bufio: avoid rescanning buffer multiple times in ReadSlice

When existing data in buffer does not have delimiter,
and new data is added with b.fill(), continue search from
previous point instead of starting from beginning.

Change-Id: Id78332afe2b0281b4a3c86bd1ffe9449cfea7848
GitHub-Last-Rev: 08e7d2f50151a00b22800e3f7020d0de8dee7dcf
GitHub-Pull-Request: golang/go#25441
Reviewed-on: https://go-review.googlesource.com/113535
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/bufio/bufio.go

@@ -314,9 +314,11 @@ func (b *Reader) Buffered() int { return b.w - b.r }
 // ReadBytes or ReadString instead.
 // ReadSlice returns err != nil if and only if line does not end in delim.
 func (b *Reader) ReadSlice(delim byte) (line []byte, err error) {
+	s := 0 // search start index
 	for {
 		// Search buffer.
-		if i := bytes.IndexByte(b.buf[b.r:b.w], delim); i >= 0 {
+		if i := bytes.IndexByte(b.buf[b.r+s:b.w], delim); i >= 0 {
+			i += s
 			line = b.buf[b.r : b.r+i+1]
 			b.r += i + 1
 			break
@@ -338,6 +340,8 @@ func (b *Reader) ReadSlice(delim byte) (line []byte, err error) {
 			break
 		}
 
+		s = b.w - b.r // do not rescan area we scanned before
+
 		b.fill() // buffer is not full
 	}