feat: aligned chunked pair iter (#6777)

Define pairwise chunked array iterator and use it in zip

---------

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

Author: joseph-isaacs

vortex-array/src/arrays/chunked/array.rs

@@ -107,8 +107,8 @@ impl ChunkedArray {
     #[inline]
     pub fn chunk(&self, idx: usize) -> &ArrayRef {
         assert!(idx < self.nchunks(), "chunk index {idx} out of bounds");
-
-        &self.chunks[idx]
+        // SAFETY: bounds checked by the assert above.
+        unsafe { self.chunks.get_unchecked(idx) }
     }
 
     pub fn nchunks(&self) -> usize {

vortex-array/src/arrays/chunked/compute/zip.rs

@@ -29,39 +29,10 @@ impl ZipKernel for Chunked {
             .union_nullability(if_false.dtype().nullability());
         let mut out_chunks = Vec::with_capacity(if_true.nchunks() + if_false.nchunks());
 
-        let mut lhs_idx = 0;
-        let mut rhs_idx = 0;
-        let mut lhs_offset = 0;
-        let mut rhs_offset = 0;
-        let mut pos = 0;
-        let total_len = if_true.len();
-
-        while pos < total_len {
-            let lhs_chunk = if_true.chunk(lhs_idx);
-            let rhs_chunk = if_false.chunk(rhs_idx);
-
-            let lhs_rem = lhs_chunk.len() - lhs_offset;
-            let rhs_rem = rhs_chunk.len() - rhs_offset;
-            let take_until = lhs_rem.min(rhs_rem);
-
-            let mask_slice = mask.slice(pos..pos + take_until)?;
-            let lhs_slice = lhs_chunk.slice(lhs_offset..lhs_offset + take_until)?;
-            let rhs_slice = rhs_chunk.slice(rhs_offset..rhs_offset + take_until)?;
-
-            out_chunks.push(mask_slice.zip(lhs_slice, rhs_slice)?);
-
-            pos += take_until;
-            lhs_offset += take_until;
-            rhs_offset += take_until;
-
-            if lhs_offset == lhs_chunk.len() {
-                lhs_idx += 1;
-                lhs_offset = 0;
-            }
-            if rhs_offset == rhs_chunk.len() {
-                rhs_idx += 1;
-                rhs_offset = 0;
-            }
+        for pair in if_true.paired_chunks(if_false) {
+            let pair = pair?;
+            let mask_slice = mask.slice(pair.pos)?;
+            out_chunks.push(mask_slice.zip(pair.left, pair.right)?);
         }
 
         // SAFETY: chunks originate from zipping slices of inputs that share dtype/nullability.

vortex-array/src/arrays/chunked/mod.rs

@@ -5,6 +5,7 @@ mod array;
 pub use array::ChunkedArray;
 
 pub(crate) mod compute;
+mod paired_chunks;
 
 mod vtable;
 pub use vtable::Chunked;

vortex-array/src/arrays/chunked/paired_chunks.rs

@@ -0,0 +1,265 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright the Vortex contributors
+
+use std::ops::Range;
+
+use vortex_error::VortexResult;
+
+use crate::ArrayRef;
+use crate::arrays::ChunkedArray;
+
+pub(crate) struct AlignedPair {
+    pub left: ArrayRef,
+    pub right: ArrayRef,
+    pub pos: Range<usize>,
+}
+
+/// Cursor over a chunk slice that maintains the invariant: `idx` always
+/// points at a non-empty chunk or is past the end.
+struct ChunkCursor<'a> {
+    chunks: &'a [ArrayRef],
+    idx: usize,
+    offset: usize,
+}
+
+impl<'a> ChunkCursor<'a> {
+    fn new(chunks: &'a [ArrayRef]) -> Self {
+        let mut cursor = Self {
+            chunks,
+            idx: 0,
+            offset: 0,
+        };
+        cursor.skip_empty();
+        cursor
+    }
+
+    fn skip_empty(&mut self) {
+        while self.idx < self.chunks.len()
+            && unsafe { self.chunks.get_unchecked(self.idx) }.is_empty()
+        {
+            self.idx += 1;
+        }
+    }
+
+    fn current_chunk(&self) -> Option<&'a ArrayRef> {
+        (self.idx < self.chunks.len()).then(|| unsafe { self.chunks.get_unchecked(self.idx) })
+    }
+
+    fn remaining(&self, chunk: &ArrayRef) -> usize {
+        chunk.len() - self.offset
+    }
+
+    fn take(&mut self, chunk: &ArrayRef, n: usize) -> VortexResult<ArrayRef> {
+        let slice = chunk.slice(self.offset..self.offset + n)?;
+        self.offset += n;
+        if self.offset == chunk.len() {
+            self.idx += 1;
+            self.offset = 0;
+            self.skip_empty();
+        }
+        Ok(slice)
+    }
+}
+
+pub(crate) struct PairedChunks<'a> {
+    left: ChunkCursor<'a>,
+    right: ChunkCursor<'a>,
+    pos: usize,
+    total_len: usize,
+}
+
+impl ChunkedArray {
+    pub(crate) fn paired_chunks<'a>(&'a self, other: &'a ChunkedArray) -> PairedChunks<'a> {
+        assert_eq!(
+            self.len(),
+            other.len(),
+            "paired_chunks requires arrays of equal length"
+        );
+        PairedChunks {
+            left: ChunkCursor::new(&self.chunks),
+            right: ChunkCursor::new(&other.chunks),
+            pos: 0,
+            total_len: self.len(),
+        }
+    }
+}
+
+impl Iterator for PairedChunks<'_> {
+    type Item = VortexResult<AlignedPair>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.pos >= self.total_len {
+            return None;
+        }
+
+        let lhs_chunk = self.left.current_chunk()?;
+        let rhs_chunk = self.right.current_chunk()?;
+
+        let take = self
+            .left
+            .remaining(lhs_chunk)
+            .min(self.right.remaining(rhs_chunk));
+
+        let (lhs_slice, rhs_slice) = match self
+            .left
+            .take(lhs_chunk, take)
+            .and_then(|l| self.right.take(rhs_chunk, take).map(|r| (l, r)))
+        {
+            Ok(pair) => pair,
+            Err(e) => return Some(Err(e)),
+        };
+
+        let start = self.pos;
+        self.pos += take;
+
+        Some(Ok(AlignedPair {
+            left: lhs_slice,
+            right: rhs_slice,
+            pos: start..self.pos,
+        }))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use vortex_buffer::buffer;
+    use vortex_error::VortexResult;
+
+    use crate::IntoArray;
+    use crate::arrays::ChunkedArray;
+    use crate::dtype::DType;
+    use crate::dtype::Nullability;
+    use crate::dtype::PType;
+
+    fn i32_dtype() -> DType {
+        DType::Primitive(PType::I32, Nullability::NonNullable)
+    }
+
+    #[allow(clippy::type_complexity)]
+    fn collect_pairs(
+        left: &ChunkedArray,
+        right: &ChunkedArray,
+    ) -> VortexResult<Vec<(Vec<i32>, Vec<i32>, std::ops::Range<usize>)>> {
+        use crate::ToCanonical;
+        let mut result = Vec::new();
+        for pair in left.paired_chunks(right) {
+            let pair = pair?;
+            let l: Vec<i32> = pair.left.to_primitive().as_slice::<i32>().to_vec();
+            let r: Vec<i32> = pair.right.to_primitive().as_slice::<i32>().to_vec();
+            result.push((l, r, pair.pos));
+        }
+        Ok(result)
+    }
+
+    #[test]
+    fn test_aligned_chunks() -> VortexResult<()> {
+        let left = ChunkedArray::try_new(
+            vec![buffer![1i32, 2].into_array(), buffer![3i32, 4].into_array()],
+            i32_dtype(),
+        )?;
+        let right = ChunkedArray::try_new(
+            vec![
+                buffer![10i32, 20].into_array(),
+                buffer![30i32, 40].into_array(),
+            ],
+            i32_dtype(),
+        )?;
+
+        let pairs = collect_pairs(&left, &right)?;
+        assert_eq!(pairs.len(), 2);
+        assert_eq!(pairs[0], (vec![1, 2], vec![10, 20], 0..2));
+        assert_eq!(pairs[1], (vec![3, 4], vec![30, 40], 2..4));
+        Ok(())
+    }
+
+    #[test]
+    fn test_misaligned_chunks() -> VortexResult<()> {
+        let left = ChunkedArray::try_new(
+            vec![
+                buffer![1i32, 2].into_array(),
+                buffer![3i32].into_array(),
+                buffer![4i32, 5].into_array(),
+            ],
+            i32_dtype(),
+        )?;
+        let right = ChunkedArray::try_new(
+            vec![
+                buffer![10i32].into_array(),
+                buffer![20i32, 30].into_array(),
+                buffer![40i32, 50].into_array(),
+            ],
+            i32_dtype(),
+        )?;
+
+        let pairs = collect_pairs(&left, &right)?;
+        assert_eq!(pairs.len(), 4);
+        assert_eq!(pairs[0], (vec![1], vec![10], 0..1));
+        assert_eq!(pairs[1], (vec![2], vec![20], 1..2));
+        assert_eq!(pairs[2], (vec![3], vec![30], 2..3));
+        assert_eq!(pairs[3], (vec![4, 5], vec![40, 50], 3..5));
+        Ok(())
+    }
+
+    #[test]
+    fn test_empty_chunks() -> VortexResult<()> {
+        let left = ChunkedArray::try_new(
+            vec![
+                buffer![0i32; 0].into_array(),
+                buffer![1i32, 2, 3].into_array(),
+            ],
+            i32_dtype(),
+        )?;
+        let right = ChunkedArray::try_new(
+            vec![
+                buffer![10i32, 20, 30].into_array(),
+                buffer![0i32; 0].into_array(),
+            ],
+            i32_dtype(),
+        )?;
+
+        let pairs = collect_pairs(&left, &right)?;
+        assert_eq!(pairs.len(), 1);
+        assert_eq!(pairs[0], (vec![1, 2, 3], vec![10, 20, 30], 0..3));
+        Ok(())
+    }
+
+    #[test]
+    fn test_single_element_chunks() -> VortexResult<()> {
+        let left = ChunkedArray::try_new(
+            vec![
+                buffer![1i32].into_array(),
+                buffer![2i32].into_array(),
+                buffer![3i32].into_array(),
+            ],
+            i32_dtype(),
+        )?;
+        let right = ChunkedArray::try_new(vec![buffer![10i32, 20, 30].into_array()], i32_dtype())?;
+
+        let pairs = collect_pairs(&left, &right)?;
+        assert_eq!(pairs.len(), 3);
+        assert_eq!(pairs[0], (vec![1], vec![10], 0..1));
+        assert_eq!(pairs[1], (vec![2], vec![20], 1..2));
+        assert_eq!(pairs[2], (vec![3], vec![30], 2..3));
+        Ok(())
+    }
+
+    #[test]
+    fn test_both_empty() -> VortexResult<()> {
+        let left = ChunkedArray::try_new(vec![], i32_dtype())?;
+        let right = ChunkedArray::try_new(vec![], i32_dtype())?;
+
+        let pairs = collect_pairs(&left, &right)?;
+        assert!(pairs.is_empty());
+        Ok(())
+    }
+
+    #[test]
+    #[should_panic(expected = "paired_chunks requires arrays of equal length")]
+    fn test_length_mismatch_panics() {
+        let left = ChunkedArray::try_new(vec![buffer![1i32, 2].into_array()], i32_dtype()).unwrap();
+        let right =
+            ChunkedArray::try_new(vec![buffer![10i32, 20, 30].into_array()], i32_dtype()).unwrap();
+
+        drop(left.paired_chunks(&right).collect::<Vec<_>>());
+    }
+}