viirya commented on code in PR #9628:
URL: https://github.com/apache/arrow-rs/pull/9628#discussion_r3012710758
##########
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
+ /// standard filters use modular hashing: `index = h(x) mod m`, so `h(x)
mod (m/2)`
+ /// maps index `i` and index `i + m/2` to the same position.
+ ///
+ /// 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` (not `i` and `i + N/2`) map to the
same position `i`
+ /// in the folded filter.
+ ///
+ /// ## 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"
+ );
Review Comment:
assert!(len % 2 == 0)?
I think `fold_once` can only work if len is not odd.
--
This is an automated message from the Apache Git Service.
To respond to the message, please log on to GitHub and use the
URL above to go to the specific comment.
To unsubscribe, e-mail: [email protected]
For queries about this service, please contact Infrastructure at:
[email protected]
