package guuid

import (
	"crypto/rand"
	"encoding/hex"
	"errors"
	"io"
	"strings"
)



const (
	uuidByteCount     = 16
	uuidDashCount     = 4
	uuidEncodedLength = (uuidByteCount * 2) + uuidDashCount
)


var (
	dashIndexes = []int { 8, 13, 18, 23 }
	emptyUUID   = UUID {}
)

var (
	BadLengthError       = errors.New("guuid: str isn't of the correct length")
	BadDashCountError    = errors.New("guuid: Bad count of dashes in string")
	BadDashPositionError = errors.New("guuid: Bad char in string")
)



type UUID [uuidByteCount]byte



func NewFrom(r io.Reader) (UUID, error) {
	var uuid UUID
	_, err := io.ReadFull(r, uuid[:])
	if err != nil {
		return emptyUUID, err
	}
	uuid[6] = (uuid[6] & 0x0f) | 0x40 // v4
	uuid[8] = (uuid[8] & 0x3f) | 0x80 // variant 10
	return uuid, nil
}

func New() UUID {
	uuid, err := NewFrom(rand.Reader)
	if err != nil {
		panic(err)
	}
	return uuid
}

func (uuid UUID) String() string {
	dst := [uuidEncodedLength]byte {
		0, 0, 0, 0,
		0, 0, 0, 0,
		'-',
		0, 0, 0, 0,
		'-',
		0, 0, 0, 0,
		'-',
		0, 0, 0, 0,
		'-',
		0, 0, 0, 0,
		0, 0, 0, 0,
		0, 0, 0, 0,
	}

	hex.Encode(dst[ 0:8],  uuid[0:4])
	hex.Encode(dst[ 9:13], uuid[4:6])
	hex.Encode(dst[14:18], uuid[6:8])
	hex.Encode(dst[19:23], uuid[8:10])
	hex.Encode(dst[24:36], uuid[10:])

	return string(dst[:])
}

func FromString(str string) (UUID, error) {
	if len(str) != uuidEncodedLength {
		return emptyUUID, BadLengthError
	}

	if strings.Count(str, "-") != uuidDashCount {
		return emptyUUID, BadDashCountError
	}

	for _, idx := range dashIndexes {
		if str[idx] != '-' {
			return emptyUUID, BadDashPositionError
		}
	}

	hexstr := strings.Join(strings.Split(str, "-"), "")
	data, err := hex.DecodeString(hexstr)
	if err != nil {
		return emptyUUID, err
	}

	return [uuidByteCount]byte(data), nil
}