rlp: add Flat

rlp/encode.go

@@ -32,6 +32,48 @@ type Encoder interface {
 	EncodeRLP(io.Writer) error
 }
 
+// Flat wraps a value (which must encode as a list) so
+// it encodes as the list's elements.
+//
+// Example: suppose you have defined a type
+//
+//     type foo struct { A, B uint }
+//
+// Under normal encoding rules,
+//
+//     rlp.Encode(foo{1, 2}) --> 0xC20102
+//
+// This function can help you achieve the following encoding:
+//
+//     rlp.Encode(rlp.Flat(foo{1, 2})) --> 0x0102
+func Flat(val interface{}) Encoder {
+	return flatenc{val}
+}
+
+type flatenc struct{ val interface{} }
+
+func (e flatenc) EncodeRLP(out io.Writer) error {
+	// record current output position
+	var (
+		eb          = out.(*encbuf)
+		prevstrsize = len(eb.str)
+		prevnheads  = len(eb.lheads)
+	)
+	if err := eb.encode(e.val); err != nil {
+		return err
+	}
+	// check that a new list header has appeared
+	if len(eb.lheads) == prevnheads || eb.lheads[prevnheads].offset == prevstrsize-1 {
+		return fmt.Errorf("rlp.Flat: %T did not encode as list", e.val)
+	}
+	// remove the new list header
+	newhead := eb.lheads[prevnheads]
+	copy(eb.lheads[prevnheads:], eb.lheads[prevnheads+1:])
+	eb.lheads = eb.lheads[:len(eb.lheads)-1]
+	eb.lhsize -= newhead.tagsize()
+	return nil
+}
+
 // Encode writes the RLP encoding of val to w. Note that Encode may
 // perform many small writes in some cases. Consider making w
 // buffered.
@@ -123,6 +165,13 @@ func (head *listhead) encode(buf []byte) []byte {
 	}
 }
 
+func (head *listhead) tagsize() int {
+	if head.size < 56 {
+		return 1
+	}
+	return 1 + intsize(uint64(head.size))
+}
+
 func newencbuf() *encbuf {
 	return &encbuf{sizebuf: make([]byte, 9)}
 }