package main

import (
	"encoding/json"
	"fmt"
	"reflect"
	"strings"
)

// Example - An example JSON-friendly struct.
type Example struct {
	A string `json:"A"`
	B int64  `json:"B"`
	C string `json:"C"`
	D struct {
		E string `json:"E"`
	} `json:"D"`
}

func main() {
	fmt.Println(CompareJSONToStruct([]byte(`{"A": "null","B": 0,"C": "null","D": {"E": "null"}}`), Example{}))
}

// CompareJSONToStruct - Compare the fields in the JSON []byte slice to a struct.
// Only top level fields are checked, nested struct fields are ignored.
func CompareJSONToStruct(bytes []byte, empty interface{}) bool {
	var mapped map[string]interface{}

	if err := json.Unmarshal(bytes, &mapped); err != nil {
		return false
	}

	emptyValue := reflect.ValueOf(empty).Type()

	// check if number of fields is the same
	if len(mapped) != emptyValue.NumField() {
		return false
	}

	// check if field names are the same
	for key := range mapped {
	// @todo go deeper into nested struct fields
		if field, found := emptyValue.FieldByName(key); found {
			if !strings.EqualFold(key, strings.Split(field.Tag.Get("json"), ",")[0]) {
				return false
			}
		}
	}

	// @todo check for field type mismatching

	return true
}