Example of using http://github.com/pquerna/ffjson

education.go

//go:generate ffjson $GOFILE
package education

type Student struct {
	FirstName string
	LastName  string
}

type University struct {
	Name     string
	Students []Student
}

education_ffjson.go

// DO NOT EDIT!
// Code generated by ffjson <https://github.com/pquerna/ffjson>
// source: education.go
// DO NOT EDIT!

package education

import (
	"bytes"
	"fmt"
	fflib "github.com/pquerna/ffjson/fflib/v1"
)

func (mj *Student) MarshalJSON() ([]byte, error) {
	var buf fflib.Buffer
	if mj == nil {
		buf.WriteString("null")
		return buf.Bytes(), nil
	}
	err := mj.MarshalJSONBuf(&buf)
	if err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}
func (mj *Student) MarshalJSONBuf(buf fflib.EncodingBuffer) error {
	if mj == nil {
		buf.WriteString("null")
		return nil
	}
	var err error
	var obj []byte
	_ = obj
	_ = err
	buf.WriteString(`{"FirstName":`)
	fflib.WriteJsonString(buf, string(mj.FirstName))
	buf.WriteString(`,"LastName":`)
	fflib.WriteJsonString(buf, string(mj.LastName))
	buf.WriteByte('}')
	return nil
}

const (
	ffj_t_Studentbase = iota
	ffj_t_Studentno_such_key

	ffj_t_Student_FirstName

	ffj_t_Student_LastName
)

var ffj_key_Student_FirstName = []byte("FirstName")

var ffj_key_Student_LastName = []byte("LastName")

func (uj *Student) UnmarshalJSON(input []byte) error {
	fs := fflib.NewFFLexer(input)
	return uj.UnmarshalJSONFFLexer(fs, fflib.FFParse_map_start)
}

func (uj *Student) UnmarshalJSONFFLexer(fs *fflib.FFLexer, state fflib.FFParseState) error {
	var err error = nil
	currentKey := ffj_t_Studentbase
	_ = currentKey
	tok := fflib.FFTok_init
	wantedTok := fflib.FFTok_init

mainparse:
	for {
		tok = fs.Scan()
		//	println(fmt.Sprintf("debug: tok: %v  state: %v", tok, state))
		if tok == fflib.FFTok_error {
			goto tokerror
		}

		switch state {

		case fflib.FFParse_map_start:
			if tok != fflib.FFTok_left_bracket {
				wantedTok = fflib.FFTok_left_bracket
				goto wrongtokenerror
			}
			state = fflib.FFParse_want_key
			continue

		case fflib.FFParse_after_value:
			if tok == fflib.FFTok_comma {
				state = fflib.FFParse_want_key
			} else if tok == fflib.FFTok_right_bracket {
				goto done
			} else {
				wantedTok = fflib.FFTok_comma
				goto wrongtokenerror
			}

		case fflib.FFParse_want_key:
			// json {} ended. goto exit. woo.
			if tok == fflib.FFTok_right_bracket {
				goto done
			}
			if tok != fflib.FFTok_string {
				wantedTok = fflib.FFTok_string
				goto wrongtokenerror
			}

			kn := fs.Output.Bytes()
			if len(kn) <= 0 {
				// "" case. hrm.
				currentKey = ffj_t_Studentno_such_key
				state = fflib.FFParse_want_colon
				goto mainparse
			} else {
				switch kn[0] {

				case 'F':

					if bytes.Equal(ffj_key_Student_FirstName, kn) {
						currentKey = ffj_t_Student_FirstName
						state = fflib.FFParse_want_colon
						goto mainparse
					}

				case 'L':

					if bytes.Equal(ffj_key_Student_LastName, kn) {
						currentKey = ffj_t_Student_LastName
						state = fflib.FFParse_want_colon
						goto mainparse
					}

				}

				if fflib.EqualFoldRight(ffj_key_Student_LastName, kn) {
					currentKey = ffj_t_Student_LastName
					state = fflib.FFParse_want_colon
					goto mainparse
				}

				if fflib.EqualFoldRight(ffj_key_Student_FirstName, kn) {
					currentKey = ffj_t_Student_FirstName
					state = fflib.FFParse_want_colon
					goto mainparse
				}

				currentKey = ffj_t_Studentno_such_key
				state = fflib.FFParse_want_colon
				goto mainparse
			}

		case fflib.FFParse_want_colon:
			if tok != fflib.FFTok_colon {
				wantedTok = fflib.FFTok_colon
				goto wrongtokenerror
			}
			state = fflib.FFParse_want_value
			continue
		case fflib.FFParse_want_value:

			if tok == fflib.FFTok_left_brace || tok == fflib.FFTok_left_bracket || tok == fflib.FFTok_integer || tok == fflib.FFTok_double || tok == fflib.FFTok_string || tok == fflib.FFTok_bool || tok == fflib.FFTok_null {
				switch currentKey {

				case ffj_t_Student_FirstName:
					goto handle_FirstName

				case ffj_t_Student_LastName:
					goto handle_LastName

				case ffj_t_Studentno_such_key:
					err = fs.SkipField(tok)
					if err != nil {
						return fs.WrapErr(err)
					}
					state = fflib.FFParse_after_value
					goto mainparse
				}
			} else {
				goto wantedvalue
			}
		}
	}

handle_FirstName:

	/* handler: uj.FirstName type=string kind=string quoted=false*/

	{

		{
			if tok != fflib.FFTok_string && tok != fflib.FFTok_null {
				return fs.WrapErr(fmt.Errorf("cannot unmarshal %s into Go value for string", tok))
			}
		}

		if tok == fflib.FFTok_null {

		} else {

			outBuf := fs.Output.Bytes()

			uj.FirstName = string(string(outBuf))

		}
	}

	state = fflib.FFParse_after_value
	goto mainparse

handle_LastName:

	/* handler: uj.LastName type=string kind=string quoted=false*/

	{

		{
			if tok != fflib.FFTok_string && tok != fflib.FFTok_null {
				return fs.WrapErr(fmt.Errorf("cannot unmarshal %s into Go value for string", tok))
			}
		}

		if tok == fflib.FFTok_null {

		} else {

			outBuf := fs.Output.Bytes()

			uj.LastName = string(string(outBuf))

		}
	}

	state = fflib.FFParse_after_value
	goto mainparse

wantedvalue:
	return fs.WrapErr(fmt.Errorf("wanted value token, but got token: %v", tok))
wrongtokenerror:
	return fs.WrapErr(fmt.Errorf("ffjson: wanted token: %v, but got token: %v output=%s", wantedTok, tok, fs.Output.String()))
tokerror:
	if fs.BigError != nil {
		return fs.WrapErr(fs.BigError)
	}
	err = fs.Error.ToError()
	if err != nil {
		return fs.WrapErr(err)
	}
	panic("ffjson-generated: unreachable, please report bug.")
done:
	return nil
}

func (mj *University) MarshalJSON() ([]byte, error) {
	var buf fflib.Buffer
	if mj == nil {
		buf.WriteString("null")
		return buf.Bytes(), nil
	}
	err := mj.MarshalJSONBuf(&buf)
	if err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}
func (mj *University) MarshalJSONBuf(buf fflib.EncodingBuffer) error {
	if mj == nil {
		buf.WriteString("null")
		return nil
	}
	var err error
	var obj []byte
	_ = obj
	_ = err
	buf.WriteString(`{"Name":`)
	fflib.WriteJsonString(buf, string(mj.Name))
	buf.WriteString(`,"Students":`)
	if mj.Students != nil {
		buf.WriteString(`[`)
		for i, v := range mj.Students {
			if i != 0 {
				buf.WriteString(`,`)
			}

			{

				err = v.MarshalJSONBuf(buf)
				if err != nil {
					return err
				}

			}
		}
		buf.WriteString(`]`)
	} else {
		buf.WriteString(`null`)
	}
	buf.WriteByte('}')
	return nil
}

const (
	ffj_t_Universitybase = iota
	ffj_t_Universityno_such_key

	ffj_t_University_Name

	ffj_t_University_Students
)

var ffj_key_University_Name = []byte("Name")

var ffj_key_University_Students = []byte("Students")

func (uj *University) UnmarshalJSON(input []byte) error {
	fs := fflib.NewFFLexer(input)
	return uj.UnmarshalJSONFFLexer(fs, fflib.FFParse_map_start)
}

func (uj *University) UnmarshalJSONFFLexer(fs *fflib.FFLexer, state fflib.FFParseState) error {
	var err error = nil
	currentKey := ffj_t_Universitybase
	_ = currentKey
	tok := fflib.FFTok_init
	wantedTok := fflib.FFTok_init

mainparse:
	for {
		tok = fs.Scan()
		//	println(fmt.Sprintf("debug: tok: %v  state: %v", tok, state))
		if tok == fflib.FFTok_error {
			goto tokerror
		}

		switch state {

		case fflib.FFParse_map_start:
			if tok != fflib.FFTok_left_bracket {
				wantedTok = fflib.FFTok_left_bracket
				goto wrongtokenerror
			}
			state = fflib.FFParse_want_key
			continue

		case fflib.FFParse_after_value:
			if tok == fflib.FFTok_comma {
				state = fflib.FFParse_want_key
			} else if tok == fflib.FFTok_right_bracket {
				goto done
			} else {
				wantedTok = fflib.FFTok_comma
				goto wrongtokenerror
			}

		case fflib.FFParse_want_key:
			// json {} ended. goto exit. woo.
			if tok == fflib.FFTok_right_bracket {
				goto done
			}
			if tok != fflib.FFTok_string {
				wantedTok = fflib.FFTok_string
				goto wrongtokenerror
			}

			kn := fs.Output.Bytes()
			if len(kn) <= 0 {
				// "" case. hrm.
				currentKey = ffj_t_Universityno_such_key
				state = fflib.FFParse_want_colon
				goto mainparse
			} else {
				switch kn[0] {

				case 'N':

					if bytes.Equal(ffj_key_University_Name, kn) {
						currentKey = ffj_t_University_Name
						state = fflib.FFParse_want_colon
						goto mainparse
					}

				case 'S':

					if bytes.Equal(ffj_key_University_Students, kn) {
						currentKey = ffj_t_University_Students
						state = fflib.FFParse_want_colon
						goto mainparse
					}

				}

				if fflib.EqualFoldRight(ffj_key_University_Students, kn) {
					currentKey = ffj_t_University_Students
					state = fflib.FFParse_want_colon
					goto mainparse
				}

				if fflib.SimpleLetterEqualFold(ffj_key_University_Name, kn) {
					currentKey = ffj_t_University_Name
					state = fflib.FFParse_want_colon
					goto mainparse
				}

				currentKey = ffj_t_Universityno_such_key
				state = fflib.FFParse_want_colon
				goto mainparse
			}

		case fflib.FFParse_want_colon:
			if tok != fflib.FFTok_colon {
				wantedTok = fflib.FFTok_colon
				goto wrongtokenerror
			}
			state = fflib.FFParse_want_value
			continue
		case fflib.FFParse_want_value:

			if tok == fflib.FFTok_left_brace || tok == fflib.FFTok_left_bracket || tok == fflib.FFTok_integer || tok == fflib.FFTok_double || tok == fflib.FFTok_string || tok == fflib.FFTok_bool || tok == fflib.FFTok_null {
				switch currentKey {

				case ffj_t_University_Name:
					goto handle_Name

				case ffj_t_University_Students:
					goto handle_Students

				case ffj_t_Universityno_such_key:
					err = fs.SkipField(tok)
					if err != nil {
						return fs.WrapErr(err)
					}
					state = fflib.FFParse_after_value
					goto mainparse
				}
			} else {
				goto wantedvalue
			}
		}
	}

handle_Name:

	/* handler: uj.Name type=string kind=string quoted=false*/

	{

		{
			if tok != fflib.FFTok_string && tok != fflib.FFTok_null {
				return fs.WrapErr(fmt.Errorf("cannot unmarshal %s into Go value for string", tok))
			}
		}

		if tok == fflib.FFTok_null {

		} else {

			outBuf := fs.Output.Bytes()

			uj.Name = string(string(outBuf))

		}
	}

	state = fflib.FFParse_after_value
	goto mainparse

handle_Students:

	/* handler: uj.Students type=[]education.Student kind=slice quoted=false*/

	{

		{
			if tok != fflib.FFTok_left_brace && tok != fflib.FFTok_null {
				return fs.WrapErr(fmt.Errorf("cannot unmarshal %s into Go value for ", tok))
			}
		}

		if tok == fflib.FFTok_null {
			uj.Students = nil
		} else {

			uj.Students = make([]Student, 0)

			wantVal := true

			for {

				var v Student

				tok = fs.Scan()
				if tok == fflib.FFTok_error {
					goto tokerror
				}
				if tok == fflib.FFTok_right_brace {
					break
				}

				if tok == fflib.FFTok_comma {
					if wantVal == true {
						// TODO(pquerna): this isn't an ideal error message, this handles
						// things like [,,,] as an array value.
						return fs.WrapErr(fmt.Errorf("wanted value token, but got token: %v", tok))
					}
					continue
				} else {
					wantVal = true
				}

				/* handler: v type=education.Student kind=struct quoted=false*/

				{
					if tok == fflib.FFTok_null {

						state = fflib.FFParse_after_value
						goto mainparse
					}

					err = v.UnmarshalJSONFFLexer(fs, fflib.FFParse_want_key)
					if err != nil {
						return err
					}
					state = fflib.FFParse_after_value
				}

				uj.Students = append(uj.Students, v)
				wantVal = false
			}
		}
	}

	state = fflib.FFParse_after_value
	goto mainparse

wantedvalue:
	return fs.WrapErr(fmt.Errorf("wanted value token, but got token: %v", tok))
wrongtokenerror:
	return fs.WrapErr(fmt.Errorf("ffjson: wanted token: %v, but got token: %v output=%s", wantedTok, tok, fs.Output.String()))
tokerror:
	if fs.BigError != nil {
		return fs.WrapErr(fs.BigError)
	}
	err = fs.Error.ToError()
	if err != nil {
		return fs.WrapErr(err)
	}
	panic("ffjson-generated: unreachable, please report bug.")
done:
	return nil
}

main.go

package main

import (
	"spike/education"

	"github.com/pquerna/ffjson/ffjson"
)

func main() {
	student := education.Student{
		FirstName: "John",
		LastName:  "Smith",
	}

	university := education.University{
		Name:     "MIT",
		Students: []education.Student{student},
	}

	json, err := ffjson.Marshal(&university)
	if err != nil {
		panic(err)
	}

	var universityCopy education.University
	err = ffjson.Unmarshal(json, &universityCopy)
	if err != nil {
		panic(err)
	}
}