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[go1.20] Repeat go1.19 strings.Repeat for go1.20 #1324

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Merged
merged 1 commit into from
Jul 17, 2024
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[go1.20] Repeat go1.19 strings.Repeat for go1.20 #1324

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45 changes: 45 additions & 0 deletions compiler/natives/src/strings/strings.go
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Original file line number Diff line number Diff line change
Expand Up @@ -73,3 +73,48 @@ func Clone(s string) string {
// memory overheads and simply return the string as-is.
return s
}

// Repeat is the go1.19 implementation of strings.Repeat.
//
// In the go1.20 implementation, the function was changed to use chunks that
// are 8KB in size to improve speed and cache access. This change is faster
// when running native Go code. However, for GopherJS, the change is much
// slower than the go1.19 implementation.
//
// The go1.20 change made tests like encoding/pem TestCVE202224675 take
// significantly longer to run for GopherJS.
// go1.19 concatenates 24 times and the test takes about 8 seconds.
// go1.20 concatenates about 15000 times and can take over a hour.
//
// We can't use `js.InternalObject(s).Call("repeat", count).String()`
// because JS performs additional UTF-8 escapes meaning tests like
// hash/adler32 TestGolden will fail because the wrong input is created.
func Repeat(s string, count int) string {
if count == 0 {
return ""
}

// Since we cannot return an error on overflow,
// we should panic if the repeat will generate
// an overflow.
// See Issue golang.org/issue/16237
if count < 0 {
panic("strings: negative Repeat count")
} else if len(s)*count/count != len(s) {
panic("strings: Repeat count causes overflow")
}

n := len(s) * count
var b Builder
b.Grow(n)
b.WriteString(s)
for b.Len() < n {
if b.Len() <= n/2 {
b.WriteString(b.String())
} else {
b.WriteString(b.String()[:n-b.Len()])
break
}
}
return b.String()
}
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