fix: slice for zstd alignment

fixes: #5715




Signed-off-by: Alexander Droste <[email protected]>

Author: 0ax1

encodings/zstd/src/array.rs

@@ -330,6 +330,11 @@ impl ZstdArray {
                 .get(i + 1)
                 .copied()
                 .unwrap_or(value_bytes.len());
+
+            // Use slice_unaligned instead of slice to handle cases where frame boundaries
+            // don't align with the buffer's alignment requirements (e.g., 64-byte aligned
+            // buffers with 1-byte frame boundaries). ZSTD compression works on raw bytes
+            // and doesn't require memory alignment.
             let uncompressed = &value_bytes.slice(frame_byte_starts[i]..frame_byte_end);
             let compressed = compressor
                 .compress(uncompressed)

encodings/zstd/src/test.rs

@@ -9,6 +9,7 @@ use vortex_array::arrays::VarBinViewArray;
 use vortex_array::assert_arrays_eq;
 use vortex_array::validity::Validity;
 use vortex_array::vtable::ValidityHelper;
+use vortex_buffer::Alignment;
 use vortex_buffer::Buffer;
 use vortex_dtype::DType;
 use vortex_dtype::Nullability;
@@ -202,3 +203,16 @@ fn test_sliced_array_children() {
     let sliced = compressed.slice(0..4);
     sliced.children();
 }
+
+#[test]
+fn test_zstd_alignment_issue() {
+    // Scenario where a u8 array has a 64-byte alignment.
+    // See: https://github.com/vortex-data/vortex/issues/5715
+    let data = vec![0u8; 2];
+
+    let aligned_buffer = Buffer::copy_from_aligned(&data, Alignment::new(8));
+    let array = PrimitiveArray::new(aligned_buffer, Validity::NonNullable);
+    let compressed = ZstdArray::from_primitive(&array, 0, 1);
+
+    assert!(compressed.is_ok());
+}