bytes: rely on utf8.AppendRune

This is both simpler and more performant.

WriteRune                                 23.9µs ± 1%   22.8µs ± 8%   -4.43%  (p=0.006 n=8+10)
ToUpper/longɐstringɐwithɐnonasciiⱯchars    573ns ± 4%    474ns ± 6%  -17.27%  (p=0.000 n=10+10)
ToUpper/ɐɐɐɐɐ                              236ns ± 6%    202ns ± 5%  -14.12%  (p=0.000 n=10+10)
ToUpper/a\u0080\U0010ffff                 98.8ns ± 6%   91.2ns ± 3%   -7.67%  (p=0.000 n=10+10)
ToLower/LONGⱯSTRINGⱯWITHⱯNONASCIIⱯCHARS    511ns ± 3%    409ns ± 4%  -20.02%  (p=0.000 n=10+10)
ToLower/ⱭⱭⱭⱭⱭ                              178ns ± 4%    173ns ± 2%   -2.76%  (p=0.005 n=10+10)
ToLower/A\u0080\U0010ffff                  100ns ± 3%     91ns ± 2%   -8.55%  (p=0.000 n=10+10)

Change-Id: I968ef194da2c115cbdcac2d3575c34c65e6a8a56
Reviewed-on: https://go-review.googlesource.com/c/go/+/412337
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>

1 files changed, 3 insertions, 16 deletions

diff --git a/src/bytes/bytes.go b/src/bytes/bytes.go
index 5cf5b477eb..7b28cf1efc 100644
--- a/src/bytes/bytes.go
+++ b/src/bytes/bytes.go
@@ -560,9 +560,7 @@ func Map(mapping func(r rune) rune, s []byte) []byte {
 	// In the worst case, the slice can grow when mapped, making
 	// things unpleasant. But it's so rare we barge in assuming it's
 	// fine. It could also shrink but that falls out naturally.
-	maxbytes := len(s) // length of b
-	nbytes := 0        // number of bytes encoded in b
-	b := make([]byte, maxbytes)
+	b := make([]byte, 0, len(s))
 	for i := 0; i < len(s); {
 		wid := 1
 		r := rune(s[i])
@@ -571,22 +569,11 @@ func Map(mapping func(r rune) rune, s []byte) []byte {
 		}
 		r = mapping(r)
 		if r >= 0 {
-			rl := utf8.RuneLen(r)
-			if rl < 0 {
-				rl = len(string(utf8.RuneError))
-			}
-			if nbytes+rl > maxbytes {
-				// Grow the buffer.
-				maxbytes = maxbytes*2 + utf8.UTFMax
-				nb := make([]byte, maxbytes)
-				copy(nb, b[0:nbytes])
-				b = nb
-			}
-			nbytes += utf8.EncodeRune(b[nbytes:maxbytes], r)
+			b = utf8.AppendRune(b, r)
 		}
 		i += wid
 	}
-	return b[0:nbytes]
+	return b
 }
 
 // Repeat returns a new byte slice consisting of count copies of b.