bytes: avoid duplicate malloc/copy in Buffer.ReadString

Twice faster and twice less garbage.

R=golang-dev, dave, daniel.morsing, bradfitz
CC=golang-dev
https://golang.org/cl/6849128

diff --git a/src/pkg/bytes/buffer.go b/src/pkg/bytes/buffer.go
index efb9798ee018b0040672648e360a7941a4e7f23a..3ae930384f6b14201b81199bdeba2c9fe3dd4d79 100644 (file)
--- a/src/pkg/bytes/buffer.go
+++ b/src/pkg/bytes/buffer.go
@@ -360,16 +360,24 @@ func (b *Buffer) UnreadByte() error {
 // ReadBytes returns err != nil if and only if the returned data does not end in
 // delim.
 func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) {
+       slice, err := b.readSlice(delim)
+       // return a copy of slice. The buffer's backing array may
+       // be overwritten by later calls.
+       line = append(line, slice...)
+       return
+}
+
+// readSlice is like readBytes but returns a reference to internal buffer data.
+func (b *Buffer) readSlice(delim byte) (line []byte, err error) {
        i := IndexByte(b.buf[b.off:], delim)
-       size := i + 1
+       end := b.off + i + 1
        if i < 0 {
-               size = len(b.buf) - b.off
+               end = len(b.buf)
                err = io.EOF
        }
-       line = make([]byte, size)
-       copy(line, b.buf[b.off:])
-       b.off += size
-       return
+       line = b.buf[b.off:end]
+       b.off = end
+       return line, err
 }
 
 // ReadString reads until the first occurrence of delim in the input,
@@ -379,8 +387,8 @@ func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) {
 // ReadString returns err != nil if and only if the returned data does not end
 // in delim.
 func (b *Buffer) ReadString(delim byte) (line string, err error) {
-       bytes, err := b.ReadBytes(delim)
-       return string(bytes), err
+       slice, err := b.readSlice(delim)
+       return string(slice), err
 }
 
 // NewBuffer creates and initializes a new Buffer using buf as its initial

diff --git a/src/pkg/bytes/buffer_test.go b/src/pkg/bytes/buffer_test.go
index 92e29146b32d5061d3e2ce4af69c7fb090ef3ba7..c53544a74a06d317757bc9760f989f97d459f5ff 100644 (file)
--- a/src/pkg/bytes/buffer_test.go
+++ b/src/pkg/bytes/buffer_test.go
@@ -375,6 +375,41 @@ func TestReadBytes(t *testing.T) {
        }
 }
 
+func TestReadString(t *testing.T) {
+       for _, test := range readBytesTests {
+               buf := NewBufferString(test.buffer)
+               var err error
+               for _, expected := range test.expected {
+                       var s string
+                       s, err = buf.ReadString(test.delim)
+                       if s != expected {
+                               t.Errorf("expected %q, got %q", expected, s)
+                       }
+                       if err != nil {
+                               break
+                       }
+               }
+               if err != test.err {
+                       t.Errorf("expected error %v, got %v", test.err, err)
+               }
+       }
+}
+
+func BenchmarkReadString(b *testing.B) {
+       const n = 32 << 10
+
+       data := make([]byte, n)
+       data[n-1] = 'x'
+       b.SetBytes(int64(n))
+       for i := 0; i < b.N; i++ {
+               buf := NewBuffer(data)
+               _, err := buf.ReadString('x')
+               if err != nil {
+                       b.Fatal(err)
+               }
+       }
+}
+
 func TestGrow(t *testing.T) {
        x := []byte{'x'}
        y := []byte{'y'}