Support zero-column mutable matrix iterators

diskann-utils/src/views/rowmajor.rs

@@ -6,9 +6,7 @@
 use std::{marker::PhantomData, mem::ManuallyDrop, num::NonZeroUsize, ptr::NonNull};
 
 #[cfg(feature = "rayon")]
-use rayon::prelude::{
-    IndexedParallelIterator, IntoParallelIterator, ParallelIterator, ParallelSliceMut,
-};
+use rayon::prelude::{IndexedParallelIterator, IntoParallelIterator, ParallelIterator};
 use thiserror::Error;
 
 pub mod iter;
@@ -525,20 +523,18 @@ pub unsafe trait MatrixMut: Matrix {
 
     /// Return a parallel iterator over the rows of the matrix.
     ///
-    /// # Panics
-    ///
-    /// Panics if `self.ncols() == 0 && self.nrows() != 0`.
+    /// A matrix with zero columns yields one empty mutable slice per row.
     #[cfg(feature = "rayon")]
     fn par_rows_mut(&mut self) -> impl IndexedParallelIterator<Item = &mut [Self::Element]>
     where
         Self::Element: Send,
     {
-        let ncols = self.ncols();
-        assert!(
-            ncols != 0 || self.nrows() == 0,
-            "`MatrixMut::par_rows_mut` does not support matrices with rows and zero columns"
-        );
-        self.as_mut_slice().par_chunks_exact_mut(ncols.max(1))
+        let matrix = iter::ParMut::new(self);
+        (0..matrix.nrows()).into_par_iter().map(move |row| {
+            // SAFETY: The range produces each in-bounds row exactly once. Distinct rows
+            // have disjoint element ranges; zero-column rows contain no elements.
+            unsafe { matrix.row_disjoint_unchecked(row) }
+        })
     }
 
     /// Return a parallel iterator that divides the matrix into mutable sub-matrices with
@@ -550,9 +546,12 @@ pub unsafe trait MatrixMut: Matrix {
     /// It is possible for yielded sub-matrices to have fewer than `batchsize` rows if the
     /// number of rows in the parent matrix is not evenly divisible by `batchsize`.
     ///
+    /// A matrix with zero columns yields zero-column sub-matrices containing up to
+    /// `batchsize` rows.
+    ///
     /// # Panics
     ///
-    /// Panics if `batchsize = 0` or `self.ncols() == 0 && self.nrows() != 0`.
+    /// Panics if `batchsize = 0`.
     #[cfg(feature = "rayon")]
     fn par_window_iter_mut(
         &mut self,
@@ -566,30 +565,17 @@ pub unsafe trait MatrixMut: Matrix {
             "par_window_iter_mut batchsize cannot be zero"
         );
 
-        let ncols = self.ncols();
-        assert!(
-            ncols != 0 || self.nrows() == 0,
-            "`MatrixMut::par_window_iter_mut` does not support matrices with rows and zero columns"
-        );
+        let matrix = iter::ParMut::new(self);
+        let nrows = matrix.nrows();
+        (0..nrows)
+            .into_par_iter()
+            .step_by(batchsize)
+            .map(move |start| {
+                let end = start.saturating_add(batchsize).min(nrows);
 
-        // Ensure that `batchsize * ncols` does not overflow.
-        let batchsize = batchsize.min(self.nrows());
-        self.as_mut_slice()
-            .par_chunks_mut((ncols * batchsize).max(1))
-            .map(move |data| {
-                let blobsize = data.len();
-                let nrows = blobsize / ncols;
-                assert_eq!(blobsize % ncols, 0);
-
-                // SAFETY:
-                //
-                // * `Layout::new_unchecked` is safe because `ncols` is the parent column
-                //   count and `nrows <= self.nrows()`, so this layout cannot exceed the
-                //   validated parent layout.
-                //
-                // * `Mut::from_data_unchecked` is safe because by construction,
-                //   `data.len() == ncols * nrows`.
-                unsafe { Mut::from_data_unchecked(data, Layout::new_unchecked(nrows, ncols)) }
+                // SAFETY: `start` comes from an in-bounds range and `end` is clamped to
+                // `nrows`. Stepping by `batchsize` makes the yielded ranges disjoint.
+                unsafe { matrix.window_disjoint_unchecked(start..end) }
             })
     }
 }
@@ -2325,7 +2311,7 @@ mod tests {
     fn parallel_mutable_iterators_match_scalar_indexing() {
         use rayon::prelude::*;
 
-        for (nrows, ncols) in [(0, 0), (0, 4), (1, 1), (1, 4), (4, 1), (5, 3)] {
+        for (nrows, ncols) in [(0, 0), (0, 4), (3, 0), (1, 1), (1, 4), (4, 1), (5, 3)] {
             let context = lazy_format!("nrows = {nrows}, ncols = {ncols}");
 
             let original = striped_matrix(nrows, ncols);
@@ -2392,26 +2378,6 @@ mod tests {
         }
     }
 
-    #[test]
-    #[cfg(feature = "rayon")]
-    #[should_panic(
-        expected = "`MatrixMut::par_rows_mut` does not support matrices with rows and zero columns"
-    )]
-    fn par_rows_mut_rejects_nonempty_zero_column_matrix() {
-        let mut m = striped_matrix(3, 0);
-        let _ = m.par_rows_mut();
-    }
-
-    #[test]
-    #[cfg(feature = "rayon")]
-    #[should_panic(
-        expected = "`MatrixMut::par_window_iter_mut` does not support matrices with rows and zero columns"
-    )]
-    fn par_window_iter_mut_rejects_nonempty_zero_column_matrix() {
-        let mut m = striped_matrix(3, 0);
-        let _ = m.par_window_iter_mut(2);
-    }
-
     #[test]
     fn matrix_iterators_match_scalar_indexing() {
         for (nrows, ncols) in [(0, 0), (0, 4), (3, 0), (1, 1), (1, 4), (4, 1), (5, 3)] {

diskann-utils/src/views/rowmajor/iter.rs

@@ -5,6 +5,8 @@
 
 use std::{marker::PhantomData, num::NonZeroUsize, ptr::NonNull};
 
+#[cfg(feature = "rayon")]
+use crate::views::rowmajor::MatrixMut;
 use crate::views::rowmajor::{Layout, Matrix, Mut, Ref};
 
 //------//
@@ -187,3 +189,153 @@ impl<'a, T> Iterator for Windows<'a, T> {
 
 impl<T> ExactSizeIterator for Windows<'_, T> {}
 impl<T> std::iter::FusedIterator for Windows<'_, T> {}
+
+//--------//
+// ParMut //
+//--------//
+
+#[cfg(feature = "rayon")]
+/// Carries an exclusive matrix borrow across Rayon workers.
+pub(super) struct ParMut<'a, T> {
+    ptr: NonNull<T>,
+    layout: Layout<T>,
+    _lifetime: PhantomData<&'a mut [T]>,
+}
+
+// SAFETY: `ParMut` owns an exclusive slice borrow, so sending it requires `T: Send`.
+#[cfg(feature = "rayon")]
+unsafe impl<T: Send> Send for ParMut<'_, T> {}
+// SAFETY: The only methods that produce mutable views are unsafe and require callers to
+// ensure disjointness. Sending those views between workers requires `T: Send`.
+#[cfg(feature = "rayon")]
+unsafe impl<T: Send> Sync for ParMut<'_, T> {}
+
+#[cfg(feature = "rayon")]
+impl<'a, T> ParMut<'a, T> {
+    pub(super) fn new<M>(matrix: &'a mut M) -> Self
+    where
+        M: MatrixMut<Element = T> + ?Sized,
+    {
+        let layout = matrix.layout();
+        let ptr = matrix.as_nonnull_mut();
+        Self {
+            ptr,
+            layout,
+            _lifetime: PhantomData,
+        }
+    }
+
+    pub(super) fn nrows(&self) -> usize {
+        self.layout.nrows()
+    }
+
+    /// # Safety
+    ///
+    /// * `row < self.nrows()`.
+    /// * No other live reference derived from this `ParMut` may overlap any element of
+    ///   row `row`. Zero-column rows never overlap, even when their addresses match.
+    pub(super) unsafe fn row_disjoint_unchecked(&self, row: usize) -> &'a mut [T] {
+        debug_assert!(row < self.layout.nrows());
+        let ncols = self.layout.ncols();
+
+        // SAFETY: The caller guarantees that `row` is in-bounds and does not overlap any
+        // other live view. The validated parent layout makes the offset representable and
+        // places the row within the initialized matrix span.
+        unsafe { std::slice::from_raw_parts_mut(self.ptr.as_ptr().add(row * ncols), ncols) }
+    }
+
+    /// # Safety
+    ///
+    /// * `rows.start <= rows.end <= self.nrows()`.
+    /// * No other live reference derived from this `ParMut` may overlap any element in
+    ///   `rows`. Zero-column windows never overlap.
+    pub(super) unsafe fn window_disjoint_unchecked(
+        &self,
+        rows: std::ops::Range<usize>,
+    ) -> Mut<'a, T> {
+        debug_assert!(rows.start <= rows.end);
+        debug_assert!(rows.end <= self.layout.nrows());
+
+        let ncols = self.layout.ncols();
+        let nrows = rows.end - rows.start;
+
+        // SAFETY: The caller guarantees an ordered, in-bounds range. The validated parent
+        // layout makes the offset representable and places it within or one past the matrix.
+        let ptr = unsafe { self.ptr.add(rows.start * ncols) };
+
+        Mut {
+            ptr,
+            // SAFETY: This window has no more rows than the validated parent and keeps its
+            // column count, so its element count and byte span cannot exceed the parent.
+            layout: unsafe { Layout::new_unchecked(nrows, ncols) },
+            _lifetime: PhantomData,
+        }
+    }
+}
+
+#[cfg(all(test, feature = "rayon"))]
+mod tests {
+    use super::ParMut;
+    use crate::views::rowmajor::{Matrix, MatrixMut, Owned};
+
+    #[test]
+    fn par_mut_zero_column_views_can_coexist() {
+        let mut matrix = Owned::from_element(usize::MAX, 0, 0);
+        let ptr = matrix.as_ptr();
+        let matrix = ParMut::new(&mut matrix);
+
+        // SAFETY: Empty views do not overlap any elements, even when their addresses match.
+        let rows = unsafe {
+            [
+                matrix.row_disjoint_unchecked(0),
+                matrix.row_disjoint_unchecked(1),
+                matrix.row_disjoint_unchecked(usize::MAX - 1),
+            ]
+        };
+        // SAFETY: Empty windows do not overlap any elements, including the live row views
+        // whose logical rows fall within these windows.
+        let windows = unsafe {
+            [
+                matrix.window_disjoint_unchecked(0..2),
+                matrix.window_disjoint_unchecked(2..usize::MAX),
+            ]
+        };
+
+        assert!(rows.iter().all(|row| row.is_empty() && row.as_ptr() == ptr));
+        assert!(windows.iter().all(|window| {
+            window.ncols() == 0 && window.as_slice().is_empty() && window.as_ptr() == ptr
+        }));
+    }
+
+    #[test]
+    fn par_mut_disjoint_nonempty_views_can_coexist() {
+        let mut matrix = Owned::from_fn(6, 2, |rc| rc.row * 100 + rc.col);
+        {
+            let matrix = ParMut::new(&mut matrix);
+
+            // SAFETY: These rows and windows are in-bounds and pairwise disjoint.
+            let (row0, row1, mut window2, mut window4) = unsafe {
+                (
+                    matrix.row_disjoint_unchecked(0),
+                    matrix.row_disjoint_unchecked(1),
+                    matrix.window_disjoint_unchecked(2..4),
+                    matrix.window_disjoint_unchecked(4..6),
+                )
+            };
+            row0[0] = 10;
+            row1[1] = 11;
+
+            let mut window2_rows: Vec<_> = window2.rows_mut().collect();
+            let mut window4_rows: Vec<_> = window4.rows_mut().collect();
+            window2_rows[0][0] = 20;
+            window2_rows[1][1] = 31;
+            window4_rows[0][0] = 40;
+            window4_rows[1][1] = 51;
+        }
+
+        assert_eq!(
+            matrix.as_slice(),
+            &[10, 1, 100, 11, 20, 201, 300, 31, 40, 401, 500, 51]
+        );
+    }
+}