jsonscan.go

package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"reflect"
	"regexp"
	"strconv"
	"strings"
)

var s = `
{
	"foo": {
		"bar": [
			{
				"baz": "bbb",
				"noo": 3 
			},
			{
				"maz": true,
				"moo": ["foo", "bar"]
			}
		],
		"boo": {
			"bag": "ddd",
			"bug": "ccc"
		}
	}
}
`

var re = regexp.MustCompile("^([^0-9\\s\\[][^\\s\\[]*)?(\\[[0-9]+\\])?$")

func jsonScan(b []byte, jp string, t interface{}) (err error) {
	var v interface{}
	err = json.Unmarshal([]byte(s), &v)
	if err != nil {
		return err
	}
	if jp == "" {
		return errors.New("invalid path")
	}
	var ok bool
	for _, token := range strings.Split(jp, "/") {
		sl := re.FindAllStringSubmatch(token, -1)
		if len(sl) == 0 {
			return errors.New("invalid path")
		}
		ss := sl[0]
		if ss[1] != "" {
			if v, ok = v.(map[string]interface{})[ss[1]]; !ok {
				return errors.New("invalid path: " + ss[1])
			}
		}
		if ss[2] != "" {
			i, err := strconv.Atoi(ss[2][1 : len(ss[2])-1])
			if err != nil {
				return errors.New("invalid path: " + ss[2])
			}
			var vl []interface{}
			if vl, ok = v.([]interface{}); !ok {
				return errors.New("invalid path: " + ss[2])
			}
			if i < 0 || i > len(vl)-1 {
				return errors.New("invalid path: " + ss[2])
			}
			v = vl[i]
		}
	}
	rt := reflect.ValueOf(t).Elem()
	rv := reflect.ValueOf(v)
	defer func() {
		if errstr := recover(); errstr != nil {
			err = errors.New(errstr.(string))
		}
	}()
	rt.Set(rv)
	return nil
}

func main() {
	var t1 string
	var err error
	err = jsonScan([]byte(s), "/foo/bar[0]/baz", &t1)
	if err == nil {
		fmt.Println(t1)
	} else {
		fmt.Println(err.Error())
	}

	var t2 float64
	err = jsonScan([]byte(s), "/foo/bar[0]/noo", &t2)
	if err == nil {
		fmt.Println(t2)
	} else {
		fmt.Println(err.Error())
	}

	var t3 bool
	err = jsonScan([]byte(s), "/foo/bar[1]/maz", &t3)
	if err == nil {
		fmt.Println(t3)
	} else {
		fmt.Println(err.Error())
	}

	var t4 []interface{}
	err = jsonScan([]byte(s), "/foo/bar[1]/moo", &t4)
	if err == nil {
		fmt.Println(t4)
	} else {
		fmt.Println(err.Error())
	}

	var t5 map[string]interface{}
	err = jsonScan([]byte(s), "/foo/boo", &t5)
	if err == nil {
		fmt.Println(t5)
	} else {
		fmt.Println(err.Error())
	}
}