emkornfield commented on code in PR #9628:
URL: https://github.com/apache/arrow-rs/pull/9628#discussion_r3024559937
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parquet/src/bloom_filter/mod.rs:
##########
@@ -431,6 +485,150 @@ impl Sbbf {
self.0.capacity() * std::mem::size_of::<Block>()
}
+ /// Returns the number of blocks in this bloom filter.
+ pub fn num_blocks(&self) -> usize {
+ self.0.len()
+ }
+
+ /// Fold the bloom filter once, halving its size by merging adjacent block
pairs.
+ ///
+ /// This implements an elementary folding operation for Split Block Bloom
+ /// Filters. Each pair of adjacent blocks is combined via bitwise OR:
+ ///
+ /// ```text
+ /// folded[i] = blocks[2*i] | blocks[2*i + 1] for 0 <= i < num_blocks/2
+ /// ```
+ ///
+ /// ## Why adjacent pairs (not halves)?
+ ///
+ /// SBBFs use **multiplicative** hashing for block selection:
+ ///
+ /// ```text
+ /// block_index = ((hash >> 32) * num_blocks) >> 32
+ /// ```
+ ///
+ /// When `num_blocks` is halved, the new index becomes
`floor(original_index / 2)`.
+ /// Therefore blocks `2i` and `2i+1` map to the same position `i` in the
folded filter.
+ ///
+ /// This differs from standard Bloom filter folding, which merges the two
halves
+ /// (`B[i] | B[i + m/2]`) because standard filters use modular hashing
where
+ /// `h(x) mod (m/2)` maps indices `i` and `i + m/2` to the same position.
+ ///
+ /// ## References
+ ///
+ /// 1. Sailhan, F. & Stehr, M-O. "Folding and Unfolding Bloom Filters",
+ /// IEEE iThings 2012. <https://doi.org/10.1109/GreenCom.2012.16>
+ ///
+ /// # Panics
+ ///
+ /// Panics if the filter has fewer than 2 blocks.
+ fn fold_once(&mut self) {
+ let len = self.0.len();
+ assert!(
+ len >= 2,
+ "Cannot fold a bloom filter with fewer than 2 blocks"
+ );
+ assert!(
+ len % 2 == 0,
+ "Cannot fold a bloom filter with an odd number of blocks"
+ );
+ let half = len / 2;
+ for i in 0..half {
+ // Copy both blocks to stack locals before writing. This eliminates
+ // aliasing between self.0[i] and self.0[2*i], allowing the
compiler
+ // to vectorize the OR into SIMD instructions.
+ let merged = self.0[2 * i] | self.0[2 * i + 1];
+ self.0[i] = merged;
+ }
+ self.0.truncate(half);
+ }
+
+ /// Estimate the FPP that would result from folding once, without mutating
the filter.
+ ///
+ /// SBBF membership checks are per-block (`k=8` bit checks within one
256-bit block),
+ /// so FPP is the average per-block false positive probability:
+ ///
+ /// ```text
+ /// FPP = (1 / num_blocks) * sum_i (set_bits_in_block_i / 256)^8
+ /// ```
+ ///
+ /// Separating the OR from the popcount lets the compiler emit vectorized
popcount
+ /// instructions (e.g., `cnt.16b` on ARM NEON) instead of per-word scalar
popcount.
+ fn estimated_fpp_after_fold(&self) -> f64 {
+ let half = self.0.len() as f64 / 2.0;
+ let mut total_fpp: f64 = 0.0;
+ for pair in self.0.chunks_exact(2) {
Review Comment:
nit: either have a remainder block here, or assert len() is evenly divisible
by 2?
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