encoding/json: add (*Encoder).Indent

Fixes #6492.

Change-Id: Ibc633c43a6d134bb140addb59780a5758b35a5c5
Reviewed-on: https://go-review.googlesource.com/21057
Run-TryBot: Caleb Spare <cespare@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/json/stream.go

@@ -168,6 +168,10 @@ func nonSpace(b []byte) bool {
 type Encoder struct {
 	w   io.Writer
 	err error
+
+	indentBuf    *bytes.Buffer
+	indentPrefix string
+	indentValue  string
 }
 
 // NewEncoder returns a new encoder that writes to w.
@@ -198,13 +202,29 @@ func (enc *Encoder) Encode(v interface{}) error {
 	// digits coming.
 	e.WriteByte('\n')
 
-	if _, err = enc.w.Write(e.Bytes()); err != nil {
+	b := e.Bytes()
+	if enc.indentBuf != nil {
+		enc.indentBuf.Reset()
+		err = Indent(enc.indentBuf, b, enc.indentPrefix, enc.indentValue)
+		if err != nil {
+			return err
+		}
+		b = enc.indentBuf.Bytes()
+	}
+	if _, err = enc.w.Write(b); err != nil {
 		enc.err = err
 	}
 	encodeStatePool.Put(e)
 	return err
 }
 
+// Indent sets the encoder to format each encoded object with Indent.
+func (enc *Encoder) Indent(prefix, indent string) {
+	enc.indentBuf = new(bytes.Buffer)
+	enc.indentPrefix = prefix
+	enc.indentValue = indent
+}
+
 // RawMessage is a raw encoded JSON object.
 // It implements Marshaler and Unmarshaler and can
 // be used to delay JSON decoding or precompute a JSON encoding.

src/encoding/json/stream_test.go

@@ -57,6 +57,36 @@ func TestEncoder(t *testing.T) {
 	}
 }
 
+var streamEncodedIndent = `0.1
+"hello"
+null
+true
+false
+[
+>."a",
+>."b",
+>."c"
+>]
+{
+>."ß": "long s",
+>."K": "Kelvin"
+>}
+3.14
+`
+
+func TestEncoderIndent(t *testing.T) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	enc.Indent(">", ".")
+	for _, v := range streamTest {
+		enc.Encode(v)
+	}
+	if have, want := buf.String(), streamEncodedIndent; have != want {
+		t.Error("indented encoding mismatch")
+		diff(t, []byte(have), []byte(want))
+	}
+}
+
 func TestDecoder(t *testing.T) {
 	for i := 0; i <= len(streamTest); i++ {
 		// Use stream without newlines as input,