strings: fix encoding of \u0080 in map

Fix encoding of PAD (U+0080) which has the same value as utf8.RuneSelf
being incorrectly encoded as \x80 in strings.Map due to using <= instead
of a < comparison operator to check one byte encodings for utf8.

Fixes #25242

Change-Id: Ib6c7d1f425a7ba81e431b6d64009e713d94ea3bc
Reviewed-on: https://go-review.googlesource.com/111286
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/strings/strings.go

@@ -479,7 +479,7 @@ func Map(mapping func(rune) rune, s string) string {
 		b = make([]byte, len(s)+utf8.UTFMax)
 		nbytes = copy(b, s[:i])
 		if r >= 0 {
-			if r <= utf8.RuneSelf {
+			if r < utf8.RuneSelf {
 				b[nbytes] = byte(r)
 				nbytes++
 			} else {
@@ -509,7 +509,7 @@ func Map(mapping func(rune) rune, s string) string {
 		r := mapping(c)
 
 		// common case
-		if (0 <= r && r <= utf8.RuneSelf) && nbytes < len(b) {
+		if (0 <= r && r < utf8.RuneSelf) && nbytes < len(b) {
 			b[nbytes] = byte(r)
 			nbytes++
 			continue

src/strings/strings_test.go

@@ -528,6 +528,7 @@ var upperTests = []StringTest{
 	{"longStrinGwitHmixofsmaLLandcAps", "LONGSTRINGWITHMIXOFSMALLANDCAPS"},
 	{"long\u0250string\u0250with\u0250nonascii\u2C6Fchars", "LONG\u2C6FSTRING\u2C6FWITH\u2C6FNONASCII\u2C6FCHARS"},
 	{"\u0250\u0250\u0250\u0250\u0250", "\u2C6F\u2C6F\u2C6F\u2C6F\u2C6F"}, // grows one byte per char
+	{"a\u0080\U0010FFFF", "A\u0080\U0010FFFF"},                           // test utf8.RuneSelf and utf8.MaxRune
 }
 
 var lowerTests = []StringTest{
@@ -538,6 +539,7 @@ var lowerTests = []StringTest{
 	{"longStrinGwitHmixofsmaLLandcAps", "longstringwithmixofsmallandcaps"},
 	{"LONG\u2C6FSTRING\u2C6FWITH\u2C6FNONASCII\u2C6FCHARS", "long\u0250string\u0250with\u0250nonascii\u0250chars"},
 	{"\u2C6D\u2C6D\u2C6D\u2C6D\u2C6D", "\u0251\u0251\u0251\u0251\u0251"}, // shrinks one byte per char
+	{"A\u0080\U0010FFFF", "a\u0080\U0010FFFF"},                           // test utf8.RuneSelf and utf8.MaxRune
 }
 
 const space = "\t\v\r\f\n\u0085\u00a0\u2000\u3000"
@@ -650,6 +652,27 @@ func TestMap(t *testing.T) {
 	if m != expect {
 		t.Errorf("replace invalid sequence: expected %q got %q", expect, m)
 	}
+
+	// 8. Check utf8.RuneSelf and utf8.MaxRune encoding
+	encode := func(r rune) rune {
+		switch r {
+		case utf8.RuneSelf:
+			return unicode.MaxRune
+		case unicode.MaxRune:
+			return utf8.RuneSelf
+		}
+		return r
+	}
+	s := string(utf8.RuneSelf) + string(utf8.MaxRune)
+	r := string(utf8.MaxRune) + string(utf8.RuneSelf) // reverse of s
+	m = Map(encode, s)
+	if m != r {
+		t.Errorf("encoding not handled correctly: expected %q got %q", r, m)
+	}
+	m = Map(encode, r)
+	if m != s {
+		t.Errorf("encoding not handled correctly: expected %q got %q", s, m)
+	}
 }
 
 func TestToUpper(t *testing.T) { runStringTests(t, ToUpper, "ToUpper", upperTests) }