Jefffrey commented on code in PR #10813:
URL: https://github.com/apache/arrow-rs/pull/10813#discussion_r3890731200
##########
arrow-select/src/take.rs:
##########
@@ -456,43 +456,229 @@ fn take_native<T: ArrowNativeType, I:
ArrowPrimitiveType>(
}
}
+/// Read the bit at `src_bit_idx` from `src` and, if it is set, write a `1` to
`dst_bit_idx`
+/// in `dst`. Leaves `dst_bit_idx` unchanged (zero) when the source bit is
unset.
+///
+/// ```text
+/// src = 0b00100000 (bit 5 is set)
+/// copy_bit_if_set(src, 5, dst, 2) → dst bit 2 becomes 1
+/// ```
+///
+/// # Safety
+/// - `src` must be valid for reads up to byte `src_bit_idx / 8`.
+/// - `dst` must be valid for writes up to byte `dst_bit_idx / 8`.
+#[inline(always)]
+unsafe fn copy_bit_if_set(src: *const u8, src_bit_idx: usize, dst: *mut u8,
dst_bit_idx: usize) {
+ unsafe {
+ if bit_util::get_bit_raw(src, src_bit_idx) {
+ bit_util::set_bit_raw(dst, dst_bit_idx);
+ }
+ }
+}
+
+/// Read the bit at `bit_idx` from `src` and return it shifted to `out_pos`,
ready to be
+/// OR'd into an output byte accumulator.
+///
+/// ```text
+/// src = 0b10100000 (bit 5 is set)
+/// pack_bit(src, 5, 2) → 0b00000100 (bit from position 5, placed at
position 2)
+/// ```
+///
+/// # Safety
+/// `src` must be valid for reads up to byte `bit_idx / 8`.
+#[inline(always)]
+unsafe fn pack_bit(src: *const u8, bit_idx: usize, out_pos: usize) -> u8 {
+ let byte = unsafe { *src.add(bit_idx >> 3) }; // byte containing bit
`bit_idx`
+ ((byte >> (bit_idx & 7)) & 1) << out_pos // extract the bit, shift to
output position
+}
+
#[inline(never)]
fn take_bits<I: ArrowPrimitiveType, const CHECKED: bool>(
values: &BooleanBuffer,
indices: &PrimitiveArray<I>,
) -> BooleanBuffer {
let len = indices.len();
+ let src_offset = values.offset();
+ let src_ptr = values.values().as_ptr();
+ let out_bytes = len.div_ceil(8);
+
+ match indices.nulls().filter(|nulls| nulls.null_count() > 0) {
+ Some(index_nulls) => {
+ let mut output = vec![0u8; out_bytes];
+ let out_ptr = output.as_mut_ptr();
+ index_nulls.valid_indices().for_each(|valid_idx| {
+ let src_idx = if CHECKED {
+ indices.value(valid_idx).as_usize()
+ } else {
+ // SAFETY: valid_idx < len (validity bitmap); caller
guarantees index values are in bounds when CHECKED=false
+ unsafe { indices.value_unchecked(valid_idx) }.as_usize()
+ } + src_offset;
+ // SAFETY: src_idx bounded by take's prior bounds check
Review Comment:
i still dont follow in the checked case; itll panic if we go beyond bounds
for indices itself, but the value returned from indices itself (the `src_idx`)
isnt guaranteed to be in bounds of the array we're taking on, right?
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