Which is the best way to process big json & replace specific values?

I have a big json (30mb) which contains "title" fields in different objects , structure of json is unknown.

Known only that json contains keys "title" and string value of this key must be translated into another.

<!-- language: lang-html -->

{
	"data1" : {
		"title" : "alpha",
		"color" : "green"
	},
	"data2" : {
		"someInnerData1" : {
			"title" : "beta"
			"color" : "red"
		},
		"someInnerData2" : {
			"someArray" : [
			{
				"title" : "gamme",
				"color" : "orange"
			},
			{
				"title" : "delta",
				"color" : "purple"
			}
			],
			"title" : "epsilon"
		}
	}
}

<!-- end snippet -->

Replace example
"alpha" -> "Α"
"beta" -> "B"
etc..

Which the best way achieve that in Golang , without decoding into struct ?

P.S. Json is received from network.

I would make a struct that implements the io.Reader interface, and use that reader as a translation ground: you can use it to get you JSON input chunk by chunk, and detect when you are on a key that need to be changed, so translate it on the fly.

Then, you just have to use a io.Copy to read the whole file into another.

See the text.transform package dependency graph for examples…

You can use a streaming JSON decoder like megajson:

// Transform 'title' strings into Title case
func TitleizeJSON(r io.Reader, w io.Writer) error {
	buf := new(bytes.Buffer)
	r = io.TeeReader(r, buf)

	s := scanner.NewScanner(r)
	var prevTok int
	var prevPos int
	wasTitle := false
	titleField := []byte("title")
	for {
		// read the next json token
		tok, data, err := s.Scan()
		if err == io.EOF {
			return nil
		} else if err != nil {
			return err
		}
		// calculate the position in the buffer
		pos := s.Pos()
		off := pos - prevPos

		switch tok {
		// if this is a string
		case scanner.TSTRING:
			// if the previous string before a : was 'title', then
			// titlelize it
			if prevTok == scanner.TCOLON && wasTitle {
				// grab the first part of the buffer and skip
				// the first ", the titleize the rest
				data = buf.Bytes()[:off][1:]
				copy(data, bytes.Title(data))
				wasTitle = false
			} else {
				wasTitle = bytes.Equal(data, titleField)
			}
		}

		// now send the data to the writer
		data = buf.Bytes()
		_, err = w.Write(data[:off])
		if err != nil {
			return err
		}

		// reset the buffer (so it doesn't grow forever)
		nbuf := make([]byte, len(data)-off)
		copy(nbuf, data[off:])
		buf.Reset()
		buf.Write(nbuf)

		// for the next go-around
		prevTok = tok
		prevPos = pos
	}
}

This should do the titleizing on the fly. The one case I can think of where it will have a problem is if you have a really really big string.