filter(|&t| t < self.rope.len())
+            .ok_or(RopeError::OutOfBounds(format!(
+                "index out of bounds: i={} + {} > {}",
+                pos, behind, self.rope.len()
+            )))?;
+        self.state
+            .get(self.rope, target)
+            .ok_or(RopeError::OutOfBounds(format!(
+                "index out of bounds: i={} + {} > {}",
+                pos, behind, self.rope.len()
+            )))
+    }
+
+    pub fn iter(&self) -> BackwardIter<'a, '_> {
+        BackwardIter { cursor: self }
+    }
+}
+
+impl<'a> RopeCursor for BackwardCursor<'a> {
+    fn get(&self, i: usize) -> Option<u8> {
+        self.state.get(self.rope, i)
+    }
+
+    fn set(&self, i: usize, value: u8) -> Result<(), RopeError> {
+        self.state.set(self.rope, i, value)
+    }
+
+    fn peek(&self) -> Option<u8> {
+        self.state.get(self.rope, self.state.current.get()?)
+    }
+
+    fn poke(&self, value: u8) -> Result<(), RopeError> {
+        let pos = self.state.current.get().ok_or(RopeError::CurrentNotSet)?;
+        self.state.set(self.rope, pos, value)
+    }
+
+    fn tell(&self) -> Option<usize> {
+        self.state.current.get()
+    }
+
+    fn len(&self) -> usize {
+        self.rope.len()
+    }
+
+    fn read_next(&self) -> Result<u8, RopeError> {
+        let next_pos = match self.state.current.get() {
+            None => self.rope.len().checked_sub(1).ok_or(RopeError::OutOfBounds(
+                "BackwardCursor: rope is empty".to_string(),
+            ))?,
+            Some(0) => return Err(RopeError::OutOfBounds(
+                "BackwardCursor: already at beginning".to_string(),
+            )),
+            Some(i) => i - 1,
+        };
+        let value = self.state
+            .get(self.rope, next_pos)
+            .ok_or(RopeError::OutOfBounds(format!(
+                "BackwardCursor: index out of bounds at i={}",
+                next_pos
+            )))?;
+        self.state.current.set(Some(next_pos));
+        Ok(value)
+    }
+
+    fn seek(&self, pos: isize, mode: SeekMode) -> Result<usize, RopeError> {
+        let pos = match mode {
+            SeekMode::Absolute    => pos,
+            SeekMode::Relative    => self.state.current.get().ok_or(RopeError::CurrentNotSet)? as isize - pos,
+            SeekMode::RelativeToEnd => self.rope.len() as isize - pos,
+        };
+        if pos < 0 {
+            return Err(RopeError::OutOfBounds(format!("index out of bounds: i={} < 0", pos)));
+        }
+        self.state.current.set(Some(pos as usize));
+        Ok(pos as usize)
+    }
+
+    fn rewind(&self, go_back_of: usize) -> Result<(), RopeError> {
+        self.seek(-(go_back_of as isize), SeekMode::Relative)?;
+        Ok(())
+    }
+}
+
+impl Iterator for BackwardCursor<'_> {
+    type Item = u8;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.read_next().ok()
+    }
+}
+
+pub struct BackwardIter<'a, 'b> {
+    cursor: &'b BackwardCursor<'a>,
+}
+
+impl Iterator for BackwardIter<'_, '_> {
+    type Item = u8;
+    fn next(&mut self) -> Option<u8> {
+        self.cursor.read_next().ok()
+    }
+}

src/obikrope/src/error.rs

@@ -0,0 +1,6 @@
+#[derive(Debug)]
+pub enum RopeError {
+    OutOfBounds(String),
+    BlockNotFound(String),
+    CurrentNotSet,
+}

src/obikrope/src/lib.rs

@@ -0,0 +1,8 @@
+mod cursor;
+mod error;
+mod rope;
+
+pub use {
+    cursor::BackwardCursor, cursor::ForwardCursor, cursor::RopeCursor, cursor::SeekMode,
+    error::RopeError, rope::Rope,
+};

src/obikrope/src/rope.rs

@@ -0,0 +1,114 @@
+use crate::{BackwardCursor, ForwardCursor, RopeError};
+use std::cell::Cell;
+
+pub struct Rope {
+    pub(crate) blocks: Vec<Vec<Cell<u8>>>,
+    pub(crate) length: usize,
+    pub(crate) start_block_idx: Vec<usize>,
+}
+
+impl Rope {
+    pub fn new() -> Self {
+        Self {
+            blocks: Vec::new(),
+            length: 0,
+            start_block_idx: Vec::new(),
+        }
+    }
+
+    pub fn push(&mut self, block: Vec<u8>) {
+        let block_len = block.len();
+        self.start_block_idx.push(self.length);
+        // Safety: Cell<u8> has the same memory layout as u8 (guaranteed by the language)
+        let cell_block: Vec<Cell<u8>> = unsafe {
+            let mut v = std::mem::ManuallyDrop::new(block);
+            Vec::from_raw_parts(v.as_mut_ptr() as *mut Cell<u8>, v.len(), v.capacity())
+        };
+        self.blocks.push(cell_block);
+        self.length += block_len;
+    }
+
+    pub fn n_blocks(&self) -> usize {
+        self.blocks.len()
+    }
+
+    pub(crate) fn get_block(&self, block_idx: usize) -> Option<&[Cell<u8>]> {
+        self.blocks.get(block_idx).map(Vec::as_slice)
+    }
+
+    pub fn len(&self) -> usize {
+        self.length
+    }
+
+    pub(crate) fn lookup(&self, i: usize) -> Option<(usize, usize, usize)> {
+        if i >= self.length || self.blocks.is_empty() {
+            return None;
+        }
+        let block_idx = self.start_block_idx.partition_point(|&s| s <= i) - 1;
+        let from = self.start_block_idx[block_idx];
+        let to = if block_idx + 1 < self.blocks.len() {
+            self.start_block_idx[block_idx + 1]
+        } else {
+            self.length
+        };
+        Some((block_idx, from, to))
+    }
+
+    pub fn split_off(&mut self, pos: usize) -> Result<Rope, RopeError> {
+        if pos > self.length {
+            return Err(RopeError::OutOfBounds(format!(
+                "split_at: pos={} > length={}",
+                pos, self.length
+            )));
+        }
+
+        // pos == length: tail is empty.
+        if pos == self.length {
+            return Ok(Rope::new());
+        }
+
+        let (block_idx, from, _) = self.lookup(pos).ok_or_else(|| {
+            RopeError::OutOfBounds(format!("split_at: lookup failed at pos={}", pos))
+        })?;
+        let cut_offset = pos - from;
+
+        // Keep block_idx in self temporarily, split it, move remainder to tail.
+        let mut tail_blocks = self.blocks.split_off(block_idx + 1);
+        self.start_block_idx.truncate(block_idx + 1);
+
+        let tail_part = self.blocks[block_idx].split_off(cut_offset);
+        if !tail_part.is_empty() {
+            tail_blocks.insert(0, tail_part);
+        }
+
+        let mut tail_length = 0;
+        let tail_starts: Vec<usize> = tail_blocks
+            .iter()
+            .map(|b| {
+                let s = tail_length;
+                tail_length += b.len();
+                s
+            })
+            .collect();
+
+        self.length = pos;
+
+        Ok(Rope {
+            blocks: tail_blocks,
+            length: tail_length,
+            start_block_idx: tail_starts,
+        })
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.blocks.is_empty()
+    }
+
+    pub fn fw_cursor(&self) -> ForwardCursor<'_> {
+        ForwardCursor::new(self)
+    }
+
+    pub fn bw_cursor(&self) -> BackwardCursor<'_> {
+        BackwardCursor::new(self)
+    }
+}

src/obiread/Cargo.toml

@@ -4,6 +4,6 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
-bytes  = "1"
+obikrope = { path = "../obikrope" }
 niffler = { version = "2", default-features = false, features = ["gz", "bz2", "lzma", "zstd"] }
 ureq   = "2"

src/obiread/src/chunk.rs

@@ -1,54 +1,40 @@
-//! Chunk iterator: yields rope slices that each end on a complete sequence record.
+//! Chunk iterator: yields Rope slices that each end on a complete sequence record.
 //!
-//! Each `Vec<Bytes>` yielded by [`SeqChunkIter`] contains one or more reference-counted
-//! byte slices that together form a self-contained block of complete sequence records.
-//! The slices are NOT contiguous in memory — the consumer iterates over them in order.
-//!
-//! The splitter operates directly on the rope via [`RopeCursor`], so no packing
-//! (flattening into a contiguous buffer) is ever required — even for sequences
-//! longer than the read block size.
+//! Each `Rope` yielded by [`SeqChunkIter`] contains one or more blocks that
+//! together form a self-contained block of complete sequence records.
 
 use std::io::{self, Read};
 
-use bytes::{Bytes, BytesMut};
+use obikrope::Rope;
 
 /// A splitter function: given the accumulated rope, returns the absolute byte
 /// offset at which to cut, or `None` if no complete-record boundary was found.
-pub type Splitter = fn(&[Bytes]) -> Option<usize>;
+pub type Splitter = fn(&Rope) -> Option<usize>;
 
-/// Iterator that reads from `R` in blocks and yields `Vec<Bytes>` chunks,
+/// Iterator that reads from `R` in blocks and yields `Rope` chunks,
 /// each ending on a complete sequence record boundary.
 pub struct SeqChunkIter<R> {
     source:     R,
-    rope:       Vec<Bytes>,
+    rope:       Rope,
     block_size: usize,
     splitter:   Splitter,
-    probe:      &'static [u8],
     eof:        bool,
 }
 
 impl<R: Read> SeqChunkIter<R> {
     /// Create a new iterator.
-    ///
-    /// - `block_size`: bytes per read call (default 1 MiB).
-    /// - `splitter`: format-specific backward boundary detector working on the rope.
-    /// - `probe`: short byte string whose presence is necessary (not sufficient)
-    ///   for a boundary to exist (e.g. `b"\n>"` for FASTA, `b"\n@"` for FASTQ).
-    pub fn new(source: R, block_size: usize, splitter: Splitter, probe: &'static [u8]) -> Self {
+    pub fn new(source: R, block_size: usize, splitter: Splitter) -> Self {
         Self {
             source,
-            rope: Vec::with_capacity(4),
+            rope: Rope::new(),
             block_size,
             splitter,
-            probe,
             eof: false,
         }
     }
 
-    /// Read one block from source into a fresh `Bytes`.
-    /// Returns `None` on EOF (zero bytes read).
-    fn read_block(&mut self) -> io::Result<Option<Bytes>> {
-        let mut buf = BytesMut::zeroed(self.block_size);
+    fn read_block(&mut self) -> io::Result<Option<Vec<u8>>> {
+        let mut buf = vec![0u8; self.block_size];
         let mut filled = 0;
         loop {
             match self.source.read(&mut buf[filled..]) {
@@ -67,46 +53,12 @@ impl<R: Read> SeqChunkIter<R> {
             return Ok(None);
         }
         buf.truncate(filled);
-        Ok(Some(buf.freeze()))
-    }
-
-    /// Check whether the boundary probe might appear in the newly added block
-    /// or at the seam between the last two blocks.
-    ///
-    /// This is a fast O(block_size) heuristic: if the probe is absent, the
-    /// splitter is not called.
-    fn probe_in_last_chunk(&self) -> bool {
-        let last = match self.rope.last() {
-            Some(b) => b,
-            None => return false,
-        };
-
-        // Within the last block.
-        if last.windows(self.probe.len()).any(|w| w == self.probe) {
-            return true;
-        }
-
-        // At the seam between the previous block and this one.
-        if self.rope.len() >= 2 {
-            let prev = &self.rope[self.rope.len() - 2];
-            let overlap = self.probe.len() - 1;
-            let from = prev.len().saturating_sub(overlap);
-            let seam: Vec<u8> = prev[from..]
-                .iter()
-                .chain(last.iter().take(overlap))
-                .copied()
-                .collect();
-            if seam.windows(self.probe.len()).any(|w| w == self.probe) {
-                return true;
-            }
-        }
-
-        false
+        Ok(Some(buf))
     }
 }
 
 impl<R: Read> Iterator for SeqChunkIter<R> {
-    type Item = io::Result<Vec<Bytes>>;
+    type Item = io::Result<Rope>;
 
     fn next(&mut self) -> Option<Self::Item> {
         loop {
@@ -114,7 +66,7 @@ impl<R: Read> Iterator for SeqChunkIter<R> {
                 if self.rope.is_empty() {
                     return None;
                 }
-                return Some(Ok(std::mem::take(&mut self.rope)));
+                return Some(Ok(std::mem::replace(&mut self.rope, Rope::new())));
             }
 
             match self.read_block() {
@@ -128,54 +80,16 @@ impl<R: Read> Iterator for SeqChunkIter<R> {
                 }
             }
 
-            if self.probe_in_last_chunk() {
-                if let Some(abs_offset) = (self.splitter)(&self.rope) {
-                    return Some(Ok(self.split_at(abs_offset)));
-                }
+            if let Some(abs_offset) = (self.splitter)(&self.rope) {
+                let tail = self.rope.split_off(abs_offset)
+                    .expect("splitter returned valid offset");
+                let chunk = std::mem::replace(&mut self.rope, tail);
+                return Some(Ok(chunk));
             }
         }
     }
 }
 
-impl<R> SeqChunkIter<R> {
-    /// Split the rope at absolute byte offset `abs_offset`.
-    ///
-    /// Returns the chunk (`rope[..abs_offset]`) as a `Vec<Bytes>` and stores
-    /// the remainder (`rope[abs_offset..]`) in `self.rope`.
-    fn split_at(&mut self, abs_offset: usize) -> Vec<Bytes> {
-        let mut chunk = Vec::with_capacity(self.rope.len());
-        let mut remaining = abs_offset;
-
-        let mut old_rope = std::mem::take(&mut self.rope);
-        let mut remainder_rope: Vec<Bytes> = Vec::with_capacity(2);
-
-        for piece in old_rope.drain(..) {
-            if remaining == 0 {
-                remainder_rope.push(piece);
-            } else if remaining >= piece.len() {
-                remaining -= piece.len();
-                chunk.push(piece);
-            } else {
-                // The cut falls inside this piece.
-                // Copy both halves so each gets its own Arc (unique ownership),
-                // which is required by RopeTape for in-place writing.
-                let head = BytesMut::from(&piece[..remaining]).freeze();
-                let tail = BytesMut::from(&piece[remaining..]).freeze();
-                remaining = 0;
-                if !head.is_empty() {
-                    chunk.push(head);
-                }
-                if !tail.is_empty() {
-                    remainder_rope.push(tail);
-                }
-            }
-        }
-
-        self.rope = remainder_rope;
-        chunk
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -183,17 +97,15 @@ mod tests {
     use crate::fastq::end_of_last_fastq_entry;
 
     fn fasta_iter(data: &'static [u8], block_size: usize) -> SeqChunkIter<&'static [u8]> {
-        SeqChunkIter::new(data, block_size, end_of_last_fasta_entry, b"\n>")
+        SeqChunkIter::new(data, block_size, end_of_last_fasta_entry)
     }
 
     fn fastq_iter(data: &'static [u8], block_size: usize) -> SeqChunkIter<&'static [u8]> {
-        SeqChunkIter::new(data, block_size, end_of_last_fastq_entry, b"\n@")
+        SeqChunkIter::new(data, block_size, end_of_last_fastq_entry)
     }
 
-    fn collect_fasta(chunks: Vec<Vec<Bytes>>) -> Vec<Vec<u8>> {
-        chunks.into_iter().map(|rope| {
-            rope.into_iter().flat_map(|b| b.to_vec()).collect()
-        }).collect()
+    fn rope_to_vec(rope: &Rope) -> Vec<u8> {
+        rope.fw_cursor().collect()
     }
 
     // ── FASTA ─────────────────────────────────────────────────────────────────
@@ -203,16 +115,14 @@ mod tests {
         let data: &[u8] = b">s1\nACGT\n";
         let chunks: Vec<_> = fasta_iter(data, 64).collect::<Result<_, _>>().unwrap();
         assert_eq!(chunks.len(), 1);
-        let flat: Vec<u8> = chunks.into_iter().flatten().flat_map(|b| b.to_vec()).collect();
-        assert_eq!(flat, b">s1\nACGT\n");
+        assert_eq!(rope_to_vec(&chunks[0]), b">s1\nACGT\n");
     }
 
     #[test]
     fn fasta_two_records_split_across_chunks() {
         let data: &[u8] = b">s1\nACGT\n>s2\nTTTT\n";
         let chunks: Vec<_> = fasta_iter(data, 10).collect::<Result<_, _>>().unwrap();
-        let flat = collect_fasta(chunks);
-        let all: Vec<u8> = flat.into_iter().flatten().collect();
+        let all: Vec<u8> = chunks.iter().flat_map(|r| rope_to_vec(r)).collect();
         assert_eq!(all, b">s1\nACGT\n>s2\nTTTT\n");
     }
 
@@ -222,7 +132,7 @@ mod tests {
         for block in [8, 12, 20, 100] {
             let chunks: Vec<_> = fasta_iter(data, block).collect::<Result<_, _>>().unwrap();
             for rope in &chunks {
-                let flat: Vec<u8> = rope.iter().flat_map(|b| b.to_vec()).collect();
+                let flat = rope_to_vec(rope);
                 assert_eq!(flat[0], b'>', "block={block}: chunk doesn't start with '>'");
                 assert_eq!(*flat.last().unwrap(), b'\n', "block={block}: chunk doesn't end with newline");
             }
@@ -258,7 +168,7 @@ mod tests {
             (b"TTTTTTTT", b"HHHHHHHH"),
         ]).into_boxed_slice());
         let chunks: Vec<_> = fastq_iter(data, 16).collect::<Result<_, _>>().unwrap();
-        let all: Vec<u8> = chunks.into_iter().flatten().flat_map(|b| b.to_vec()).collect();
+        let all: Vec<u8> = chunks.iter().flat_map(|r| rope_to_vec(r)).collect();
         assert_eq!(all, *data);
     }
 
@@ -273,7 +183,7 @@ mod tests {
         for block in [18, 30, 60] {
             let chunks: Vec<_> = fastq_iter(data, block).collect::<Result<_, _>>().unwrap();
             for rope in &chunks {
-                let first_byte = rope.iter().flat_map(|b| b.iter().copied()).next().unwrap();
+                let first_byte = rope_to_vec(rope)[0];
                 assert_eq!(first_byte, b'@', "block={block}: chunk doesn't start with '@'");
             }
         }

src/obiread/src/fasta.rs

@@ -1,8 +1,6 @@
 //! Backward boundary detection for FASTA chunks.
 
-use bytes::Bytes;
-
-use crate::tape::RopeCursor;
+use obikrope::{Rope, RopeCursor};
 
 /// Scan the rope backward for the start of the last complete FASTA entry.
 ///
@@ -10,57 +8,51 @@ use crate::tape::RopeCursor;
 /// record, so that `rope[..offset]` is a self-contained chunk and
 /// `rope[offset..]` is the remainder carried into the next chunk.
 /// Returns `None` if no valid boundary is found (need more data).
-///
-/// Port of Go's `EndOfLastFastaEntry`, now working directly on a rope
-/// via [`RopeCursor`] — no contiguous packing required.
-pub fn end_of_last_fasta_entry(rope: &[Bytes]) -> Option<usize> {
-    let total_len: usize = rope.iter().map(|b| b.len()).sum();
-    if total_len == 0 {
-        return None;
-    }
-
-    let mut cursor = RopeCursor::end(rope)?;
+pub fn end_of_last_fasta_entry(rope: &Rope) -> Option<usize> {
+    let cursor = rope.bw_cursor();
     let mut state: u8 = 0;
     let mut last: usize = 0;
-    let mut i = total_len as isize - 1;
 
-    while i >= 0 && state < 2 {
-        let c = cursor.peek(rope).unwrap();
+    for c in cursor.iter() {
         match state {
             0 if c == b'>' => {
+                last = cursor.tell()?;
                 state = 1;
-                last = i as usize;
             }
             1 if c == b'\n' || c == b'\r' => {
-                state = 2;
+                if last > 0 {
+                    return Some(last);
+                }
+                return None;
             }
-            _ => {
+            1 => {
                 state = 0;
             }
-        }
-        i -= 1;
-        if i >= 0 {
-            cursor.previous(rope);
+            _ => {}
         }
     }
-
-    if i <= 0 || state != 2 {
-        return None;
-    }
-    Some(last)
+    None
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
-    use bytes::BytesMut;
 
-    fn rope(data: &[u8]) -> Vec<Bytes> {
-        vec![BytesMut::from(data).freeze()]
+    fn rope(data: &[u8]) -> Rope {
+        let mut r = Rope::new();
+        r.push(data.to_vec());
+        r
     }
 
-    fn rope2(a: &[u8], b: &[u8]) -> Vec<Bytes> {
-        vec![BytesMut::from(a).freeze(), BytesMut::from(b).freeze()]
+    fn rope2(a: &[u8], b: &[u8]) -> Rope {
+        let mut r = Rope::new();
+        r.push(a.to_vec());
+        r.push(b.to_vec());
+        r
+    }
+
+    fn flat(r: &Rope) -> Vec<u8> {
+        r.fw_cursor().collect()
     }
 
     #[test]
@@ -71,39 +63,43 @@ mod tests {
     #[test]
     fn two_entries_cuts_at_second_header() {
         let data = b">seq1\nACGT\n>seq2\nTTTT\n";
-        let pos = end_of_last_fasta_entry(&rope(data)).unwrap();
-        assert_eq!(&data[pos..], b">seq2\nTTTT\n");
-        assert_eq!(&data[..pos], b">seq1\nACGT\n");
+        let r = rope(data);
+        let pos = end_of_last_fasta_entry(&r).unwrap();
+        assert_eq!(&flat(&r)[pos..], b">seq2\nTTTT\n");
+        assert_eq!(&flat(&r)[..pos], b">seq1\nACGT\n");
     }
 
     #[test]
     fn three_entries_cuts_at_last_header() {
         let data = b">s1\nAA\n>s2\nCC\n>s3\nGG\n";
-        let pos = end_of_last_fasta_entry(&rope(data)).unwrap();
-        assert_eq!(&data[pos..], b">s3\nGG\n");
+        let r = rope(data);
+        let pos = end_of_last_fasta_entry(&r).unwrap();
+        assert_eq!(&flat(&r)[pos..], b">s3\nGG\n");
     }
 
     #[test]
     fn multiline_sequence() {
         let data = b">s1\nACGT\nACGT\n>s2\nTTTT\n";
-        let pos = end_of_last_fasta_entry(&rope(data)).unwrap();
-        assert_eq!(&data[pos..], b">s2\nTTTT\n");
+        let r = rope(data);
+        let pos = end_of_last_fasta_entry(&r).unwrap();
+        assert_eq!(&flat(&r)[pos..], b">s2\nTTTT\n");
     }
 
     #[test]
     fn crlf_line_endings() {
         let data = b">s1\r\nACGT\r\n>s2\r\nTTTT\r\n";
-        let pos = end_of_last_fasta_entry(&rope(data)).unwrap();
-        assert_eq!(&data[pos..], b">s2\r\nTTTT\r\n");
+        let r = rope(data);
+        let pos = end_of_last_fasta_entry(&r).unwrap();
+        assert_eq!(&flat(&r)[pos..], b">s2\r\nTTTT\r\n");
     }
 
     #[test]
     fn boundary_spans_two_blocks() {
-        // Split so that "\n" is at end of first block and ">" at start of second.
         let a = b">s1\nACGT\n";
         let b = b">s2\nTTTT\n";
-        let total: Vec<u8> = a.iter().chain(b.iter()).copied().collect();
-        let pos = end_of_last_fasta_entry(&rope2(a, b)).unwrap();
-        assert_eq!(&total[pos..], b">s2\nTTTT\n");
+        let r = rope2(a, b);
+        let all: Vec<u8> = flat(&r);
+        let pos = end_of_last_fasta_entry(&r).unwrap();
+        assert_eq!(&all[pos..], b">s2\nTTTT\n");
     }
 }

src/obiread/src/fastq.rs

@@ -9,13 +9,8 @@
 //! The `@` in quality lines (Phred 31 = ASCII 64) makes forward heuristics
 //! unreliable. The backward scanner identifies a genuine record start by
 //! verifying the structural context around each `@` candidate.
-//!
-//! Port of Go's `EndOfLastFastqEntry` (7-state machine), now working directly
-//! on a rope via [`RopeCursor`] — no contiguous packing required.
 
-use bytes::Bytes;
-
-use crate::tape::RopeCursor;
+use obikrope::{Rope, RopeCursor, SeekMode};
 
 #[inline]
 fn is_eol(c: u8) -> bool {
@@ -38,42 +33,28 @@ fn is_seq_char(c: u8) -> bool {
 /// record, so that `rope[..offset]` is a self-contained chunk and
 /// `rope[offset..]` is the remainder for the next chunk.
 /// Returns `None` if no valid boundary is found (need more data).
-pub fn end_of_last_fastq_entry(rope: &[Bytes]) -> Option<usize> {
-    let total_len: usize = rope.iter().map(|b| b.len()).sum();
-    if total_len == 0 {
-        return None;
-    }
-
-    let mut cursor = RopeCursor::end(rope)?;
+pub fn end_of_last_fastq_entry(rope: &Rope) -> Option<usize> {
+    let mut cursor = rope.bw_cursor();
     let mut state: u8 = 0;
-    let mut restart: isize = total_len as isize - 1;
-    let mut restart_cursor = cursor;
-    let mut cut: usize = total_len;
-    let mut i: isize = total_len as isize - 1;
-
-    while i >= 0 && state < 7 {
-        let c = cursor.peek(rope).unwrap();
+    let mut restart: usize = 0;
+    let mut cut: usize = rope.len();
 
+    while let Some(c) = cursor.next() {
         match state {
-            // Looking for `+` separator line content
             0 => {
                 if c == b'+' {
                     state = 1;
-                    restart = i;
-                    restart_cursor = cursor;
+                    restart = cursor.tell()?;
                 }
             }
-            // Found `+` — expect end-of-line immediately before it (going backward)
             1 => {
                 if is_eol(c) {
                     state = 2;
                 } else {
                     state = 0;
-                    i = restart;
-                    cursor = restart_cursor;
+                    cursor.seek(restart as isize, SeekMode::Absolute).ok();
                 }
             }
-            // After `\n+`: skip separators, then expect sequence characters
             2 => {
                 if is_sep(c) {
                     // stay
@@ -81,11 +62,9 @@ pub fn end_of_last_fastq_entry(rope: &[Bytes]) -> Option<usize> {
                     state = 3;
                 } else {
                     state = 0;
-                    i = restart;
-                    cursor = restart_cursor;
+                    cursor.seek(restart as isize, SeekMode::Absolute).ok();
                 }
             }
-            // Scanning sequence characters backward
             3 => {
                 if is_eol(c) {
                     state = 4;
@@ -93,60 +72,48 @@ pub fn end_of_last_fastq_entry(rope: &[Bytes]) -> Option<usize> {
                     // stay
                 } else {
                     state = 0;
-                    i = restart;
-                    cursor = restart_cursor;
+                    cursor.seek(restart as isize, SeekMode::Absolute).ok();
                 }
             }
-            // Found end-of-line before sequence — skip any extra newlines
             4 => {
-                if is_eol(c) {
-                    // stay
-                } else {
+                if !is_eol(c) {
                     state = 5;
                 }
             }
-            // Scanning header content — looking for `@` at start of line
             5 => {
                 if is_eol(c) {
                     state = 0;
-                    i = restart;
-                    cursor = restart_cursor;
+                    cursor.seek(restart as isize, SeekMode::Absolute).ok();
                 } else if c == b'@' {
+                    cut = cursor.tell()?;
                     state = 6;
-                    cut = i as usize;
                 }
-                // else: stay
+                // else stay
             }
-            // Found `@` — expect end-of-line before it
             6 => {
                 if is_eol(c) {
-                    state = 7; // success
+                    if cut > 0 {
+                        return Some(cut);
+                    }
+                    return None;
                 } else {
                     state = 5;
                 }
             }
             _ => unreachable!(),
         }
-
-        i -= 1;
-        if i >= 0 {
-            cursor.previous(rope);
-        }
     }
-
-    if i <= 0 || state != 7 {
-        return None;
-    }
-    Some(cut)
+    None
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
-    use bytes::BytesMut;
 
-    fn rope(data: &[u8]) -> Vec<Bytes> {
-        vec![BytesMut::from(data).freeze()]
+    fn rope(data: &[u8]) -> Rope {
+        let mut r = Rope::new();
+        r.push(data.to_vec());
+        r
     }
 
     fn make_fastq(records: &[(&[u8], &[u8])]) -> Vec<u8> {
@@ -162,6 +129,10 @@ mod tests {
         buf
     }
 
+    fn flat(r: &Rope) -> Vec<u8> {
+        r.fw_cursor().collect()
+    }
+
     #[test]
     fn single_record_no_boundary() {
         let buf = make_fastq(&[(b"ACGT", b"IIII")]);
@@ -171,9 +142,10 @@ mod tests {
     #[test]
     fn two_records_cuts_at_second() {
         let buf = make_fastq(&[(b"ACGT", b"IIII"), (b"TTTT", b"HHHH")]);
-        let pos = end_of_last_fastq_entry(&rope(&buf)).unwrap();
-        assert_eq!(buf[pos], b'@');
-        assert_eq!(&buf[pos..], make_fastq(&[(b"TTTT", b"HHHH")]).as_slice());
+        let r = rope(&buf);
+        let pos = end_of_last_fastq_entry(&r).unwrap();
+        assert_eq!(flat(&r)[pos], b'@');
+        assert_eq!(&flat(&r)[pos..], make_fastq(&[(b"TTTT", b"HHHH")]).as_slice());
     }
 
     #[test]
@@ -183,8 +155,9 @@ mod tests {
             (b"CCCC", b"JJJJ"),
             (b"GGGG", b"KKKK"),
         ]);
-        let pos = end_of_last_fastq_entry(&rope(&buf)).unwrap();
-        assert_eq!(&buf[pos..], make_fastq(&[(b"GGGG", b"KKKK")]).as_slice());
+        let r = rope(&buf);
+        let pos = end_of_last_fastq_entry(&r).unwrap();
+        assert_eq!(&flat(&r)[pos..], make_fastq(&[(b"GGGG", b"KKKK")]).as_slice());
     }
 
     #[test]
@@ -193,15 +166,17 @@ mod tests {
             (b"ACGTACGT", b"@@@@IIII"),
             (b"TTTT",     b"HHHH"),
         ]);
-        let pos = end_of_last_fastq_entry(&rope(&buf)).unwrap();
-        assert_eq!(&buf[pos..], make_fastq(&[(b"TTTT", b"HHHH")]).as_slice());
+        let r = rope(&buf);
+        let pos = end_of_last_fastq_entry(&r).unwrap();
+        assert_eq!(&flat(&r)[pos..], make_fastq(&[(b"TTTT", b"HHHH")]).as_slice());
     }
 
     #[test]
     fn crlf_line_endings() {
-        let buf = b"@h\r\nACGT\r\n+\r\nIIII\r\n@h\r\nTTTT\r\n+\r\nHHHH\r\n";
-        let pos = end_of_last_fastq_entry(&rope(buf)).unwrap();
-        assert_eq!(buf[pos], b'@');
-        assert_eq!(&buf[pos..], b"@h\r\nTTTT\r\n+\r\nHHHH\r\n");
+        let data = b"@h\r\nACGT\r\n+\r\nIIII\r\n@h\r\nTTTT\r\n+\r\nHHHH\r\n";
+        let r = rope(data);
+        let pos = end_of_last_fastq_entry(&r).unwrap();
+        assert_eq!(flat(&r)[pos], b'@');
+        assert_eq!(&flat(&r)[pos..], b"@h\r\nTTTT\r\n+\r\nHHHH\r\n");
     }
 }

src/obiread/src/lib.rs

@@ -1,7 +1,7 @@
 //! Streaming sequence file reader for obikmer.
 //!
-//! Yields rope chunks (`Vec<Bytes>`) from FASTA or FASTQ files, each ending
-//! on a complete sequence record boundary. Zero allocation in the common case.
+//! Yields rope chunks from FASTA or FASTQ files, each ending
+//! on a complete sequence record boundary.
 
 #![deny(missing_docs)]
 
@@ -9,7 +9,6 @@ mod fasta;
 mod fastq;
 pub mod chunk;
 pub mod normalize;
-pub mod tape;
 pub mod xopen;
 
 use std::io::Read;
@@ -22,20 +21,10 @@ pub const DEFAULT_BLOCK_SIZE: usize = 1024 * 1024;
 
 /// Create a FASTA chunk iterator over `source`.
 pub fn fasta_chunks<R: Read>(source: R) -> SeqChunkIter<R> {
-    SeqChunkIter::new(
-        source,
-        DEFAULT_BLOCK_SIZE,
-        fasta::end_of_last_fasta_entry,
-        b"\n>",
-    )
+    SeqChunkIter::new(source, DEFAULT_BLOCK_SIZE, fasta::end_of_last_fasta_entry)
 }
 
 /// Create a FASTQ chunk iterator over `source`.
 pub fn fastq_chunks<R: Read>(source: R) -> SeqChunkIter<R> {
-    SeqChunkIter::new(
-        source,
-        DEFAULT_BLOCK_SIZE,
-        fastq::end_of_last_fastq_entry,
-        b"\n@",
-    )
+    SeqChunkIter::new(source, DEFAULT_BLOCK_SIZE, fastq::end_of_last_fastq_entry)
 }

src/obiread/src/normalize.rs

@@ -1,141 +1,114 @@
 //! Sequence normalisation automata for FASTA and FASTQ rope chunks.
 //!
-//! Both automata operate on a [`RopeTape`] in place (two-cursor, zero
-//! reallocation in the common case) and produce a compact byte stream:
-//! ACGT segments of length ≥ k separated by `0x00`, uppercased.
-//!
-//! Ambiguous bases terminate the current segment. Segments shorter than k
-//! are silently discarded by retreating the write cursor.
+//! Both automata operate on a Rope in place (two-cursor, zero reallocation)
+//! and produce a compact byte stream: ACGT segments of length ≥ k separated
+//! by `0x00`, uppercased. Ambiguous bases terminate the current segment.
+//! Segments shorter than k are silently discarded.
 
-use bytes::Bytes;
-
-use crate::tape::{RopeCursor, RopeTape};
+use obikrope::{ForwardCursor, Rope, RopeCursor};
 
 // ── public entry points ───────────────────────────────────────────────────────
 
-/// Normalise a FASTA chunk: skip headers; copy, filter and concatenate
-/// sequence lines across multi-line records.
-///
-/// Returns the written region of the tape as a rope of `Bytes`.
-pub fn normalize_fasta_chunk(chunks: Vec<Bytes>, k: usize) -> Vec<Bytes> {
-    let mut tape = RopeTape::new(chunks);
-    normalize_fasta(&mut tape, k);
-    tape.into_output()
+/// Normalise a FASTA chunk into a compact ACGT\x00-separated rope.
+pub fn normalize_fasta_chunk(mut rope: Rope, k: usize) -> Rope {
+    let write_pos = {
+        let read = rope.fw_cursor();
+        let write = rope.fw_cursor();
+        normalize_fasta(&read, &write, k);
+        write.tell().unwrap_or(0)
+    };
+    let _ = rope.split_off(write_pos);
+    rope
 }
 
-/// Normalise a FASTQ chunk: skip headers, `+` lines and quality lines;
-/// copy and filter sequence lines.
-///
-/// Returns the written region of the tape as a rope of `Bytes`.
-pub fn normalize_fastq_chunk(chunks: Vec<Bytes>, k: usize) -> Vec<Bytes> {
-    let mut tape = RopeTape::new(chunks);
-    normalize_fastq(&mut tape, k);
-    tape.into_output()
+/// Normalise a FASTQ chunk into a compact ACGT\x00-separated rope.
+pub fn normalize_fastq_chunk(mut rope: Rope, k: usize) -> Rope {
+    let write_pos = {
+        let read = rope.fw_cursor();
+        let write = rope.fw_cursor();
+        normalize_fastq(&read, &write, k);
+        write.tell().unwrap_or(0)
+    };
+    let _ = rope.split_off(write_pos);
+    rope
 }
 
 // ── FASTA automaton ───────────────────────────────────────────────────────────
 
-/// Drive the FASTA normalisation automaton on `tape`.
-///
-/// After the initial header skip, the automaton reads sequence characters
-/// one by one. At each newline run it peeks at the next character to decide:
-/// `>` → new record (close segment, skip header); anything else → line
-/// continuation within the same sequence.
-fn normalize_fasta(tape: &mut RopeTape, k: usize) {
-    // Skip the first header line (includes the leading `>`).
-    skip_line(tape);
+fn normalize_fasta(read: &ForwardCursor<'_>, write: &ForwardCursor<'_>, k: usize) {
+    skip_line(read);
 
     loop {
-        let mut seg_start = tape.write_snapshot();
+        let mut seg_start = write.tell().unwrap_or(0);
 
         'seq: loop {
-            let Some(c) = tape.read_next() else {
-                end_segment(tape, &mut seg_start, k);
+            let Some(c) = read.read_next().ok() else {
+                end_segment(write, &mut seg_start, k);
                 return;
             };
 
             if is_newline(c) {
-                // Peek at the very next character — no newline run to skip.
-                // \r\n is handled naturally: \r → peek \n → continue; \n → peek content.
-                match tape.peek() {
+                match read.read_ahead(1).ok() {
                     None => {
-                        end_segment(tape, &mut seg_start, k);
+                        end_segment(write, &mut seg_start, k);
                         return;
                     }
                     Some(b'>') => {
-                        // New record: close current segment, skip next header.
-                        end_segment(tape, &mut seg_start, k);
-                        skip_line(tape);
-                        break 'seq; // restart outer loop
-                    }
-                    Some(_) => {
-                        // Line continuation or \r before \n: next char is a nucleotide.
-                        continue 'seq;
+                        end_segment(write, &mut seg_start, k);
+                        skip_line(read);
+                        break 'seq;
                     }
+                    Some(_) => continue 'seq,
                 }
             }
 
             let upper = c & !0x20u8;
             if is_acgt(upper) {
-                tape.write_next(upper);
+                write.write(upper).ok();
             } else {
-                // Ambiguous base: close segment, skip the non-ACGT run.
-                end_segment(tape, &mut seg_start, k);
-                skip_until_acgt_or_newline(tape);
-                seg_start = tape.write_snapshot();
+                end_segment(write, &mut seg_start, k);
+                skip_until_acgt_or_newline(read);
+                seg_start = write.tell().unwrap_or(0);
             }
         }
-        // Outer loop restarts for the next record.
     }
 }
 
 // ── FASTQ automaton ───────────────────────────────────────────────────────────
 
-/// Drive the FASTQ normalisation automaton on `tape`.
-///
-/// The FASTQ 4-line structure (`@header`, sequence, `+`, quality) is rigid,
-/// so the automaton cycles through four fixed phases without backtracking.
-fn normalize_fastq(tape: &mut RopeTape, k: usize) {
+fn normalize_fastq(read: &ForwardCursor<'_>, write: &ForwardCursor<'_>, k: usize) {
     loop {
-        // ── Phase 1: skip header line (@…\n) ─────────────────────────────
-        skip_line(tape);
+        skip_line(read); // skip header
 
-        // ── Phase 2: copy sequence ────────────────────────────────────────
-        let mut seg_start = tape.write_snapshot();
+        let mut seg_start = write.tell().unwrap_or(0);
 
         'seq: loop {
-            let Some(c) = tape.read_next() else {
-                // EOF inside a sequence: flush whatever we have.
-                end_segment(tape, &mut seg_start, k);
+            let Some(c) = read.read_next().ok() else {
+                end_segment(write, &mut seg_start, k);
                 return;
             };
 
             if is_newline(c) {
-                skip_newlines(tape);
+                skip_newlines(read);
                 break 'seq;
             }
 
-            let upper = c & !0x20u8; // ASCII uppercase trick
+            let upper = c & !0x20u8;
             if is_acgt(upper) {
-                tape.write_next(upper);
+                write.write(upper).ok();
             } else {
-                // Ambiguous base: close the current segment, skip non-ACGT run.
-                end_segment(tape, &mut seg_start, k);
-                skip_until_acgt_or_newline(tape);
-                seg_start = tape.write_snapshot();
+                end_segment(write, &mut seg_start, k);
+                skip_until_acgt_or_newline(read);
+                seg_start = write.tell().unwrap_or(0);
             }
         }
 
-        // Sequence line ended: close the last segment.
-        end_segment(tape, &mut seg_start, k);
+        end_segment(write, &mut seg_start, k);
 
-        // ── Phase 3: skip + line ──────────────────────────────────────────
-        skip_line(tape);
+        skip_line(read); // skip + line
+        skip_line(read); // skip quality line
 
-        // ── Phase 4: skip quality line ────────────────────────────────────
-        skip_line(tape);
-
-        if tape.peek().is_none() {
+        if read.read_ahead(1).ok().is_none() {
             return;
         }
     }
@@ -143,46 +116,38 @@ fn normalize_fastq(tape: &mut RopeTape, k: usize) {
 
 // ── shared helpers ────────────────────────────────────────────────────────────
 
-/// Skip to the end of the current line, consuming the newline run.
-fn skip_line(tape: &mut RopeTape) {
-    while let Some(c) = tape.read_next() {
+fn skip_line(read: &ForwardCursor<'_>) {
+    while let Some(c) = read.read_next().ok() {
         if is_newline(c) {
-            skip_newlines(tape);
+            skip_newlines(read);
             return;
         }
     }
 }
 
-/// Consume a contiguous run of `\n` / `\r` characters.
-fn skip_newlines(tape: &mut RopeTape) {
-    while matches!(tape.peek(), Some(c) if is_newline(c)) {
-        tape.read_next();
+fn skip_newlines(read: &ForwardCursor<'_>) {
+    while matches!(read.read_ahead(1).ok(), Some(c) if is_newline(c)) {
+        read.read_next().ok();
     }
 }
 
-/// Consume characters until the next ACGT base or newline (leaving it unread).
-fn skip_until_acgt_or_newline(tape: &mut RopeTape) {
-    while let Some(c) = tape.peek() {
+fn skip_until_acgt_or_newline(read: &ForwardCursor<'_>) {
+    while let Some(c) = read.read_ahead(1).ok() {
         if is_newline(c) || is_acgt(c & !0x20u8) {
             return;
         }
-        tape.read_next();
+        read.read_next().ok();
     }
 }
 
-/// Close the current segment.
-///
-/// - If `seg_len >= k`: write `0x00` terminator and advance `seg_start`.
-/// - If `0 < seg_len < k`: erase by retreating the write cursor.
-/// - If `seg_len == 0`: nothing to do.
-fn end_segment(tape: &mut RopeTape, seg_start: &mut RopeCursor, k: usize) {
-    let seg_len = tape.written_since(*seg_start);
+fn end_segment(write: &ForwardCursor<'_>, seg_start: &mut usize, k: usize) {
+    let seg_len = write.tell().unwrap_or(0) - *seg_start;
     if seg_len >= k {
-        tape.write_next(0x00);
+        write.write(0x00).ok();
     } else if seg_len > 0 {
-        tape.write_retreat(seg_len);
+        write.rewind(seg_len).ok();
     }
-    *seg_start = tape.write_snapshot();
+    *seg_start = write.tell().unwrap_or(0);
 }
 
 #[inline] fn is_newline(c: u8) -> bool { c == b'\n' || c == b'\r' }
@@ -193,14 +158,23 @@ fn end_segment(tape: &mut RopeTape, seg_start: &mut RopeCursor, k: usize) {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use bytes::BytesMut;
 
-    fn run(data: &[u8], k: usize) -> Vec<u8> {
-        let chunks = vec![BytesMut::from(data).freeze()];
-        normalize_fastq_chunk(chunks, k)
-            .into_iter()
-            .flat_map(|b| b.to_vec())
-            .collect()
+    fn make_rope(data: &[u8]) -> Rope {
+        let mut r = Rope::new();
+        r.push(data.to_vec());
+        r
+    }
+
+    fn flat(r: Rope) -> Vec<u8> {
+        r.fw_cursor().collect()
+    }
+
+    fn run_fastq(data: &[u8], k: usize) -> Vec<u8> {
+        flat(normalize_fastq_chunk(make_rope(data), k))
+    }
+
+    fn run_fasta(data: &[u8], k: usize) -> Vec<u8> {
+        flat(normalize_fasta_chunk(make_rope(data), k))
     }
 
     fn make_fastq(records: &[&[u8]]) -> Vec<u8> {
@@ -216,107 +190,6 @@ mod tests {
         buf
     }
 
-    // ── basic output format ───────────────────────────────────────────────────
-
-    #[test]
-    fn single_record_produces_seq_then_null() {
-        let out = run(&make_fastq(&[b"ACGTACGT"]), 4);
-        assert_eq!(out, b"ACGTACGT\x00");
-    }
-
-    #[test]
-    fn two_records_concatenated() {
-        let out = run(&make_fastq(&[b"ACGTACGT", b"TTTTTTTT"]), 4);
-        assert_eq!(out, b"ACGTACGT\x00TTTTTTTT\x00");
-    }
-
-    // ── uppercase normalisation ───────────────────────────────────────────────
-
-    #[test]
-    fn lowercase_input_uppercased() {
-        let out = run(&make_fastq(&[b"acgtacgt"]), 4);
-        assert_eq!(out, b"ACGTACGT\x00");
-    }
-
-    #[test]
-    fn mixed_case_uppercased() {
-        let out = run(&make_fastq(&[b"AcGtAcGt"]), 4);
-        assert_eq!(out, b"ACGTACGT\x00");
-    }
-
-    // ── k filter ─────────────────────────────────────────────────────────────
-
-    #[test]
-    fn sequence_shorter_than_k_discarded() {
-        let out = run(&make_fastq(&[b"ACG"]), 4);
-        assert_eq!(out, b"");
-    }
-
-    #[test]
-    fn sequence_exactly_k_kept() {
-        let out = run(&make_fastq(&[b"ACGT"]), 4);
-        assert_eq!(out, b"ACGT\x00");
-    }
-
-    #[test]
-    fn short_record_among_valid_ones_discarded() {
-        let out = run(&make_fastq(&[b"ACGTACGT", b"AC", b"TTTTTTTT"]), 4);
-        assert_eq!(out, b"ACGTACGT\x00TTTTTTTT\x00");
-    }
-
-    // ── ambiguous bases ───────────────────────────────────────────────────────
-
-    #[test]
-    fn ambiguous_splits_into_two_segments() {
-        // 'N' in the middle splits "ACGT" + "ACGT"
-        let out = run(&make_fastq(&[b"ACGTNACGT"]), 4);
-        assert_eq!(out, b"ACGT\x00ACGT\x00");
-    }
-
-    #[test]
-    fn segment_after_ambiguous_too_short_discarded() {
-        // "ACGTACGT" + 'N' + "AC" (< k=4)
-        let out = run(&make_fastq(&[b"ACGTACGTNAC"]), 4);
-        assert_eq!(out, b"ACGTACGT\x00");
-    }
-
-    #[test]
-    fn consecutive_ambiguous_produce_no_empty_segment() {
-        let out = run(&make_fastq(&[b"ACGTNNNNACGT"]), 4);
-        assert_eq!(out, b"ACGT\x00ACGT\x00");
-    }
-
-    #[test]
-    fn ambiguous_at_start_skipped() {
-        let out = run(&make_fastq(&[b"NNACGTACGT"]), 4);
-        assert_eq!(out, b"ACGTACGT\x00");
-    }
-
-    #[test]
-    fn ambiguous_at_end_produces_no_trailing_empty() {
-        let out = run(&make_fastq(&[b"ACGTACGTNN"]), 4);
-        assert_eq!(out, b"ACGTACGT\x00");
-    }
-
-    // ── CRLF line endings ─────────────────────────────────────────────────────
-
-    #[test]
-    fn crlf_handled() {
-        let data = b"@hdr\r\nACGTACGT\r\n+\r\nIIIIIIII\r\n";
-        let out = run(data, 4);
-        assert_eq!(out, b"ACGTACGT\x00");
-    }
-
-    // ── FASTA ─────────────────────────────────────────────────────────────────
-
-    fn run_fasta(data: &[u8], k: usize) -> Vec<u8> {
-        let chunks = vec![BytesMut::from(data).freeze()];
-        normalize_fasta_chunk(chunks, k)
-            .into_iter()
-            .flat_map(|b| b.to_vec())
-            .collect()
-    }
-
     fn make_fasta(records: &[(&[u8], &[u8])]) -> Vec<u8> {
         let mut buf = Vec::new();
         for (id, seq) in records {
@@ -329,112 +202,160 @@ mod tests {
         buf
     }
 
+    // ── FASTQ basic ──────────────────────────────────────────────────────────
+
+    #[test]
+    fn single_record_produces_seq_then_null() {
+        assert_eq!(run_fastq(&make_fastq(&[b"ACGTACGT"]), 4), b"ACGTACGT\x00");
+    }
+
+    #[test]
+    fn two_records_concatenated() {
+        assert_eq!(
+            run_fastq(&make_fastq(&[b"ACGTACGT", b"TTTTTTTT"]), 4),
+            b"ACGTACGT\x00TTTTTTTT\x00"
+        );
+    }
+
+    #[test]
+    fn lowercase_input_uppercased() {
+        assert_eq!(run_fastq(&make_fastq(&[b"acgtacgt"]), 4), b"ACGTACGT\x00");
+    }
+
+    #[test]
+    fn mixed_case_uppercased() {
+        assert_eq!(run_fastq(&make_fastq(&[b"AcGtAcGt"]), 4), b"ACGTACGT\x00");
+    }
+
+    #[test]
+    fn sequence_shorter_than_k_discarded() {
+        assert_eq!(run_fastq(&make_fastq(&[b"ACG"]), 4), b"");
+    }
+
+    #[test]
+    fn sequence_exactly_k_kept() {
+        assert_eq!(run_fastq(&make_fastq(&[b"ACGT"]), 4), b"ACGT\x00");
+    }
+
+    #[test]
+    fn short_record_among_valid_ones_discarded() {
+        assert_eq!(
+            run_fastq(&make_fastq(&[b"ACGTACGT", b"AC", b"TTTTTTTT"]), 4),
+            b"ACGTACGT\x00TTTTTTTT\x00"
+        );
+    }
+
+    #[test]
+    fn ambiguous_splits_into_two_segments() {
+        assert_eq!(run_fastq(&make_fastq(&[b"ACGTNACGT"]), 4), b"ACGT\x00ACGT\x00");
+    }
+
+    #[test]
+    fn segment_after_ambiguous_too_short_discarded() {
+        assert_eq!(run_fastq(&make_fastq(&[b"ACGTACGTNAC"]), 4), b"ACGTACGT\x00");
+    }
+
+    #[test]
+    fn consecutive_ambiguous_produce_no_empty_segment() {
+        assert_eq!(run_fastq(&make_fastq(&[b"ACGTNNNNACGT"]), 4), b"ACGT\x00ACGT\x00");
+    }
+
+    #[test]
+    fn ambiguous_at_start_skipped() {
+        assert_eq!(run_fastq(&make_fastq(&[b"NNACGTACGT"]), 4), b"ACGTACGT\x00");
+    }
+
+    #[test]
+    fn ambiguous_at_end_produces_no_trailing_empty() {
+        assert_eq!(run_fastq(&make_fastq(&[b"ACGTACGTNN"]), 4), b"ACGTACGT\x00");
+    }
+
+    #[test]
+    fn crlf_handled() {
+        let data = b"@hdr\r\nACGTACGT\r\n+\r\nIIIIIIII\r\n";
+        assert_eq!(run_fastq(data, 4), b"ACGTACGT\x00");
+    }
+
+    #[test]
+    fn multi_slice_rope() {
+        let data = make_fastq(&[b"ACGTACGT", b"TTTTTTTT"]);
+        let mid = data.len() / 2;
+        let mut rope = Rope::new();
+        rope.push(data[..mid].to_vec());
+        rope.push(data[mid..].to_vec());
+        assert_eq!(flat(normalize_fastq_chunk(rope, 4)), b"ACGTACGT\x00TTTTTTTT\x00");
+    }
+
+    // ── FASTA ─────────────────────────────────────────────────────────────────
+
     #[test]
     fn fasta_single_record() {
-        let out = run_fasta(&make_fasta(&[(b"s1", b"ACGTACGT")]), 4);
-        assert_eq!(out, b"ACGTACGT\x00");
+        assert_eq!(run_fasta(&make_fasta(&[(b"s1", b"ACGTACGT")]), 4), b"ACGTACGT\x00");
     }
 
     #[test]
     fn fasta_two_records() {
-        let out = run_fasta(&make_fasta(&[(b"s1", b"ACGTACGT"), (b"s2", b"TTTTTTTT")]), 4);
-        assert_eq!(out, b"ACGTACGT\x00TTTTTTTT\x00");
+        assert_eq!(
+            run_fasta(&make_fasta(&[(b"s1", b"ACGTACGT"), (b"s2", b"TTTTTTTT")]), 4),
+            b"ACGTACGT\x00TTTTTTTT\x00"
+        );
     }
 
     #[test]
     fn fasta_multiline_sequence_concatenated() {
-        let data = b">s1\nACGT\nACGT\nACGT\n";
-        let out = run_fasta(data, 4);
-        assert_eq!(out, b"ACGTACGTACGT\x00");
+        assert_eq!(run_fasta(b">s1\nACGT\nACGT\nACGT\n", 4), b"ACGTACGTACGT\x00");
     }
 
     #[test]
     fn fasta_lowercase_uppercased() {
-        let out = run_fasta(&make_fasta(&[(b"s1", b"acgtacgt")]), 4);
-        assert_eq!(out, b"ACGTACGT\x00");
+        assert_eq!(run_fasta(&make_fasta(&[(b"s1", b"acgtacgt")]), 4), b"ACGTACGT\x00");
     }
 
     #[test]
     fn fasta_short_record_discarded() {
-        let out = run_fasta(&make_fasta(&[(b"s1", b"ACG")]), 4);
-        assert_eq!(out, b"");
+        assert_eq!(run_fasta(&make_fasta(&[(b"s1", b"ACG")]), 4), b"");
     }
 
     #[test]
     fn fasta_short_among_valid_discarded() {
-        let out = run_fasta(
-            &make_fasta(&[(b"s1", b"ACGTACGT"), (b"s2", b"AC"), (b"s3", b"TTTTTTTT")]),
-            4,
+        assert_eq!(
+            run_fasta(&make_fasta(&[(b"s1", b"ACGTACGT"), (b"s2", b"AC"), (b"s3", b"TTTTTTTT")]), 4),
+            b"ACGTACGT\x00TTTTTTTT\x00"
         );
-        assert_eq!(out, b"ACGTACGT\x00TTTTTTTT\x00");
     }
 
     #[test]
     fn fasta_ambiguous_splits_segments() {
-        let data = b">s1\nACGTNACGT\n";
-        let out = run_fasta(data, 4);
-        assert_eq!(out, b"ACGT\x00ACGT\x00");
+        assert_eq!(run_fasta(b">s1\nACGTNACGT\n", 4), b"ACGT\x00ACGT\x00");
     }
 
     #[test]
     fn fasta_ambiguous_across_line_boundary() {
-        // 'N' at start of second line — still ambiguous
-        let data = b">s1\nACGT\nNACGT\n";
-        let out = run_fasta(data, 4);
-        assert_eq!(out, b"ACGT\x00ACGT\x00");
+        assert_eq!(run_fasta(b">s1\nACGT\nNACGT\n", 4), b"ACGT\x00ACGT\x00");
     }
 
     #[test]
     fn fasta_ambiguous_short_segment_discarded() {
-        let data = b">s1\nACGTACGTNAC\n";
-        let out = run_fasta(data, 4);
-        assert_eq!(out, b"ACGTACGT\x00");
+        assert_eq!(run_fasta(b">s1\nACGTACGTNAC\n", 4), b"ACGTACGT\x00");
     }
 
     #[test]
     fn fasta_no_trailing_newline() {
-        let data = b">s1\nACGTACGT";
-        let out = run_fasta(data, 4);
-        assert_eq!(out, b"ACGTACGT\x00");
+        assert_eq!(run_fasta(b">s1\nACGTACGT", 4), b"ACGTACGT\x00");
     }
 
     #[test]
     fn fasta_crlf_line_endings() {
-        let data = b">s1\r\nACGT\r\nACGT\r\n>s2\r\nTTTT\r\n";
-        let out = run_fasta(data, 4);
-        assert_eq!(out, b"ACGTACGT\x00TTTT\x00");
+        assert_eq!(run_fasta(b">s1\r\nACGT\r\nACGT\r\n>s2\r\nTTTT\r\n", 4), b"ACGTACGT\x00TTTT\x00");
     }
 
     #[test]
     fn fasta_multi_slice_rope() {
         let data = make_fasta(&[(b"s1", b"ACGTACGT"), (b"s2", b"TTTTTTTT")]);
         let mid = data.len() / 2;
-        let chunks = vec![
-            BytesMut::from(&data[..mid]).freeze(),
-            BytesMut::from(&data[mid..]).freeze(),
-        ];
-        let out: Vec<u8> = normalize_fasta_chunk(chunks, 4)
-            .into_iter()
-            .flat_map(|b| b.to_vec())
-            .collect();
-        assert_eq!(out, b"ACGTACGT\x00TTTTTTTT\x00");
-    }
-
-    // ── multi-record rope (multiple Bytes slices) ─────────────────────────────
-
-    #[test]
-    fn multi_slice_rope() {
-        let data = make_fastq(&[b"ACGTACGT", b"TTTTTTTT"]);
-        // Split into two slices at an arbitrary boundary.
-        let mid = data.len() / 2;
-        let chunks = vec![
-            BytesMut::from(&data[..mid]).freeze(),
-            BytesMut::from(&data[mid..]).freeze(),
-        ];
-        let out: Vec<u8> = normalize_fastq_chunk(chunks, 4)
-            .into_iter()
-            .flat_map(|b| b.to_vec())
-            .collect();
-        assert_eq!(out, b"ACGTACGT\x00TTTTTTTT\x00");
+        let mut rope = Rope::new();
+        rope.push(data[..mid].to_vec());
+        rope.push(data[mid..].to_vec());
+        assert_eq!(flat(normalize_fasta_chunk(rope, 4)), b"ACGTACGT\x00TTTTTTTT\x00");
     }
 }

src/obiread/src/tape.rs

@@ -1,427 +0,0 @@
-//! Two-cursor tape over a rope of `BytesMut` slices.
-//!
-//! A [`RopeCursor`] is a position `(slice, byte)` within a rope.
-//! Read and write cursors are the same type — the only difference is which
-//! primitive they call: [`RopeCursor::peek`] vs [`RopeCursor::poke`].
-//!
-//! The read-only methods (`peek`, `advance`, `retreat`, `distance`) are generic
-//! over `T: std::ops::Deref<Target = [u8]>` and work on both `Vec<Bytes>` and
-//! `Vec<BytesMut>`. The write method (`poke`) is specific to `BytesMut`.
-//!
-//! [`RopeTape`] owns the rope and two cursors, and exposes the four Turing-machine
-//! primitives used by the sequence normalisation automata.
-
-use std::ops::Deref;
-use bytes::{Bytes, BytesMut};
-
-// ── RopeCursor ────────────────────────────────────────────────────────────────
-
-/// A position within a rope (`Vec<BytesMut>`), usable as either a read or write head.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct RopeCursor {
-    slice: usize,
-    byte:  usize,
-}
-
-impl RopeCursor {
-    /// Position at the very start of the rope.
-    #[inline]
-    pub fn new() -> Self {
-        Self { slice: 0, byte: 0 }
-    }
-
-    /// Read the byte at the current position without moving.
-    /// Returns `None` at end of rope.
-    #[inline]
-    pub fn peek<T: Deref<Target = [u8]>>(&self, rope: &[T]) -> Option<u8> {
-        rope.get(self.slice)?.get(self.byte).copied()
-    }
-
-    /// Write `b` at the current position without moving.
-    #[inline]
-    pub fn poke(&self, rope: &mut [BytesMut], b: u8) {
-        rope[self.slice][self.byte] = b;
-    }
-
-    /// Advance by one byte, crossing slice boundaries transparently.
-    /// Returns `false` when the end of the rope is reached.
-    #[inline]
-    pub fn advance<T: Deref<Target = [u8]>>(&mut self, rope: &[T]) -> bool {
-        if self.slice >= rope.len() {
-            return false;
-        }
-        self.byte += 1;
-        if self.byte >= rope[self.slice].len() {
-            self.slice += 1;
-            self.byte = 0;
-        }
-        self.slice < rope.len()
-    }
-
-    /// Position at the last byte of the rope.
-    ///
-    /// Returns `None` if the rope is empty or all slices are empty.
-    pub fn end<T: Deref<Target = [u8]>>(rope: &[T]) -> Option<Self> {
-        for (i, slice) in rope.iter().enumerate().rev() {
-            if !slice.is_empty() {
-                return Some(Self { slice: i, byte: slice.len() - 1 });
-            }
-        }
-        None
-    }
-
-    /// Retreat by one byte and return the byte now under the cursor.
-    ///
-    /// Returns `None` if already at the beginning of the rope.
-    /// This is the inverse of the read-advance pattern used by `RopeTape::read_next`.
-    pub fn previous<T: Deref<Target = [u8]>>(&mut self, rope: &[T]) -> Option<u8> {
-        if self.slice == 0 && self.byte == 0 {
-            return None;
-        }
-        self.retreat(1, rope);
-        self.peek(rope)
-    }
-
-    /// Retreat by `n` bytes, crossing slice boundaries transparently.
-    ///
-    /// Panics in debug mode if retreating past the beginning of the rope.
-    pub fn retreat<T: Deref<Target = [u8]>>(&mut self, mut n: usize, rope: &[T]) {
-        while n > 0 {
-            if self.byte >= n {
-                self.byte -= n;
-                return;
-            }
-            // Consume all bytes in current slice and cross boundary.
-            n -= self.byte + 1;
-            debug_assert!(self.slice > 0, "retreat past beginning of rope");
-            self.slice -= 1;
-            self.byte = rope[self.slice].len() - 1;
-        }
-    }
-
-    /// Number of bytes from `from` (inclusive) to `to` (exclusive).
-    ///
-    /// Requires `from` ≤ `to` in rope order.
-    pub fn distance<T: Deref<Target = [u8]>>(from: &Self, to: &Self, rope: &[T]) -> usize {
-        if from.slice == to.slice {
-            to.byte - from.byte
-        } else {
-            let mut d = rope[from.slice].len() - from.byte;
-            for s in (from.slice + 1)..to.slice {
-                d += rope[s].len();
-            }
-            d += to.byte;
-            d
-        }
-    }
-}
-
-impl Default for RopeCursor {
-    fn default() -> Self { Self::new() }
-}
-
-// ── RopeTape ──────────────────────────────────────────────────────────────────
-
-/// A mutable rope with independent read and write cursors.
-///
-/// The write cursor always stays at or behind the read cursor — this invariant
-/// is guaranteed structurally by the formats (FASTA headers and FASTQ
-/// non-sequence lines are consumed before any sequence byte reaches the same
-/// position).
-pub struct RopeTape {
-    rope:  Vec<BytesMut>,
-    read:  RopeCursor,
-    write: RopeCursor,
-}
-
-impl RopeTape {
-    /// Build a tape from a chunk produced by [`crate::chunk::SeqChunkIter`].
-    ///
-    /// Each `Bytes` is converted to `BytesMut` in O(1): succeeds because the
-    /// chunk reader yields uniquely-owned, non-shared buffers.
-    pub fn new(chunks: Vec<Bytes>) -> Self {
-        let rope = chunks
-            .into_iter()
-            .map(|b| b.try_into_mut().expect("Bytes not uniquely owned"))
-            .collect();
-        Self { rope, read: RopeCursor::new(), write: RopeCursor::new() }
-    }
-
-    /// Peek at the byte under the read cursor without advancing.
-    #[inline]
-    pub fn peek(&self) -> Option<u8> {
-        self.read.peek(&self.rope)
-    }
-
-    /// Read the byte under the read cursor and advance it.
-    #[inline]
-    pub fn read_next(&mut self) -> Option<u8> {
-        let b = self.read.peek(&self.rope)?;
-        self.read.advance(&self.rope);
-        Some(b)
-    }
-
-    /// Write `b` at the write cursor and advance it.
-    #[inline]
-    pub fn write_next(&mut self, b: u8) {
-        self.write.poke(&mut self.rope, b);
-        self.write.advance(&self.rope);
-    }
-
-    /// Retreat the read cursor by one byte and return the byte now under it.
-    ///
-    /// This is the inverse of `read_next`: move back one position and read.
-    /// Returns `None` if already at the beginning of the tape.
-    #[inline]
-    pub fn previous(&mut self) -> Option<u8> {
-        self.read.previous(&self.rope)
-    }
-
-    /// Retreat the write cursor by `n` bytes (to erase a sequence shorter than k).
-    #[inline]
-    pub fn write_retreat(&mut self, n: usize) {
-        self.write.retreat(n, &self.rope);
-    }
-
-    /// Snapshot the current write position, typically used as `seq_start`.
-    #[inline]
-    pub fn write_snapshot(&self) -> RopeCursor {
-        self.write
-    }
-
-    /// Number of bytes written since `snapshot`.
-    #[inline]
-    pub fn written_since(&self, snapshot: RopeCursor) -> usize {
-        RopeCursor::distance(&snapshot, &self.write, &self.rope)
-    }
-
-    /// Consume the tape and return the written region as a `Vec<Bytes>`.
-    ///
-    /// Slices fully overwritten are returned in full; the last partial slice is
-    /// truncated to the write position; all remaining (unread) slices are dropped.
-    pub fn into_output(mut self) -> Vec<Bytes> {
-        let ws = self.write.slice;
-        let wb = self.write.byte;
-
-        if ws < self.rope.len() {
-            self.rope.truncate(ws + 1);
-            if let Some(last) = self.rope.last_mut() {
-                last.truncate(wb);
-            }
-        }
-
-        self.rope
-            .into_iter()
-            .filter(|s| !s.is_empty())
-            .map(BytesMut::freeze)
-            .collect()
-    }
-}
-
-// ── tests ─────────────────────────────────────────────────────────────────────
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    fn rope(slices: &[&[u8]]) -> Vec<BytesMut> {
-        slices.iter().map(|s| BytesMut::from(*s)).collect()
-    }
-
-    /// Mimics what SeqChunkIter produces: Bytes allocated from BytesMut (uniquely owned).
-    fn owned_bytes(data: &[u8]) -> Bytes {
-        BytesMut::from(data).freeze()
-    }
-
-    // ── RopeCursor::peek ──────────────────────────────────────────────────────
-
-    #[test]
-    fn peek_first_byte() {
-        let r = rope(&[b"ACGT"]);
-        assert_eq!(RopeCursor::new().peek(&r), Some(b'A'));
-    }
-
-    #[test]
-    fn peek_past_end_returns_none() {
-        let r = rope(&[b"A"]);
-        let mut c = RopeCursor::new();
-        c.advance(&r);
-        assert_eq!(c.peek(&r), None);
-    }
-
-    // ── RopeCursor::advance ───────────────────────────────────────────────────
-
-    #[test]
-    fn advance_crosses_slice_boundary() {
-        let r = rope(&[b"AC", b"GT"]);
-        let mut c = RopeCursor::new();
-        c.advance(&r); // → 'C'
-        c.advance(&r); // → 'G' (slice 1)
-        assert_eq!(c.peek(&r), Some(b'G'));
-    }
-
-    #[test]
-    fn advance_at_end_returns_false() {
-        let r = rope(&[b"A"]);
-        let mut c = RopeCursor::new();
-        let still_in = c.advance(&r);
-        assert!(!still_in);
-        assert_eq!(c.peek(&r), None);
-    }
-
-    // ── RopeCursor::poke ──────────────────────────────────────────────────────
-
-    #[test]
-    fn poke_writes_without_advancing() {
-        let mut r = rope(&[b"XXXX"]);
-        let c = RopeCursor::new();
-        c.poke(&mut r, b'A');
-        assert_eq!(r[0][0], b'A');
-        assert_eq!(r[0][1], b'X'); // unchanged
-        // cursor position unchanged: peek still at 0
-        assert_eq!(c.peek(&r), Some(b'A'));
-    }
-
-    // ── RopeCursor::retreat ───────────────────────────────────────────────────
-
-    #[test]
-    fn retreat_within_slice() {
-        let r = rope(&[b"ACGT"]);
-        let mut c = RopeCursor::new();
-        c.advance(&r); c.advance(&r); c.advance(&r); // → 'T'
-        c.retreat(2, &r);
-        assert_eq!(c.peek(&r), Some(b'C'));
-    }
-
-    #[test]
-    fn retreat_crosses_slice_boundary() {
-        let r = rope(&[b"ACGT", b"TTTT"]);
-        let mut c = RopeCursor::new();
-        for _ in 0..4 { c.advance(&r); } // → first 'T' of slice 1
-        c.retreat(1, &r);
-        assert_eq!(c.peek(&r), Some(b'T')); // last byte of "ACGT"
-    }
-
-    #[test]
-    fn retreat_exactly_to_slice_start() {
-        let r = rope(&[b"ACGT", b"TTTT"]);
-        let mut c = RopeCursor::new();
-        for _ in 0..5 { c.advance(&r); } // → second 'T' of slice 1
-        c.retreat(5, &r); // back to very first byte
-        assert_eq!(c.peek(&r), Some(b'A'));
-    }
-
-    #[test]
-    fn retreat_zero_is_noop() {
-        let r = rope(&[b"ACGT"]);
-        let mut c = RopeCursor::new();
-        c.advance(&r); c.advance(&r); // → 'G'
-        c.retreat(0, &r);
-        assert_eq!(c.peek(&r), Some(b'G'));
-    }
-
-    // ── RopeCursor::distance ──────────────────────────────────────────────────
-
-    #[test]
-    fn distance_same_slice() {
-        let r = rope(&[b"ACGTACGT"]);
-        let mut a = RopeCursor::new();
-        let mut b = RopeCursor::new();
-        b.advance(&r); b.advance(&r); b.advance(&r); // at offset 3
-        a.advance(&r);                                // at offset 1
-        assert_eq!(RopeCursor::distance(&a, &b, &r), 2);
-    }
-
-    #[test]
-    fn distance_across_slices() {
-        let r = rope(&[b"ACGT", b"TTTT"]);
-        let from = RopeCursor::new(); // (0,0)
-        let mut to = RopeCursor::new();
-        for _ in 0..6 { to.advance(&r); } // (1,2)
-        assert_eq!(RopeCursor::distance(&from, &to, &r), 6);
-    }
-
-    #[test]
-    fn distance_zero() {
-        let r = rope(&[b"ACGT"]);
-        let c = RopeCursor::new();
-        assert_eq!(RopeCursor::distance(&c, &c, &r), 0);
-    }
-
-    // ── RopeTape ──────────────────────────────────────────────────────────────
-
-    #[test]
-    fn tape_read_skips_write_copies() {
-        // Simulate: 5 bytes header, then 4 bytes sequence.
-        // Read 5 without writing, then copy 4.
-        let chunks = vec![owned_bytes(b">hdr\nACGT")];
-        let mut tape = RopeTape::new(chunks);
-
-        for _ in 0..5 { tape.read_next(); } // skip ">hdr\n"
-        for _ in 0..4 {
-            let b = tape.read_next().unwrap();
-            tape.write_next(b);
-        }
-
-        let out: Vec<u8> = tape.into_output().into_iter().flat_map(|b| b.to_vec()).collect();
-        assert_eq!(out, b"ACGT");
-    }
-
-    #[test]
-    fn tape_write_retreat_erases() {
-        let chunks = vec![owned_bytes(b"XXXXXXACGT")];
-        let mut tape = RopeTape::new(chunks);
-
-        // Write 3 bytes, then retreat — simulates a sequence < k being discarded.
-        let snap = tape.write_snapshot();
-        for _ in 0..3 {
-            let b = tape.read_next().unwrap();
-            tape.write_next(b);
-        }
-        assert_eq!(tape.written_since(snap), 3);
-        tape.write_retreat(3); // erase the 3 bytes
-        assert_eq!(tape.written_since(snap), 0);
-
-        // Now write 4 more bytes (the "ACGT").
-        for _ in 0..3 { tape.read_next(); } // skip "XXX"
-        for _ in 0..4 {
-            let b = tape.read_next().unwrap();
-            tape.write_next(b);
-        }
-
-        let out: Vec<u8> = tape.into_output().into_iter().flat_map(|b| b.to_vec()).collect();
-        assert_eq!(out, b"ACGT");
-    }
-
-    #[test]
-    fn tape_into_output_exact_boundary() {
-        // Write exactly until the last byte of the first slice, nothing more.
-        let chunks = vec![
-            owned_bytes(b"ACGT"),
-            owned_bytes(b"TTTT"),
-        ];
-        let mut tape = RopeTape::new(chunks);
-        for _ in 0..4 {
-            let b = tape.read_next().unwrap();
-            tape.write_next(b);
-        }
-        // Skip the rest
-        let out: Vec<u8> = tape.into_output().into_iter().flat_map(|b| b.to_vec()).collect();
-        assert_eq!(out, b"ACGT");
-    }
-
-    #[test]
-    fn tape_written_since_across_slices() {
-        let chunks = vec![
-            owned_bytes(b"XXXX"),
-            owned_bytes(b"YYYY"),
-        ];
-        let mut tape = RopeTape::new(chunks);
-        let snap = tape.write_snapshot();
-        for _ in 0..6 {
-            let b = tape.read_next().unwrap();
-            tape.write_next(b);
-        }
-        assert_eq!(tape.written_since(snap), 6);
-    }
-}

src/obirope/src/cursor.rs

@@ -1,7 +0,0 @@
-/// A position within a rope (`Vec<BytesMut>`), usable as either a read or write head.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct RopeCursor {
-    rope: &Rope,
-    slice: usize,
-    byte: usize,
-}

src/obirope/src/lib.rs

@@ -1,3 +0,0 @@
-mod rope;
-
-use rope::Rope;

src/obirope/src/rope.rs

@@ -1,100 +0,0 @@
-use bytes::Bytes;
-
-pub struct Rope {
-    blocks: Vec<Bytes>,
-    length: usize,
-    cum_length: Vec<usize>,
-    last_block_start: usize,
-    last_block_end: usize,
-    last_block_idx: usize,
-}
-
-impl Rope {
-    pub fn new() -> Self {
-        Self {
-            blocks: Vec::new(),
-            length: 0,
-            cum_length: Vec::new(),
-            last_block_start: 0,
-            last_block_end: 0,
-            last_block_idx: 0,
-        }
-    }
-
-    pub fn push(&mut self, block: Bytes) {
-        self.blocks.push(block);
-        self.length += block.len();
-        self.cum_length.push(self.length);
-        if self.blocks.len() == 1 {
-            self.last_block_start = 0;
-            self.last_block_end = block.len();
-            self.last_block_idx = 0;
-        }
-    }
-
-    pub fn n_blocks(&self) -> usize {
-        self.blocks.len()
-    }
-
-    pub fn len(&self) -> usize {
-        self.length
-    }
-
-    fn lookup(&mut self, i: usize) -> Option<(usize, usize, usize)> {
-        if i >= self.length {
-            return None;
-        }
-
-        let (block_idx, from, to) = match self.blocks.len() {
-            0 => return None,
-            1 => (0, 0, self.cum_length[0]),
-            2 => {
-                if i < self.cum_length[0] {
-                    (0, 0, self.cum_length[0])
-                } else {
-                    (1, self.cum_length[0], self.blocks[1].len())
-                }
-            }
-            _ => {
-                let begin_idx = 0;
-                let end_idx = self.blocks.len() - 1;
-                while begin_idx < end_idx {
-                    let pivot_idx = (begin_idx + end_idx) / 2;
-                    let offset = self.cum_length[pivot_idx];
-                    if i < offset {
-                        end_idx = pivot_idx;
-                    } else {
-                        begin_idx = pivot_idx + 1;
-                    }
-                }
-                (
-                    begin_idx - 1,
-                    self.cum_length[begin_idx - 1],
-                    self.blocks[begin_idx].len(),
-                )
-            }
-        };
-        self.last_block_idx = block_idx;
-        self.last_block_start = from;
-        self.last_block_end = to;
-        Some((block_idx, from, to))
-    }
-
-    pub fn coordinates(&self, i: usize) -> Option<(usize, usize)> {
-        let (block_idx, pos) = if i < self.last_block_end && i >= self.last_block_start {
-            (self.last_block_idx, i - self.last_block_start)
-        } else {
-            let (block_idx, from, to) = self.lookup(i)?;
-            (block_idx, i - from)
-        };
-        Some((block_idx, pos))
-    }
-
-    pub fn block(&self, i: usize) -> Option<&Bytes> {
-        self.blocks.get(i)
-    }
-
-    pub fn is_empty(&self) -> bool {
-        self.blocks.is_empty()
-    }
-}

src/obiskbuilder/Cargo.toml

@@ -5,5 +5,4 @@ edition = "2024"
 
 [dependencies]
 obikseq = { path = "../obikseq" }
-obiread = { path = "../obiread" }
-bytes = "1"
+obikrope = { path = "../obikrope" }

src/obiskbuilder/src/iter.rs

@@ -15,8 +15,7 @@
 //! | minimizer changed      | k              |
 //! | super-kmer length = 256| k              |
 
-use bytes::Bytes;
-use obiread::tape::RopeCursor;
+use obikrope::{ForwardCursor, Rope, RopeCursor};
 use obikseq::superkmer::SuperKmer;
 
 use crate::encoding::encode_base;
@@ -26,9 +25,8 @@ use crate::scratch::SuperKmerScratch;
 use crate::window::KmerWindow;
 
 /// Iterator over `(minimizer, SuperKmer)` pairs.
-pub struct SuperKmerIter {
-    rope: Vec<Bytes>,
-    cursor: RopeCursor,
+pub struct SuperKmerIter<'a> {
+    cursor: ForwardCursor<'a>,
     k: usize,
     scratch: SuperKmerScratch,
     window: KmerWindow,
@@ -38,23 +36,16 @@ pub struct SuperKmerIter {
     prev_minimizer_pos: u8,
 }
 
-impl SuperKmerIter {
+impl<'a> SuperKmerIter<'a> {
     /// Build an iterator from a normalised rope chunk.
     ///
     /// - `k`: k-mer size (1–31)
     /// - `m`: minimizer size (1 < m < k)
     /// - `level_max`: maximum sub-word size for entropy (typically 6)
     /// - `theta`: entropy threshold; k-mers with score ≤ theta are rejected
-    pub fn new(
-        chunks: Vec<Bytes>,
-        k: usize,
-        m: usize,
-        level_max: usize,
-        theta: f64,
-    ) -> Self {
+    pub fn new(rope: &'a Rope, k: usize, m: usize, level_max: usize, theta: f64) -> Self {
         Self {
-            rope: chunks,
-            cursor: RopeCursor::new(),
+            cursor: rope.fw_cursor(),
             k,
             scratch: SuperKmerScratch::new(),
             window: KmerWindow::new(k),
@@ -65,13 +56,6 @@ impl SuperKmerIter {
         }
     }
 
-    #[inline]
-    fn read_byte(&mut self) -> Option<u8> {
-        let b = self.cursor.peek(&self.rope)?;
-        self.cursor.advance(&self.rope);
-        Some(b)
-    }
-
     fn reset_state(&mut self) {
         self.window.reset();
         self.minimizer.reset();
@@ -80,8 +64,6 @@ impl SuperKmerIter {
         self.prev_minimizer_pos = 0;
     }
 
-    /// Emit the current scratch as a super-kmer if it holds at least k nucleotides.
-    /// Returns `None` and silently discards short buffers (< k nt, no complete kmer).
     fn try_emit(&mut self, min: u64) -> Option<(u64, SuperKmer)> {
         if self.scratch.len() < self.k {
             self.scratch.reset();
@@ -93,19 +75,17 @@ impl SuperKmerIter {
     }
 }
 
-impl Iterator for SuperKmerIter {
+impl<'a> Iterator for SuperKmerIter<'a> {
     type Item = (u64, SuperKmer);
 
     fn next(&mut self) -> Option<(u64, SuperKmer)> {
         loop {
-            let byte = match self.read_byte() {
+            let byte = match self.cursor.read_next().ok() {
                 None => {
-                    // EOF: flush whatever we have.
                     let min = self.prev_minimizer.unwrap_or(0);
                     return self.try_emit(min);
                 }
                 Some(0x00) => {
-                    // Segment boundary: flush current super-kmer, then reset.
                     let min = self.prev_minimizer.unwrap_or(0);
                     let result = self.try_emit(min);
                     self.reset_state();
@@ -118,25 +98,22 @@ impl Iterator for SuperKmerIter {
             };
 
             let base2 = encode_base(byte);
-
-            // Update sliding windows; kmer_ready is true once we have k bases.
             let kmer_ready = self.window.push(base2);
             let current_min = self.minimizer.push(base2);
 
             if !kmer_ready {
-                // Warm-up phase: just accumulate.
                 self.scratch.push(byte);
                 continue;
             }
 
             let kmer = self.window.kmer_u64();
-            let min = current_min.unwrap(); // guaranteed when kmer_ready
+            let min = current_min.unwrap();
 
             // ── 1. Entropy check ─────────────────────────────────────────────
             if !self.entropy.accept(kmer) {
                 let prev_min = self.prev_minimizer.unwrap_or(0);
                 let result = self.try_emit(prev_min);
-                self.cursor.retreat(self.k - 1, &self.rope);
+                self.cursor.rewind(self.k - 1).ok();
                 self.reset_state();
                 if result.is_some() {
                     return result;
@@ -148,7 +125,7 @@ impl Iterator for SuperKmerIter {
             if let Some(prev) = self.prev_minimizer {
                 if min != prev {
                     let result = self.try_emit(prev);
-                    self.cursor.retreat(self.k, &self.rope);
+                    self.cursor.rewind(self.k).ok();
                     self.reset_state();
                     if result.is_some() {
                         return result;
@@ -158,10 +135,9 @@ impl Iterator for SuperKmerIter {
             }
 
             // ── 3. Super-kmer length check ────────────────────────────────────
-            // S[j] would be the 257th nucleotide → don't add it.
             if self.scratch.len() == 256 {
                 let result = self.try_emit(min);
-                self.cursor.retreat(self.k, &self.rope);
+                self.cursor.rewind(self.k).ok();
                 self.reset_state();
                 if result.is_some() {
                     return result;
@@ -169,7 +145,6 @@ impl Iterator for SuperKmerIter {
                 continue;
             }
 
-            // All checks passed: accept S[j].
             self.prev_minimizer = Some(min);
             self.prev_minimizer_pos = self.minimizer.minimizer_pos();
             self.scratch.push(byte);
@@ -182,29 +157,24 @@ impl Iterator for SuperKmerIter {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use bytes::BytesMut;
 
-    fn chunks(data: &[u8]) -> Vec<Bytes> {
-        vec![BytesMut::from(data).freeze()]
+    fn make_rope(data: &[u8]) -> Rope {
+        let mut r = Rope::new();
+        r.push(data.to_vec());
+        r
     }
 
-    /// theta=0: accept everything; level_max=1 (minimal entropy computation).
     fn run_nofilter(data: &[u8], k: usize, m: usize) -> Vec<Vec<u8>> {
-        SuperKmerIter::new(chunks(data), k, m, 1, 0.0)
+        let rope = make_rope(data);
+        SuperKmerIter::new(&rope, k, m, 1, 0.0)
             .map(|(_, sk)| sk.to_ascii())
             .collect()
     }
 
-    // ── basic segmentation (no entropy / minimizer cut) ───────────────────────
-
     #[test]
     fn single_segment_one_superkmer() {
-        // k=4, m=2; one segment "ACGTACGT\x00"
-        // All consecutive 4-mers share the same minimizer (or not — but we
-        // just check that we get the full sequence back with theta=0).
         let out = run_nofilter(b"ACGTACGT\x00", 4, 2);
         assert!(!out.is_empty());
-        // The concatenation of all emitted superkmers covers the segment.
         let total: Vec<u8> = out.into_iter().flatten().collect();
         assert!(total.len() >= 4);
     }
@@ -227,54 +197,37 @@ mod tests {
         assert!(!out.is_empty());
     }
 
-    // ── entropy cut ───────────────────────────────────────────────────────────
-
     #[test]
     fn low_complexity_kmer_is_rejected() {
-        // "AAAAAAAAACGT\x00": the run of A's is low-complexity;
-        // with a strict threshold the A-only kmers get cut.
-        // With theta=0 everything passes → we get one superkmer.
         let out_pass = run_nofilter(b"AAAAAAAAACGT\x00", 4, 2);
         assert!(!out_pass.is_empty());
 
-        // With a high threshold polyA should be rejected.
-        let out_reject: Vec<Vec<u8>> =
-            SuperKmerIter::new(chunks(b"AAAAAAAA\x00"), 4, 2, 6, 0.9)
-                .map(|(_, sk)| sk.to_ascii())
-                .collect();
-        // The sequence is pure polyA: all kmers should fail entropy → no emission.
+        let rope = make_rope(b"AAAAAAAA\x00");
+        let out_reject: Vec<Vec<u8>> = SuperKmerIter::new(&rope, 4, 2, 6, 0.9)
+            .map(|(_, sk)| sk.to_ascii())
+            .collect();
         assert!(out_reject.is_empty());
     }
 
-    // ── multi-slice rope ──────────────────────────────────────────────────────
-
     #[test]
     fn multi_slice_rope() {
         let data = b"ACGTACGTACGT\x00";
         let mid = data.len() / 2;
-        let rope = vec![
-            BytesMut::from(&data[..mid]).freeze(),
-            BytesMut::from(&data[mid..]).freeze(),
-        ];
-        let out: Vec<Vec<u8>> = SuperKmerIter::new(rope, 4, 2, 1, 0.0)
+        let mut rope = Rope::new();
+        rope.push(data[..mid].to_vec());
+        rope.push(data[mid..].to_vec());
+        let out: Vec<Vec<u8>> = SuperKmerIter::new(&rope, 4, 2, 1, 0.0)
             .map(|(_, sk)| sk.to_ascii())
             .collect();
         assert!(!out.is_empty());
     }
 
-    // ── minimizer is returned ─────────────────────────────────────────────────
-
     #[test]
     fn yields_minimizer_value() {
-        let data = b"ACGTACGT\x00";
-        let results: Vec<(u64, Vec<u8>)> =
-            SuperKmerIter::new(chunks(data), 4, 2, 1, 0.0)
-                .map(|(min, sk)| (min, sk.to_ascii()))
-                .collect();
+        let rope = make_rope(b"ACGTACGT\x00");
+        let results: Vec<(u64, Vec<u8>)> = SuperKmerIter::new(&rope, 4, 2, 1, 0.0)
+            .map(|(min, sk)| (min, sk.to_ascii()))
+            .collect();
         assert!(!results.is_empty());
-        // The minimizer is a non-trivial u64 (just check it's been computed).
-        for (min, _) in &results {
-            let _ = min; // value is format-specific; just verify no panic
-        }
     }
 }