alamb commented on code in PR #9137:
URL: https://github.com/apache/arrow-rs/pull/9137#discussion_r2678872270
##########
arrow-buffer/src/builder/boolean.rs:
##########
@@ -259,6 +259,135 @@ impl BooleanBufferBuilder {
pub fn finish_cloned(&self) -> BooleanBuffer {
BooleanBuffer::new(Buffer::from_slice_ref(self.as_slice()), 0,
self.len)
}
+
+ /// Advances the buffer by `additional` bits without initializing the new
bytes.
+ ///
+ /// # Safety
+ /// Callers must ensure that all newly added bits are written before the
buffer is read.
+ #[inline]
+ unsafe fn advance_uninit(&mut self, additional: usize) {
+ let new_len = self.len + additional;
+ let new_len_bytes = bit_util::ceil(new_len, 8);
+ if new_len_bytes > self.buffer.len() {
+ self.buffer.reserve(new_len_bytes - self.buffer.len());
+ // SAFETY: caller will initialize all newly exposed bytes
+ unsafe { self.buffer.set_len(new_len_bytes) };
+ }
+ self.len = new_len;
+ }
+
+ /// Extends this builder with boolean values.
+ ///
+ /// This requires `iter` to report an exact size via `size_hint`.
+ ///
+ /// # Safety
+ /// Callers must ensure that `iter` reports an exact size via `size_hint`.
+ #[inline]
+ pub unsafe fn extend_trusted_len<I: Iterator<Item = bool>>(&mut self,
iter: I) {
+ let (lower, upper) = iter.size_hint();
+ let len = upper.expect("Iterator must have exact size_hint");
+ assert_eq!(lower, len, "Iterator must have exact size_hint");
+
+ if len == 0 {
+ return;
+ }
+
+ let start_len = self.len;
+ let end_bit = start_len + len;
+
+ // SAFETY: we will initialize all newly exposed bytes before they are
read
+ unsafe { self.advance_uninit(len) };
+ let slice = self.buffer.as_slice_mut();
+
+ let mut iter = iter;
+ let mut bit_idx = start_len;
+
+ // ---- Unaligned prefix: advance to the next 64-bit boundary ----
+ let misalignment = bit_idx & 63;
+ let prefix_bits = if misalignment == 0 {
+ 0
+ } else {
+ (64 - misalignment).min(end_bit - bit_idx)
+ };
+
+ if prefix_bits != 0 {
+ let byte_start = bit_idx / 8;
+ let byte_end = bit_util::ceil(bit_idx + prefix_bits, 8);
+ let bit_offset = bit_idx % 8;
+
+ // Clear any newly-visible bits in the existing partial byte
+ if bit_offset != 0 {
+ let keep_mask = (1u8 << bit_offset).wrapping_sub(1);
+ slice[byte_start] &= keep_mask;
+ }
+
+ // Zero any new bytes we will partially fill in this prefix
+ let zero_from = if bit_offset == 0 {
+ byte_start
+ } else {
+ byte_start + 1
+ };
+ if byte_end > zero_from {
+ slice[zero_from..byte_end].fill(0);
+ }
+
+ for _ in 0..prefix_bits {
+ let v = iter.next().unwrap();
+ if v {
+ let byte_idx = bit_idx / 8;
+ let bit = bit_idx % 8;
+ slice[byte_idx] |= 1 << bit;
+ }
+ bit_idx += 1;
+ }
+ }
+
+ if bit_idx < end_bit {
+ // ---- Aligned middle: write u64 chunks ----
+ debug_assert_eq!(bit_idx & 63, 0);
+ let remaining_bits = end_bit - bit_idx;
+ let chunks = remaining_bits / 64;
+
+ let words_start = bit_idx / 8;
+ let words_end = words_start + chunks * 8;
+ for dst in slice[words_start..words_end].chunks_exact_mut(8) {
Review Comment:
makes sense
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