package main

import (
    "bytes"
    "fmt"
    "reflect"
    "strconv"
    "strings"
)

type User struct {
    Name string `en:"name" it:"nome"`
    Age  int64  `en:"age" it:"eta"`
}

type City struct {
    Name       string `en:"name" it:"nome"`
    Population int64  `en:"pop" it:"pop"`
    GDP        int64  `en:"gdp" it:"pil"`
    Mayor      string `en:"mayor" it:"sindaco"`
}

func main() {
    var x float64 = 3.14
    var u User = User{"bob", 10}

    refObjValPtr := reflect.ValueOf(&x)
    refObjtyp := reflect.TypeOf(&x)
    fmt.Printf("ref obj val for × %s\n", refObjValPtr.String())
    fmt.Printf("ref obj typ for × %s\n", refObjtyp.String())

    refObjVal := refObjValPtr.Elem()
    fmt.Printf("ref obj val for float64 x: %f\n", refObjVal.Float())
    fmt.Printf("can set new val for refl obj? %v\n", refObjVal.CanSet())

    refObjVal.Set(reflect.ValueOf(4.25))
    fmt.Printf("updated × using ptr and reflection obj, now val %f \n", x)

    res, err := JSONEncode(u, "en")
    if err != nil {
        panic(err)
    }
    fmt.Println(string(res))

    c := City{"sf", 5000000, 567896, "mr jones"}
    res, err = JSONEncode(c, "it")
    if err != nil {
        panic(err)
    }
    fmt.Println(string(res))
}

func JSONEncode(v interface{}, tagKey string) ([]byte, error) {
    refObjVal := reflect.ValueOf(v)
    refObjTyp := reflect.TypeOf(v)
    buf := bytes.Buffer{}
    if refObjVal.Kind() != reflect.Struct {
        return buf.Bytes(), fmt.Errorf(
            "val of kind %s is not supported",
            refObjVal.Kind(),
        )
    }
    buf.WriteString("{")
    pairs := []string{}
    for i := 0; i < refObjVal.NumField(); i++ {
        structFieldRefObj := refObjVal.Field(i)
        structFieldRefObjTyp := refObjTyp.Field(i)

        tag := structFieldRefObjTyp.Tag.Get(tagKey)
        switch structFieldRefObj.Kind() {
        case reflect.String:
            strVal := structFieldRefObj.Interface().(string)
            pairs = append(pairs, `"`+tag+`":"`+strVal+`"`)
        case reflect.Int64:
            intVal := structFieldRefObj.Interface().(int64)
            pairs = append(pairs, `"`+tag+`":`+strconv.FormatInt(intVal, 10))
        default:
            return buf.Bytes(), fmt.Errorf(
                "struct field with name %s and kind %s is not supprted",
                structFieldRefObjTyp.Name,
                structFieldRefObj.Kind(),
            )
        }
    }

    buf.WriteString(strings.Join(pairs, ","))
    buf.WriteString("}")

    return buf.Bytes(), nil
}