bufio: simplify bufio.Reader.ReadBytes to avoid an extra loop over a slice

Change-Id: Icb1c3eb30147180ba5949a25c65b48307b14c1ca
GitHub-Last-Rev: 937ae8641321139b9165ce7d57abeac5a67dc24d
GitHub-Pull-Request: golang/go#34704
Reviewed-on: https://go-review.googlesource.com/c/go/+/199157
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/bufio/bufio.go

@@ -432,6 +432,7 @@ func (b *Reader) ReadBytes(delim byte) ([]byte, error) {
 	var frag []byte
 	var full [][]byte
 	var err error
+	n := 0
 	for {
 		var e error
 		frag, e = b.ReadSlice(delim)
@@ -447,18 +448,15 @@ func (b *Reader) ReadBytes(delim byte) ([]byte, error) {
 		buf := make([]byte, len(frag))
 		copy(buf, frag)
 		full = append(full, buf)
+		n += len(buf)
 	}
 
-	// Allocate new buffer to hold the full pieces and the fragment.
-	n := 0
-	for i := range full {
-		n += len(full[i])
-	}
 	n += len(frag)
 
-	// Copy full pieces and fragment in.
+	// Allocate new buffer to hold the full pieces and the fragment.
 	buf := make([]byte, n)
 	n = 0
+	// Copy full pieces and fragment in.
 	for i := range full {
 		n += copy(buf[n:], full[i])
 	}