package protocol

import (

	"crypto/rand"

	"fmt"

	"reflect"

)

// RandReader is the random reader the protocol package will use to read

// random bytes from. This is exported for testing, and should not be used.

var RandReader = rand.Reader

const idempotencyTokenFillTag = `idempotencyToken`

// CanSetIdempotencyToken returns true if the struct field should be

// automatically populated with a Idempotency token.

//

// Only *string and string type fields that are tagged with idempotencyToken

// which are not already set can be auto filled.

func CanSetIdempotencyToken(v reflect.Value, f reflect.StructField) bool {

	switch u := v.Interface().(type) {

	// To auto fill an Idempotency token the field must be a string,

	// tagged for auto fill, and have a zero value.

	case *string:

		return u == nil && len(f.Tag.Get(idempotencyTokenFillTag)) != 0

	case string:

		return len(u) == 0 && len(f.Tag.Get(idempotencyTokenFillTag)) != 0

	}

	return false

}

// GetIdempotencyToken returns a randomly generated idempotency token.

func GetIdempotencyToken() string {

	b := make([]byte, 16)

	RandReader.Read(b)

	return UUIDVersion4(b)

}

// SetIdempotencyToken will set the value provided with a Idempotency Token.

// Given that the value can be set. Will panic if value is not setable.

func SetIdempotencyToken(v reflect.Value) {

	if v.Kind() == reflect.Ptr {

		if v.IsNil() && v.CanSet() {

			v.Set(reflect.New(v.Type().Elem()))

		}

		v = v.Elem()

	}

	v = reflect.Indirect(v)

	if !v.CanSet() {

		panic(fmt.Sprintf("unable to set idempotnecy token %v", v))

	}

	b := make([]byte, 16)

	_, err := rand.Read(b)

	if err != nil {

		// TODO handle error

		return

	}

	v.Set(reflect.ValueOf(UUIDVersion4(b)))

}

// UUIDVersion4 returns a Version 4 random UUID from the byte slice provided

func UUIDVersion4(u []byte) string {

	// https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_.28random.29

	// 13th character is "4"

	u[6] = (u[6] | 0x40) & 0x4F

	// 17th character is "8", "9", "a", or "b"

	u[8] = (u[8] | 0x80) & 0xBF

	return fmt.Sprintf(`%X-%X-%X-%X-%X`, u[0:4], u[4:6], u[6:8], u[8:10], u[10:])

}