strings: reduce allocation for byteReplacer

Though it increases the execution time, the function is already quite
fast for most users, the allocation is much more important.

name                       old time/op    new time/op    delta
ByteReplacerWriteString-8    1.23µs ± 0%    2.16µs ± 1%   +75.31%  (p=0.000 n=10+10)

name                       old alloc/op   new alloc/op   delta
ByteReplacerWriteString-8    2.69kB ± 0%    0.00kB       -100.00%  (p=0.000 n=10+10)

name                       old allocs/op  new allocs/op  delta
ByteReplacerWriteString-8      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)

Change-Id: I6a36df5fcb8e11ef27e6c7b252aa88e869592f3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/424136
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>

1 files changed, 23 insertions, 14 deletions

diff --git a/src/strings/replace.go b/src/strings/replace.go
index 73bc78a07e..f504fb48df 100644
--- a/src/strings/replace.go
+++ b/src/strings/replace.go
@@ -455,21 +455,30 @@ func (r *byteReplacer) Replace(s string) string {
 }
 
 func (r *byteReplacer) WriteString(w io.Writer, s string) (n int, err error) {
-	// TODO(bradfitz): use io.WriteString with slices of s, avoiding allocation.
-	bufsize := 32 << 10
-	if len(s) < bufsize {
-		bufsize = len(s)
-	}
-	buf := make([]byte, bufsize)
-
-	for len(s) > 0 {
-		ncopy := copy(buf, s)
-		s = s[ncopy:]
-		for i, b := range buf[:ncopy] {
-			buf[i] = r[b]
+	sw := getStringWriter(w)
+	last := 0
+	for i := 0; i < len(s); i++ {
+		b := s[i]
+		if r[b] == b {
+			continue
 		}
-		wn, err := w.Write(buf[:ncopy])
-		n += wn
+		if last != i {
+			wn, err := sw.WriteString(s[last:i])
+			n += wn
+			if err != nil {
+				return n, err
+			}
+		}
+		last = i + 1
+		nw, err := w.Write(r[b : int(b)+1])
+		n += nw
+		if err != nil {
+			return n, err
+		}
+	}
+	if last != len(s) {
+		nw, err := sw.WriteString(s[last:])
+		n += nw
 		if err != nil {
 			return n, err
 		}