emkornfield commented on code in PR #9628:
URL: https://github.com/apache/arrow-rs/pull/9628#discussion_r3012755964
##########
parquet/src/bloom_filter/mod.rs:
##########
@@ -431,6 +451,162 @@ 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)?
+ ///
+ /// Standard Bloom filter folding merges the two halves (`B[i] | B[i +
m/2]`) because
Review Comment:
nit: as an explanation it might pay to reverse, this I'm not sure whether
readers would commonly be aware of bloom filter folding. So it might be better
to explain why half first and then indicate why this is different then the
linked paper.
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