Add Truncate method to replace Reset. Add PutFunc method.

buffer.go

@@ -3,9 +3,9 @@ package drlp
 // Buffer is essentially a byte slice. It holds encoded elements.
 type Buffer []byte
 
-// Reset resets all content.
-func (b *Buffer) Reset() {
-	*b = (*b)[:0]
+// Truncate truncates the buffer to size.
+func (b *Buffer) Truncate(size int) {
+	*b = (*b)[:size]
 }
 
 // PutUint puts the uint value.
@@ -36,6 +36,19 @@ func (b *Buffer) PutString(str []byte) {
 	}
 }
 
+// PutFunc puts the string value returned by func build.
+// The build func should not modify existing content of buf,
+// and should follow the behavior like append.
+func (b *Buffer) PutFunc(build func(buf []byte) []byte) {
+	offset := len(*b)
+	// make room for string header
+	*b = append(*b, make([]byte, 9)...)
+	*b = build(*b)
+	str := (*b)[offset+9:]
+	b.Truncate(offset)
+	b.PutString(str)
+}
+
 // List starts a RLP list.
 func (b *Buffer) List() List {
 	return List{b, len(*b)}

buffer_test.go

@@ -121,10 +121,22 @@ func encode(v interface{}, b *Buffer) {
 func TestBuffer(t *testing.T) {
 	var buf Buffer
 	for i, tt := range tests {
-		buf.Reset()
+		buf.Truncate(0)
 		encode(tt.val, &buf)
 		if got := strings.ToUpper(hex.EncodeToString(buf)); got != tt.want {
 			t.Errorf("#%v: got %v, want %v", i, got, tt.want)
 		}
 	}
 }
+
+func TestBuffer_PutFunc(t *testing.T) {
+	var buf Buffer
+	buf.PutFunc(func(buf []byte) []byte {
+		return append(buf, "Lorem ipsum dolor sit amet, consectetur adipisicing eli"...)
+	})
+	want := "B74C6F72656D20697073756D20646F6C6F722073697420616D65742C20636F6E7365637465747572206164697069736963696E6720656C69"
+	if got := strings.ToUpper(hex.EncodeToString(buf)); got != want {
+		t.Errorf("got %v, want %v", got, want)
+	}
+
+}