Parallelize partition iteration using Rayon

Introduce thread-local `Vec<u8>` buffers to eliminate concurrent I/O contention. Replace the mutable row counter with an `AtomicUsize` and `fetch_update` to enable lock-free early termination when the limit is reached. Collected chunks are then written sequentially to preserve partition ordering.

src/obikindex/src/dump.rs

@@ -1,4 +1,7 @@
 use std::io::Write;
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+use rayon::prelude::*;
 
 use crate::error::{OKIError, OKIResult};
 use crate::index::KmerIndex;
@@ -13,6 +16,9 @@ impl KmerIndex {
     /// `force_presence` overrides `with_counts`: even if the index stores counts,
     /// the output uses 0/1 presence columns.
     ///
+    /// Partitions are scanned in parallel; each partition buffers its output locally
+    /// before the main thread writes the chunks in partition order.
+    ///
     /// The caller must have set the global kmer length (`obikseq::set_k`) before
     /// calling this method.
     pub fn dump<W: Write>(
@@ -38,37 +44,57 @@ impl KmerIndex {
         }
         writeln!(out)?;
 
-        // ── Rows ──────────────────────────────────────────────────────────────
+        // ── Rows — parallel over partitions ───────────────────────────────────
         let n         = self.n_partitions();
-        let mut remaining = head.unwrap_or(usize::MAX);
-        for i in 0..n {
-            if remaining == 0 { break; }
-            let cont = if debug {
+        let remaining = AtomicUsize::new(head.unwrap_or(usize::MAX));
+
+        let chunks: Vec<OKIResult<Vec<u8>>> = (0..n)
+            .into_par_iter()
+            .map(|i| {
+                if remaining.load(Ordering::Relaxed) == 0 {
+                    return Ok(vec![]);
+                }
+                let mut buf = Vec::<u8>::new();
+
+                let write_kmer = |buf: &mut Vec<u8>, row: &[u32], prefix: &str| -> bool {
+                    match remaining.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |cur| {
+                        if cur > 0 { Some(cur - 1) } else { None }
+                    }) {
+                        Err(_) => false,
+                        Ok(_)  => {
+                            let _ = buf.write_all(prefix.as_bytes());
+                            for &v in row { let _ = write!(buf, ",{v}"); }
+                            let _ = buf.write_all(b"\n");
+                            true
+                        }
+                    }
+                };
+
+                if debug {
                     self.partition
                         .iter_partition_kmers_located(i, use_counts, n_genomes, filters, |part, layer, kmer, row| {
                             let seq = String::from_utf8(kmer.to_ascii())
                                 .unwrap_or_else(|_| "?".repeat(kmer_size));
-                        let _ = write!(out, "{part},{layer},{seq}");
-                        for &v in row.iter() { let _ = write!(out, ",{v}"); }
-                        let _ = writeln!(out);
-                        remaining -= 1;
-                        remaining > 0
+                            write_kmer(&mut buf, &row, &format!("{part},{layer},{seq}"))
                         })
-                    .map_err(OKIError::Partition)?
+                        .map_err(OKIError::Partition)?;
                 } else {
                     self.partition
                         .iter_partition_kmers(i, use_counts, n_genomes, filters, |kmer, row| {
                             let seq = String::from_utf8(kmer.to_ascii())
                                 .unwrap_or_else(|_| "?".repeat(kmer_size));
-                        let _ = write!(out, "{seq}");
-                        for &v in row.iter() { let _ = write!(out, ",{v}"); }
-                        let _ = writeln!(out);
-                        remaining -= 1;
-                        remaining > 0
+                            write_kmer(&mut buf, &row, &seq)
                         })
-                    .map_err(OKIError::Partition)?
-            };
-            if !cont { break; }
+                        .map_err(OKIError::Partition)?;
+                }
+
+                Ok(buf)
+            })
+            .collect();
+
+        // ── Sequential write ──────────────────────────────────────────────────
+        for chunk in chunks {
+            out.write_all(&chunk?)?;
         }
 
         out.flush()?;