bytes: add Next method to Buffer, simplify Read.

R=r
CC=golang-dev
https://golang.org/cl/980043

src/pkg/bytes/buffer.go

@@ -196,17 +196,23 @@ func (b *Buffer) Read(p []byte) (n int, err os.Error) {
 		b.Truncate(0)
 		return 0, os.EOF
 	}
-	m := b.Len()
-	n = len(p)
+	n = copy(p, b.buf[b.off:])
+	b.off += n
+	return
+}
 
+// Next returns a slice containing the next n bytes from the buffer,
+// advancing the buffer as if the bytes had been returned by Read.
+// If there are fewer than n bytes in the buffer, Next returns the entire buffer.
+// The slice is only valid until the next call to a read or write method.
+func (b *Buffer) Next(n int) []byte {
+	m := b.Len()
 	if n > m {
-		// more bytes requested than available
 		n = m
 	}
-
-	copy(p, b.buf[b.off:b.off+n])
+	data := b.buf[b.off : b.off+n]
 	b.off += n
-	return n, err
+	return data
 }
 
 // ReadByte reads and returns the next byte from the buffer.

src/pkg/bytes/buffer_test.go

@@ -264,6 +264,7 @@ func TestWriteTo(t *testing.T) {
 	}
 }
 
+
 func TestRuneIO(t *testing.T) {
 	const NRune = 1000
 	// Built a test array while we write the data
@@ -297,3 +298,37 @@ func TestRuneIO(t *testing.T) {
 		}
 	}
 }
+
+
+func TestNext(t *testing.T) {
+	b := []byte{0, 1, 2, 3, 4}
+	tmp := make([]byte, 5)
+	for i := 0; i <= 5; i++ {
+		for j := i; j <= 5; j++ {
+			for k := 0; k <= 6; k++ {
+				// 0 <= i <= j <= 5; 0 <= k <= 6
+				// Check that if we start with a buffer
+				// of length j at offset i and ask for
+				// Next(k), we get the right bytes.
+				buf := NewBuffer(b[0:j])
+				n, _ := buf.Read(tmp[0:i])
+				if n != i {
+					t.Fatalf("Read %d returned %d", i, n)
+				}
+				bb := buf.Next(k)
+				want := k
+				if want > j-i {
+					want = j - i
+				}
+				if len(bb) != want {
+					t.Fatalf("in %d,%d: len(Next(%d)) == %d", i, j, k, len(bb))
+				}
+				for l, v := range bb {
+					if v != byte(l+i) {
+						t.Fatalf("in %d,%d: Next(%d)[%d] = %d, want %d", i, j, k, l, v, l+i)
+					}
+				}
+			}
+		}
+	}
+}