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perf(builtin): count rev_foldi's index up instead of deriving it per iteration #4117

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Merged
bobzhang merged 1 commit into from
Aug 20, 2026
Merged

perf(builtin): count rev_foldi's index up instead of deriving it per iteration #4117

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15 changes: 12 additions & 3 deletions builtin/fixedarray.mbt
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Original file line number Diff line number Diff line change
Expand Up @@ -774,9 +774,8 @@ pub fn[A, B] FixedArray::rev_foldi(
init~ : B,
f : (Int, B, A) -> B raise?,
) -> B raise? {
let len = self.length()
for i in len>..0; acc = init {
continue f(len - i - 1, acc, self.unsafe_get(i))
for i in self.length()>..0; index = 0, acc = init {
continue index + 1, f(index, acc, self.unsafe_get(i))
} nobreak {
acc
}
Expand All @@ -797,6 +796,16 @@ test "rev_foldi" {
inspect(sum, content="25")
}

///|
test "rev_foldi preserves reversed index element pairs" {
let trace = ([10, 20, 30] : FixedArray[_]).rev_foldi(init=0, (
index,
acc,
elem,
) => acc * 1000 + index * 100 + elem)
inspect(trace, content="30120210")
}

///|
/// Reverses the array in place by swapping elements from both ends until
/// reaching the middle.
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