[GitHub] [arrow-rs] Jimexist commented on a change in pull request #585: use exponential search in lexico partition to speed up

[GitBox <gi...@apache.org>]

Jimexist commented on a change in pull request #585:
URL: https://github.com/apache/arrow-rs/pull/585#discussion_r674016802



##########
File path: arrow/src/compute/kernels/partition.rs
##########
@@ -73,6 +73,25 @@ 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;
+    }
+    (bound / 2)

Review comment:
       invariant:
   
   `indices[bound / 2] <= target < indices[min(indices.len(), bound + 1)]` where `<=` and `<` are defined by the `comparator`; note the `bound + 1` because `while` might exit when `target = indices[bound]`




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