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perf(hashset): fix from_array capacity and make shift_back iterative #3334

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perf(hashset): fix from_array capacity and make shift_back iterative #3334

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perf(hashset): fix from_array capacity and make shift_back iterative
bobzhang Mar 23, 2026
138342f
fix: use overflow-safe capacity calculation in from_array
bobzhang Mar 23, 2026
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7 changes: 6 additions & 1 deletion hashset/hashset.mbt
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Original file line number Diff line number Diff line change
Expand Up @@ -37,7 +37,12 @@ pub fn[K] HashSet::new(capacity? : Int = default_init_capacity) -> HashSet[K] {
#alias(of, deprecated="Use from_array instead")
#as_free_fn(of, deprecated="Use from_array instead")
pub fn[K : Hash + Eq] HashSet::from_array(arr : ArrayView[K]) -> HashSet[K] {
let m = new()
let length = arr.length()
let mut capacity = length.next_power_of_two()
if length >= calc_grow_threshold(capacity) {
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@devin-ai-integration devin-ai-integration Bot Apr 14, 2026

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🟡 Off-by-one: >= should be > in capacity pre-allocation check, causing unnecessary doubling

The condition length >= calc_grow_threshold(capacity) at line 42 uses >= instead of >. Since add_with_hash (hashset.mbt:80) checks self.size >= self.grow_at before inserting, the set can hold exactly grow_at elements without triggering growth. Therefore, length == calc_grow_threshold(capacity) does not require a capacity increase. The equivalent Set::from_array in set/linked_hash_set.mbt:80 correctly uses >. For example, with length=6 and capacity=8, calc_grow_threshold(8) = 6, and 6 elements fit in capacity 8 without triggering growth—but the >= condition unnecessarily doubles capacity to 16.

Suggested change
if length >= calc_grow_threshold(capacity) {
if length > calc_grow_threshold(capacity) {
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capacity = capacity * 2
}
let m = new(capacity~)
arr.each(e => m.add(e))
m
}
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