@@ -73,3 +73,48 @@ func Clone(s string) string {
7373 // memory overheads and simply return the string as-is.
7474 return s
7575}
76+
77+ // Repeat is the go1.19 implementation of strings.Repeat.
78+ //
79+ // In the go1.20 implementation, the function was changed to use chunks that
80+ // are 8KB in size to improve speed and cache access. This change is faster
81+ // when running native Go code. However, for GopherJS, the change is much
82+ // slower than the go1.19 implementation.
83+ //
84+ // The go1.20 change made tests like encoding/pem TestCVE202224675 take
85+ // significantly longer to run for GopherJS.
86+ // go1.19 concatenates 24 times and the test takes about 8 seconds.
87+ // go1.20 concatenates about 15000 times and can take over a hour.
88+ //
89+ // We can't use `js.InternalObject(s).Call("repeat", count).String()`
90+ // because JS performs additional UTF-8 escapes meaning tests like
91+ // hash/adler32 TestGolden will fail because the wrong input is created.
92+ func Repeat (s string , count int ) string {
93+ if count == 0 {
94+ return ""
95+ }
96+
97+ // Since we cannot return an error on overflow,
98+ // we should panic if the repeat will generate
99+ // an overflow.
100+ // See Issue golang.org/issue/16237
101+ if count < 0 {
102+ panic ("strings: negative Repeat count" )
103+ } else if len (s )* count / count != len (s ) {
104+ panic ("strings: Repeat count causes overflow" )
105+ }
106+
107+ n := len (s ) * count
108+ var b Builder
109+ b .Grow (n )
110+ b .WriteString (s )
111+ for b .Len () < n {
112+ if b .Len () <= n / 2 {
113+ b .WriteString (b .String ())
114+ } else {
115+ b .WriteString (b .String ()[:n - b .Len ()])
116+ break
117+ }
118+ }
119+ return b .String ()
120+ }
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