rlp: add functions for encoding

I'm reasonably confident that the encoding matches the output of
ethutil.Encode for values that it supports. Some of the tests have been
adpated from the Ethereum testing repository.

There are still TODOs in the code.

rlp/decode.go

@@ -329,16 +329,10 @@ type field struct {
 }
 
 func makeStructDecoder(typ reflect.Type) (decoder, error) {
-	var fields []field
-	for i := 0; i < typ.NumField(); i++ {
-		if f := typ.Field(i); f.PkgPath == "" { // exported
-			info, err := cachedTypeInfo1(f.Type)
+	fields, err := structFields(typ)
 	if err != nil {
 		return nil, err
 	}
-			fields = append(fields, field{i, info})
-		}
-	}
 	dec := func(s *Stream, val reflect.Value) (err error) {
 		if _, err = s.List(); err != nil {
 			return wrapStreamError(err, typ)

rlp/decode_test.go

@@ -509,7 +509,7 @@ func ExampleStream() {
 }
 
 func BenchmarkDecode(b *testing.B) {
-	enc := encTest(90000)
+	enc := encodeTestSlice(90000)
 	b.SetBytes(int64(len(enc)))
 	b.ReportAllocs()
 	b.ResetTimer()
@@ -524,7 +524,7 @@ func BenchmarkDecode(b *testing.B) {
 }
 
 func BenchmarkDecodeIntSliceReuse(b *testing.B) {
-	enc := encTest(100000)
+	enc := encodeTestSlice(100000)
 	b.SetBytes(int64(len(enc)))
 	b.ReportAllocs()
 	b.ResetTimer()
@@ -538,7 +538,7 @@ func BenchmarkDecodeIntSliceReuse(b *testing.B) {
 	}
 }
 
-func encTest(n int) []byte {
+func encodeTestSlice(n int) []byte {
 	s := make([]interface{}, n)
 	for i := 0; i < n; i++ {
 		s[i] = i

rlp/encoder_example_test.go

+package rlp
+
+import (
+	"fmt"
+	"io"
+)
+
+type MyCoolType struct {
+	Name string
+	a, b uint
+}
+
+// EncodeRLP writes x as RLP list [a, b] that omits the Name field.
+func (x *MyCoolType) EncodeRLP(w io.Writer) (err error) {
+	// Note: the receiver can be a nil pointer. This allows you to
+	// control the encoding of nil, but it also means that you have to
+	// check for a nil receiver.
+	if x == nil {
+		err = Encode(w, []uint{0, 0})
+	} else {
+		err = Encode(w, []uint{x.a, x.b})
+	}
+	return err
+}
+
+func ExampleEncoder() {
+	var t *MyCoolType // t is nil pointer to MyCoolType
+	bytes, _ := EncodeToBytes(t)
+	fmt.Printf("%v → %X\n", t, bytes)
+
+	t = &MyCoolType{Name: "foobar", a: 5, b: 6}
+	bytes, _ = EncodeToBytes(t)
+	fmt.Printf("%v → %X\n", t, bytes)
+
+	// Output:
+	// <nil> → C28080
+	// &{foobar 5 6} → C20506
+}

rlp/typecache.go

@@ -5,16 +5,19 @@ import (
 	"sync"
 )
 
-type decoder func(*Stream, reflect.Value) error
+var (
+	typeCacheMutex sync.RWMutex
+	typeCache      = make(map[reflect.Type]*typeinfo)
+)
 
 type typeinfo struct {
 	decoder
+	writer
 }
 
-var (
-	typeCacheMutex sync.RWMutex
-	typeCache      = make(map[reflect.Type]*typeinfo)
-)
+type decoder func(*Stream, reflect.Value) error
+
+type writer func(reflect.Value, *encbuf) error
 
 func cachedTypeInfo(typ reflect.Type) (*typeinfo, error) {
 	typeCacheMutex.RLock()
@@ -49,11 +52,27 @@ func cachedTypeInfo1(typ reflect.Type) (*typeinfo, error) {
 	return typeCache[typ], err
 }
 
+func structFields(typ reflect.Type) (fields []field, err error) {
+	for i := 0; i < typ.NumField(); i++ {
+		if f := typ.Field(i); f.PkgPath == "" { // exported
+			info, err := cachedTypeInfo1(f.Type)
+			if err != nil {
+				return nil, err
+			}
+			fields = append(fields, field{i, info})
+		}
+	}
+	return fields, nil
+}
+
 func genTypeInfo(typ reflect.Type) (info *typeinfo, err error) {
 	info = new(typeinfo)
 	if info.decoder, err = makeDecoder(typ); err != nil {
 		return nil, err
 	}
+	if info.writer, err = makeWriter(typ); err != nil {
+		return nil, err
+	}
 	return info, nil
 }