Optimize sparse bit matrix packing to avoid intermediate copies

Refactor `pack_sparse_bit_matrix` to conditionally open either a packed or columnar dense representation based on file existence. This eliminates the previous unconditional pre-conversion step that forced a full packed copy. Update cleanup logic and add tests to verify direct transposition, proper file handling, and idempotency for both formats.

src/obicompactvec/src/bitmatrix/sparse.rs

@@ -34,6 +34,8 @@ use crate::meta::field;
 use crate::rankselect::{PersistentRankSelectBitVec, PersistentRankSelectBitVecBuilder};
 
 use super::PersistentBitMatrix;
+use super::col_path;
+use super::columnar::ColumnarBitMatrix;
 use super::packed::PackedBitMatrix;
 
 fn is_multi_path(dir: &Path) -> PathBuf { dir.join("is_multi.prsb") }
@@ -485,24 +487,33 @@ impl BitPartials for PersistentSparseBitMatrix {
 /// convention (old-format files removed only after the new format is
 /// fully written, so a crash mid-conversion leaves the previous, still
 /// valid format rather than a half-written one). Idempotent — does
-/// nothing if `is_multi.prsb` already exists. Packs to dense
-/// (`matrix.pbmx`) first via [`super::pack_bit_matrix`] if that hasn't
-/// happened yet, since the dense→sparse transpose (`build_from_dense`)
-/// needs random row access, which only the packed/columnar dense forms
-/// give — not a further reason to keep the dense file around afterward.
+/// nothing if `is_multi.prsb` already exists.
+///
+/// The dense→sparse transpose (`build_from_dense`) only needs `fill_row`,
+/// which costs the same `O(n_cols)` per row on `Columnar` as on `Packed`
+/// (see `ColumnarBitMatrix::fill_row`) — so this reads straight from
+/// whichever dense form is already on disk. In particular, when this runs
+/// right after a build (the common case: `presence/` is still `Columnar`,
+/// `matrix.pbmx` doesn't exist yet), it never materialises a full packed
+/// copy of the presence matrix just to read it back once and delete it.
 pub fn pack_sparse_bit_matrix(dir: &Path) -> io::Result<()> {
     if is_multi_path(dir).exists() {
         return Ok(());
     }
 
     let packed_path = dir.join("matrix.pbmx");
-    if !packed_path.exists() {
-        super::pack_bit_matrix(dir)?;
+    if packed_path.exists() {
+        let dense = PersistentBitMatrix::Packed(PackedBitMatrix::open(&packed_path)?);
+        PersistentSparseBitMatrixBuilder::build_from_dense(&dense, dir)?.close()?;
+        drop(dense);
+        fs::remove_file(&packed_path)?;
+    } else {
+        let dense = PersistentBitMatrix::Columnar(ColumnarBitMatrix::open(dir)?);
+        let n_cols = dense.n_cols();
+        PersistentSparseBitMatrixBuilder::build_from_dense(&dense, dir)?.close()?;
+        drop(dense);
+        for c in 0..n_cols { let _ = fs::remove_file(col_path(dir, c)); }
+        let _ = fs::remove_file(dir.join("meta.json"));
     }
-
-    let dense = PersistentBitMatrix::Packed(PackedBitMatrix::open(&packed_path)?);
-    PersistentSparseBitMatrixBuilder::build_from_dense(&dense, dir)?.close()?;
-    drop(dense);
-    fs::remove_file(&packed_path)?;
     Ok(())
 }

src/obicompactvec/src/tests/sparse.rs

@@ -1,6 +1,6 @@
 use tempfile::tempdir;
 
-use crate::{BinaryMatrix, PersistentBitMatrix, PersistentBitMatrixBuilder, PersistentSparseBitMatrix, PersistentSparseBitMatrixBuilder};
+use crate::{pack_bit_matrix, pack_sparse_bit_matrix, BinaryMatrix, PersistentBitMatrix, PersistentBitMatrixBuilder, PersistentSparseBitMatrix, PersistentSparseBitMatrixBuilder};
 
 /// Builds a dense `PersistentBitMatrix` from column-major `bool` data —
 /// mirrors `tests/bitmatrix.rs`'s own `make_matrix` helper.
@@ -307,3 +307,68 @@ fn reopen_large_real_shaped_data_with_heavy_dedup() {
         assert_eq!(&row_as_bool(&m, slot), expected, "row {slot}");
     }
 }
+
+/// Common case: `pack_sparse_bit_matrix` runs right after a build, while
+/// `presence/` is still `Columnar` (`matrix.pbmx` doesn't exist yet). Must
+/// transpose straight from the per-column `.pbiv` files — never write a
+/// full packed copy just to read it back and delete it.
+#[test]
+fn pack_sparse_bit_matrix_transposes_columnar_directly() {
+    let cols: &[&[bool]] = &[
+        &[true, false, true, false],
+        &[true, true, false, false],
+        &[false, true, true, true],
+    ];
+    let (dir, dense) = make_dense(cols);
+    let presence_dir = dir.path().join("presence");
+    assert!(!presence_dir.join("matrix.pbmx").exists(), "still Columnar before packing");
+
+    let expected_rows: Vec<Box<[bool]>> = (0..dense.n()).map(|s| dense.row(s)).collect();
+    let n_cols = dense.n_cols();
+    drop(dense);
+
+    pack_sparse_bit_matrix(&presence_dir).unwrap();
+
+    assert!(presence_dir.join("is_multi.prsb").exists());
+    assert!(!presence_dir.join("meta.json").exists(), "columnar meta.json cleaned up");
+    assert!(!presence_dir.join("col_000000.pbiv").exists(), "columnar column files cleaned up");
+    assert!(!presence_dir.join("matrix.pbmx").exists(), "never materialised");
+
+    let sparse = PersistentSparseBitMatrix::open(&presence_dir).unwrap();
+    assert_eq!(sparse.n(), expected_rows.len());
+    assert_eq!(sparse.n_cols(), n_cols);
+    for (slot, expected) in expected_rows.iter().enumerate() {
+        assert_eq!(&*sparse.row(slot), &**expected, "slot {slot}");
+    }
+
+    // Idempotent — a second call is a no-op, doesn't error.
+    pack_sparse_bit_matrix(&presence_dir).unwrap();
+}
+
+/// Regression for the pre-existing path: if `matrix.pbmx` is already there
+/// (e.g. `pack_bit_matrix` ran earlier for a non-sparse consumer), it's
+/// still used as the transpose source and cleaned up afterward.
+#[test]
+fn pack_sparse_bit_matrix_from_already_packed_matrix() {
+    let cols: &[&[bool]] = &[
+        &[true, false, true],
+        &[false, true, true],
+    ];
+    let (dir, dense) = make_dense(cols);
+    let presence_dir = dir.path().join("presence");
+    let expected_rows: Vec<Box<[bool]>> = (0..dense.n()).map(|s| dense.row(s)).collect();
+    let n_cols = dense.n_cols();
+    drop(dense);
+
+    pack_bit_matrix(&presence_dir).unwrap();
+    assert!(presence_dir.join("matrix.pbmx").exists());
+
+    pack_sparse_bit_matrix(&presence_dir).unwrap();
+    assert!(!presence_dir.join("matrix.pbmx").exists(), "packed intermediate cleaned up");
+
+    let sparse = PersistentSparseBitMatrix::open(&presence_dir).unwrap();
+    assert_eq!(sparse.n_cols(), n_cols);
+    for (slot, expected) in expected_rows.iter().enumerate() {
+        assert_eq!(&*sparse.row(slot), &**expected, "slot {slot}");
+    }
+}