if i >= 0 && s[i] >= utf8.RuneSelf {
		_, wid := utf8.DecodeRune(s[i:])
		i += wid
	} else {
		i++
	}
	return s[0:i]
}

// TrimFunc returns a subslice of s by slicing off all leading and trailing
// UTF-8-encoded code points c that satisfy f(c).
func TrimFunc(s []byte, f func(r rune) bool) []byte {
	return TrimRightFunc(TrimLeftFunc(s, f), f)
}

// TrimPrefix returns s without the provided leading prefix string.

func TestTrimFunc(t *testing.T) {
	for _, tc := range trimFuncTests {
		trimmers := []struct {
			name string
			trim func(s []byte, f func(r rune) bool) []byte
			out  []byte
		}{
			{"TrimFunc", TrimFunc, tc.trimOut},
			{"TrimLeftFunc", TrimLeftFunc, tc.leftOut},
			{"TrimRightFunc", TrimRightFunc, tc.rightOut},
		}
		for _, trimmer := range trimmers {
			actual := trimmer.trim([]byte(tc.in), tc.f.f)