refactor: centralize nonzero slot iteration logic across views

Extract duplicated traversal logic into a shared `NonzeroSlotsView` trait and `nonzero_triples` helper. Update slice view iterators to consistently yield `(position, value)` tuples and use a temporary buffer with explicit permutation mapping for sorted access. Delegate manual slot sorting and column iteration to the new shared helper, eliminating per-format duplication and eager collection while preserving existing public API signatures.

src/obicompactvec/src/bitmatrix/persistent.rs

@@ -7,7 +7,7 @@ use crate::bitvec::{PersistentBitVec, PersistentBitVecBuilder};
 use crate::layer_meta::LayerMeta;
 use crate::meta::MatrixMeta;
 use crate::traits::{BitPartials, ColumnWeights};
-use crate::views::BitSliceView;
+use crate::views::{nonzero_triples, BitSliceView};
 
 use super::columnar::ColumnarBitMatrix;
 use super::packed::PackedBitMatrix;
@@ -199,10 +199,9 @@ impl PersistentBitMatrix {
     /// `Sparse` delegates to its native row-major decode (no `n_cols`-wide
     /// buffer, ever — the whole point, see
     /// `PersistentSparseBitMatrix::nonzero_iter`); `Columnar`/`Packed`
-    /// reuse the same sorted-slot batching `fill_sub_matrix` always used
+    /// reuse the shared sorted-slot batching in `views::nonzero_triples`
-    /// (`BitSliceView::enumerate_nonzero_slots`), just eagerly collected here
+    /// (same as `IntMatrix::nonzero_iter`) — same `get` calls, same mmap
-    /// rather than filled into a dense buffer — same `get` calls, same
+    /// locality, no per-format duplication of that logic elsewhere.
-    /// mmap locality, no per-format duplication of that logic elsewhere.
     /// `Implicit` is trivial (one column, always present).
     ///
     /// Boxed (not `impl Iterator`) because the match arms are genuinely
@@ -219,20 +218,7 @@ impl PersistentBitMatrix {
                 Box::new(slots.iter().enumerate().map(|(i, _)| (i, 0usize, 1u32)))
             }
             Self::Columnar(_) | Self::Packed(_) => {
-                let n = slots.len();
+                Box::new(nonzero_triples(slots, self.n_cols(), |c| self.col_view(c)))
-                let mut perm: Vec<usize> = (0..n).collect();
-                perm.sort_by_key(|&i| slots[i]);
-                let sorted_slots: Vec<usize> = perm.iter().map(|&i| slots[i]).collect();
-                let n_cols = self.n_cols();
-                let mut hits: Vec<(usize, usize, u32)> = Vec::new();
-                for c in 0..n_cols {
-                    let view = self.col_view(c);
-                    hits.extend(
-                        view.enumerate_nonzero_slots(&sorted_slots)
-                            .map(|pos| (perm[pos], c, 1u32)),
-                    );
-                }
-                Box::new(hits.into_iter())
             }
         }
     }

src/obicompactvec/src/intmatrix.rs

@@ -14,7 +14,7 @@ use crate::meta::MatrixMeta;
 use crate::reader::PersistentCompactIntVec;
 use crate::tempbitvec::{TempBitVec, TempBitVecBuilder};
 use crate::tempintvec::{TempCompactIntVec, TempCompactIntVecBuilder};
-use crate::views::IntSliceView;
+use crate::views::{nonzero_triples, IntSliceView};
 
 fn col_path(dir: &Path, col: usize) -> PathBuf {
     dir.join(format!("col_{col:06}.pciv"))
@@ -403,23 +403,10 @@ impl PersistentCompactIntMatrix {
     /// sparse row-major access" — counts explicitly not excluded from that
     /// design even though no sparse count format exists yet). No native
     /// low-effort case here the way `PersistentSparseBitMatrix` has one —
-    /// both variants reuse the same sorted-slot batching `fill_sub_matrix`
+    /// both variants reuse the shared sorted-slot batching in
-    /// already used, via `IntSliceView::enumerate_nonzero_slots`.
+    /// `views::nonzero_triples`.
     pub fn nonzero_iter<'a>(&'a self, slots: &'a [usize]) -> impl Iterator<Item = (usize, usize, u32)> + 'a {
-        let n = slots.len();
+        nonzero_triples(slots, self.n_cols(), |c| self.col_view(c))
-        let mut perm: Vec<usize> = (0..n).collect();
-        perm.sort_by_key(|&i| slots[i]);
-        let sorted_slots: Vec<usize> = perm.iter().map(|&i| slots[i]).collect();
-        let n_cols = self.n_cols();
-        let mut hits: Vec<(usize, usize, u32)> = Vec::new();
-        for c in 0..n_cols {
-            hits.extend(
-                self.col_view(c)
-                    .enumerate_nonzero_slots(&sorted_slots)
-                    .map(|(pos, v)| (perm[pos], c, v)),
-            );
-        }
-        hits.into_iter()
     }
 
     #[inline]

src/obicompactvec/src/views.rs

@@ -43,10 +43,8 @@ impl<'a> BitSliceView<'a> {
         let mut perm: Vec<usize> = (0..n).collect();
         perm.sort_by_key(|&i| slots[i]);
         let sorted: Vec<usize> = perm.iter().map(|&i| slots[i]).collect();
-        let mut tmp = vec![false; n];
+        for (i, v) in self.enumerate_slots_values(&sorted) {
-        self.fill_slots_values_sorted(&sorted, &mut tmp);
+            out[perm[i]] = v;
-        for (i, &orig_idx) in perm.iter().enumerate() {
-            out[orig_idx] = tmp[i];
         }
     }
 
@@ -62,25 +60,20 @@ impl<'a> BitSliceView<'a> {
         slots.iter().enumerate().map(move |(pos, &slot)| (pos, self.get(slot)))
     }
 
-    /// Fill `out` assuming `sorted_slots` is already in ascending order.
+    /// `(position in slots, 1)` for every slot whose bit is set — the value
-    /// Results are written in `sorted_slots` order (no reordering).
+    /// is always `1u32` (a set bit has no other value to carry), kept in
-    pub(crate) fn fill_slots_values_sorted(&self, sorted_slots: &[usize], out: &mut [bool]) {
+    /// the tuple only so this has the same shape as
-        assert_eq!(sorted_slots.len(), out.len());
+    /// [`IntSliceView::enumerate_nonzero_slots`] and both can feed the same
-        for (pos, v) in self.enumerate_slots_values(sorted_slots) {
+    /// generic triple-collecting code (see `views::NonzeroSlotsView`).
-            out[pos] = v;
-        }
-    }
-
-    /// Positions in `slots` (not the slots themselves) whose bit is set.
     /// Order of `slots` never matters for correctness; callers batching
     /// several columns over the same slot set sort it once beforehand
     /// purely for cache-friendlier `get` access (see
     /// `PersistentBitMatrix::nonzero_iter`).
-    pub fn enumerate_nonzero_slots<'b>(self, slots: &'b [usize]) -> impl Iterator<Item = usize> + 'b
+    pub fn enumerate_nonzero_slots<'b>(self, slots: &'b [usize]) -> impl Iterator<Item = (usize, u32)> + 'b
     where
         Self: 'b,
     {
-        self.enumerate_slots_values(slots).filter_map(|(pos, v)| v.then_some(pos))
+        self.enumerate_slots_values(slots).filter_map(|(pos, v)| v.then_some((pos, 1u32)))
     }
 
     pub fn count_ones(&self) -> u64 {
@@ -209,10 +202,8 @@ impl<'a> IntSliceView<'a> {
         let mut perm: Vec<usize> = (0..n).collect();
         perm.sort_by_key(|&i| slots[i]);
         let sorted: Vec<usize> = perm.iter().map(|&i| slots[i]).collect();
-        let mut tmp = vec![0u32; n];
+        for (i, v) in self.enumerate_slots_values(&sorted) {
-        self.fill_slots_values_sorted(&sorted, &mut tmp);
+            out[perm[i]] = v;
-        for (i, &orig_idx) in perm.iter().enumerate() {
-            out[orig_idx] = tmp[i];
         }
     }
 
@@ -227,15 +218,6 @@ impl<'a> IntSliceView<'a> {
         slots.iter().enumerate().map(move |(pos, &slot)| (pos, self.get(slot)))
     }
 
-    /// Fill `out` assuming `sorted_slots` is already in ascending order.
-    /// Results are written in `sorted_slots` order (no reordering).
-    pub(crate) fn fill_slots_values_sorted(&self, sorted_slots: &[usize], out: &mut [u32]) {
-        assert_eq!(sorted_slots.len(), out.len());
-        for (pos, v) in self.enumerate_slots_values(sorted_slots) {
-            out[pos] = v;
-        }
-    }
-
     /// Like [`BitSliceView::enumerate_nonzero_slots`]: `(position in slots,
     /// value)` for every nonzero value — a `filter_map` over `get`, not a
     /// new traversal.
@@ -370,6 +352,63 @@ impl<'a> IntSliceView<'a> {
     }
 }
 
+// ── NonzeroSlotsView / nonzero_triples ──────────────────────────────────────
+
+/// Column views (`BitSliceView`, `IntSliceView`) that can enumerate their
+/// nonzero slots as `(position in slots, value)`. Mirrors each type's own
+/// inherent `enumerate_nonzero_slots` — exists only so [`nonzero_triples`]
+/// can be written once and reused by every matrix type instead of
+/// reimplementing the same sort-then-scan-columns loop per format.
+pub(crate) trait NonzeroSlotsView: Sized {
+    fn enumerate_nonzero_slots<'b>(self, slots: &'b [usize]) -> impl Iterator<Item = (usize, u32)> + 'b
+    where
+        Self: 'b;
+}
+
+impl<'a> NonzeroSlotsView for BitSliceView<'a> {
+    #[inline]
+    fn enumerate_nonzero_slots<'b>(self, slots: &'b [usize]) -> impl Iterator<Item = (usize, u32)> + 'b
+    where
+        Self: 'b,
+    {
+        BitSliceView::enumerate_nonzero_slots(self, slots)
+    }
+}
+
+impl<'a> NonzeroSlotsView for IntSliceView<'a> {
+    #[inline]
+    fn enumerate_nonzero_slots<'b>(self, slots: &'b [usize]) -> impl Iterator<Item = (usize, u32)> + 'b
+    where
+        Self: 'b,
+    {
+        IntSliceView::enumerate_nonzero_slots(self, slots)
+    }
+}
+
+/// Shared body of every matrix `nonzero_iter`: sort `slots` once, scan each
+/// column with the sorted slice for mmap locality, remap positions back to
+/// the caller's original order. Generic over the column view type so
+/// `IntMatrix` and `PersistentBitMatrix` (Columnar/Packed) don't each carry
+/// their own copy of this traversal.
+pub(crate) fn nonzero_triples<V: NonzeroSlotsView>(
+    slots: &[usize],
+    n_cols: usize,
+    col_view: impl Fn(usize) -> V,
+) -> impl Iterator<Item = (usize, usize, u32)> {
+    let n = slots.len();
+    let mut perm: Vec<usize> = (0..n).collect();
+    perm.sort_by_key(|&i| slots[i]);
+    let sorted_slots: Vec<usize> = perm.iter().map(|&i| slots[i]).collect();
+    let mut hits: Vec<(usize, usize, u32)> = Vec::new();
+    for c in 0..n_cols {
+        hits.extend(
+            col_view(c).enumerate_nonzero_slots(&sorted_slots)
+                .map(|(pos, v)| (perm[pos], c, v)),
+        );
+    }
+    hits.into_iter()
+}
+
 // ── IntSliceViewIter ──────────────────────────────────────────────────────────
 
 pub struct IntSliceViewIter<'a> {