ByteBuffer.Truncate(n int)

R=r
DELTA=22  (17 added, 0 deleted, 5 changed)
OCL=28781
CL=28815

src/lib/io/bytebuffer.go

diff --git a/src/lib/io/bytebuffer.go b/src/lib/io/bytebuffer.go
index 011166e..88371c4 100644
--- a/src/lib/io/bytebuffer.go
+++ b/src/lib/io/bytebuffer.go
@@ -39,9 +39,17 @@
 	return len(b.buf) - b.off
 }
 
+// Truncates the buffer so it contains n bytes.
+// It preserves the data in the buffer at positions [0 : n].
+// It is an error to call b.Truncate(n) with n > b.Len().
+func (b *ByteBuffer) Truncate(n int) {
+	b.buf = b.buf[0 : b.off + n];
+}
+
 // Reset resets the buffer so it has no content.
+// b.Reset() is the same as b.Truncate(0).
 func (b *ByteBuffer) Reset() {
-	b.off = len(b.buf)
+	b.buf = b.buf[0 : b.off];
 }
 
 // Write appends the contents of p to the buffer.  The return

src/lib/io/bytebuffer_test.go

diff --git a/src/lib/io/bytebuffer_test.go b/src/lib/io/bytebuffer_test.go
index 6a4f3c9..e91a641 100644
--- a/src/lib/io/bytebuffer_test.go
+++ b/src/lib/io/bytebuffer_test.go
@@ -89,6 +89,9 @@
 		buf.Reset();
 		check(t, "TestBasicOperations (2)", &buf, "");
 
+		buf.Truncate(0);
+		check(t, "TestBasicOperations (3)", &buf, "");
+
 		n, err := buf.Write(data[0 : 1]);
 		if n != 1 {
 			t.Errorf("wrote 1 byte, but n == %d\n", n);
@@ -96,16 +99,22 @@
 		if err != nil {
 			t.Errorf("err should always be nil, but err == %s\n", err);
 		}
-		check(t, "TestBasicOperations (3)", &buf, "a");
+		check(t, "TestBasicOperations (4)", &buf, "a");
 
 		n, err = buf.Write(data[1 : 26]);
 		if n != 25 {
 			t.Errorf("wrote 25 bytes, but n == %d\n", n);
 		}
-		check(t, "TestBasicOperations (4)", &buf, string(data[0 : 26]));
+		check(t, "TestBasicOperations (5)", &buf, string(data[0 : 26]));
 
-		empty(t, "TestBasicOperations (5)", &buf, string(data[0 : 26]), make([]byte, 5));
-		empty(t, "TestBasicOperations (6)", &buf, "", make([]byte, 100));
+		buf.Truncate(26);
+		check(t, "TestBasicOperations (6)", &buf, string(data[0 : 26]));
+
+		buf.Truncate(20);
+		check(t, "TestBasicOperations (7)", &buf, string(data[0 : 20]));
+
+		empty(t, "TestBasicOperations (8)", &buf, string(data[0 : 20]), make([]byte, 5));
+		empty(t, "TestBasicOperations (9)", &buf, "", make([]byte, 100));
 	}
 }