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Possible fix for #3: bounds check in heap_sort::sink #6

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Possible fix for #3: bounds check in heap_sort::sink #6

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2 changes: 1 addition & 1 deletion algorithmica/src/sort/heap_sort.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -39,7 +39,7 @@ pub fn sink<T: Ord, F: Fn(&T, &T) -> bool>(arr: &mut [T], k: usize, n: usize, p:
let mut k = k;
while 2 * k < n {
let mut j = 2 * k + 1;
if j < n && p(&arr[j], &arr[j + 1]) {
if j + 1 < n && p(&arr[j], &arr[j + 1]) {
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Copilot AI Apr 27, 2026

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The n parameter is currently used as an inclusive last index (e.g., sort_by passes arr.len() - 1 and decrements l before calling sink). With that contract, the right-child-exists check should be j < n (so j + 1 <= n is in-bounds). Changing this to j + 1 < n skips the right child when it is exactly at index n (e.g., n=6, k=2 => left=5 right=6), which can break the heap property and lead to incorrect sorting. Either revert this condition back to j < n, or change sink’s contract to treat n as an exclusive upper bound/length and update all call sites (sort_by, loop over l, and tests) + add a regression test for the boundary case where right child is at the last index.

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if j + 1 < n && p(&arr[j], &arr[j + 1]) {
if j < n && p(&arr[j], &arr[j + 1]) {

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j += 1;
}
if !p(&arr[j], &arr[k]) {
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