feat: add kmer iterators and optimize layered map performance

Replace `ph` with `ptr_hash` and introduce `epserde` and `rayon` dependencies. Refactor MPHF construction to leverage parallel iteration, eliminating intermediate `Vec<u64>` allocations and reducing memory footprint. Add a `n_kmers` field to track and serialize total kmer counts, alongside three zero-allocation iterators for efficient chunk traversal. Include comprehensive unit tests for the new iterators and update CLAUDE.md to enforce explicit dependency validation policies.

src/obiskio/src/unitig_index.rs

@@ -3,7 +3,7 @@ use std::io::{BufWriter, Write as _};
 use std::path::{Path, PathBuf};
 
 use memmap2::Mmap;
-use obikseq::{CanonicalKmer, Unitig};
+use obikseq::{CanonicalKmer, Kmer, Unitig};
 
 pub use obikseq::MAX_KMERS_PER_CHUNK;
 
@@ -13,6 +13,7 @@ use crate::error::{SKError, SKResult};
 //
 //   magic:           [u8; 4]  = b"UIDX"
 //   n_unitigs:       u32 LE
+//   n_kmers:         u64 LE   total kmer count across all chunks
 //   seqls:           [u8; n_unitigs]       max kmer index per chunk (= n_kmers − 1)
 //   packed_offsets:  [u32; n_unitigs + 1]  byte offsets to packed bytes in the
 //                                          sequence file; last entry is sentinel
@@ -44,6 +45,7 @@ pub struct UnitigFileWriter {
     seqls: Vec<u8>,
     packed_offsets: Vec<u32>,
     next_offset: u32,
+    n_kmers: usize,
     k: usize,
 }
 
@@ -56,6 +58,7 @@ impl UnitigFileWriter {
             seqls: Vec::new(),
             packed_offsets: Vec::new(),
             next_offset: 0,
+            n_kmers: 0,
             k: obikseq::params::k(),
         })
     }
@@ -98,6 +101,7 @@ impl UnitigFileWriter {
         debug_assert!(seql - self.k <= u8::MAX as usize, "chunk exceeds MAX_KMERS_PER_CHUNK");
         self.packed_offsets.push(self.next_offset + 1);
         self.seqls.push((seql - self.k) as u8);
+        self.n_kmers += seql - self.k + 1;
 
         unitig
             .write_to_binary(&mut self.file)
@@ -122,7 +126,7 @@ impl UnitigFileWriter {
         };
         self.packed_offsets.push(sentinel);
 
-        write_idx(&idx_path(&self.path), &self.seqls, &self.packed_offsets)
+        write_idx(&idx_path(&self.path), &self.seqls, &self.packed_offsets, self.n_kmers)
     }
 
     pub fn len(&self) -> usize {
@@ -134,10 +138,11 @@ impl UnitigFileWriter {
     }
 }
 
-fn write_idx(path: &Path, seqls: &[u8], packed_offsets: &[u32]) -> SKResult<()> {
+fn write_idx(path: &Path, seqls: &[u8], packed_offsets: &[u32], n_kmers: usize) -> SKResult<()> {
     let mut w = BufWriter::new(File::create(path).map_err(SKError::Io)?);
     w.write_all(&MAGIC).map_err(SKError::Io)?;
     w.write_all(&(seqls.len() as u32).to_le_bytes()).map_err(SKError::Io)?;
+    w.write_all(&(n_kmers as u64).to_le_bytes()).map_err(SKError::Io)?;
     w.write_all(seqls).map_err(SKError::Io)?;
     for &off in packed_offsets {
         w.write_all(&off.to_le_bytes()).map_err(SKError::Io)?;
@@ -155,6 +160,7 @@ pub struct UnitigFileReader {
     mmap: Mmap,
     seqls: Vec<u8>,
     packed_offsets: Vec<u32>,
+    n_kmers: usize,
     k: usize,
 }
 
@@ -162,9 +168,9 @@ impl UnitigFileReader {
     pub fn open(path: &Path) -> SKResult<Self> {
         let file = File::open(path).map_err(SKError::Io)?;
         let mmap = unsafe { Mmap::map(&file).map_err(SKError::Io)? };
-        let (seqls, packed_offsets) = read_idx(&idx_path(path))?;
+        let (seqls, packed_offsets, n_kmers) = read_idx(&idx_path(path))?;
         let k = obikseq::params::k();
-        Ok(Self { mmap, seqls, packed_offsets, k })
+        Ok(Self { mmap, seqls, packed_offsets, n_kmers, k })
     }
 
     pub fn len(&self) -> usize {
@@ -175,6 +181,11 @@ impl UnitigFileReader {
         self.seqls.is_empty()
     }
 
+    /// Total number of kmers across all chunks.
+    pub fn n_kmers(&self) -> usize {
+        self.n_kmers
+    }
+
     /// Return the nucleotide length of chunk `i`.
     #[inline]
     pub fn seql(&self, i: usize) -> usize {
@@ -206,9 +217,42 @@ impl UnitigFileReader {
     pub fn verify_canonical_kmer(&self, i: usize, j: usize, query: CanonicalKmer) -> bool {
         canonical_raw(self.raw_kmer(i, j), self.k) == query.raw()
     }
+
+    /// Iterate over all kmers in file order (all positions of chunk 0, then chunk 1, …).
+    ///
+    /// Each chunk is copied from the mmap once; iteration within the chunk is
+    /// zero-allocation (sliding-window via [`OwnedPackedSeqKmerIter`]).
+    pub fn iter_kmers(&self) -> impl Iterator<Item = Kmer> + '_ {
+        (0..self.len()).flat_map(move |i| self.unitig(i).into_kmers())
+    }
+
+    /// Iterate over all canonical kmers in file order.
+    ///
+    /// Equivalent to `iter_kmers().map(|km| km.canonical())` but uses the
+    /// built-in canonical iterator on each chunk, which avoids a separate
+    /// canonicalization pass.
+    pub fn iter_canonical_kmers(&self) -> impl Iterator<Item = CanonicalKmer> + '_ {
+        (0..self.len()).flat_map(move |i| self.unitig(i).into_canonical_kmers())
+    }
+
+    /// Iterate over `(kmer, chunk_id, rank)` for every canonical kmer in the file.
+    ///
+    /// `chunk_id` is the index of the chunk within this file; `rank` is the
+    /// 0-based position of the kmer within that chunk.  Used to build the
+    /// evidence table in `obilayeredmap`.
+    pub fn iter_indexed_canonical_kmers(
+        &self,
+    ) -> impl Iterator<Item = (CanonicalKmer, usize, usize)> + '_ {
+        (0..self.len()).flat_map(move |chunk_id| {
+            self.unitig(chunk_id)
+                .into_canonical_kmers()
+                .enumerate()
+                .map(move |(rank, kmer)| (kmer, chunk_id, rank))
+        })
+    }
 }
 
-fn read_idx(path: &Path) -> SKResult<(Vec<u8>, Vec<u32>)> {
+fn read_idx(path: &Path) -> SKResult<(Vec<u8>, Vec<u32>, usize)> {
     let data = std::fs::read(path).map_err(SKError::Io)?;
     let mut pos = 0;
 
@@ -227,6 +271,11 @@ fn read_idx(path: &Path) -> SKResult<(Vec<u8>, Vec<u32>)> {
     let n = u32::from_le_bytes(n_bytes.try_into().unwrap()) as usize;
     pos += 4;
 
+    let nk_bytes = data.get(pos..pos + 8)
+        .ok_or(SKError::Truncated { context: "unitig index: n_kmers" })?;
+    let n_kmers = u64::from_le_bytes(nk_bytes.try_into().unwrap()) as usize;
+    pos += 8;
+
     let seqls = data.get(pos..pos + n)
         .ok_or(SKError::Truncated { context: "unitig index: seqls" })?
         .to_vec();
@@ -240,7 +289,7 @@ fn read_idx(path: &Path) -> SKResult<(Vec<u8>, Vec<u32>)> {
         pos += 4;
     }
 
-    Ok((seqls, packed_offsets))
+    Ok((seqls, packed_offsets, n_kmers))
 }
 
 // ── Kmer utilities ────────────────────────────────────────────────────────────