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>

Author: cespare

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,