alamb commented on a change in pull request #585:
URL: https://github.com/apache/arrow-rs/pull/585#discussion_r676125985
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File path: arrow/src/compute/kernels/partition.rs
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@@ -73,6 +73,30 @@ impl<'a> LexicographicalPartitionIterator<'a> {
}
}
+/// Exponential search is to remedy for the case when array size and
cardinality are both large
+/// see <https://en.wikipedia.org/wiki/Exponential_search>
+#[inline]
+fn exponential_search(
+ indices: &[usize],
+ target: &usize,
+ comparator: &LexicographicalComparator<'_>,
+) -> usize {
+ let mut bound = 1;
+ while bound < indices.len()
+ && comparator.compare(&indices[bound], target) != Ordering::Greater
+ {
+ bound *= 2;
+ }
+ // invariant after while loop:
+ // indices[bound / 2] <= target < indices[min(indices.len(), bound + 1)]
+ // where <= and < are defined by the comparator;
+ // note here we have right = min(indices.len(), bound + 1) because
indices[bound] might
+ // actually be considered and must be included.
+ (bound / 2)
+ + indices[(bound / 2)..indices.len().min(bound + 1)]
Review comment:
I wonder if some of the performance improvement also comes from
potentially making a smaller sequence -- aka `(bound/2) .. len` rather than
`partition_point .. len`.
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