package utils

import (
	"crypto/rand"
	"encoding/hex"
	"fmt"
	"io"
	"sync"
)

func GenerateUUID() string {
	return Must(NewRandom()).String()
}

// NewRandom returns a Random (Version 4) UUID.
//
// The strength of the UUIDs is based on the strength of the crypto/rand
// package.
//
// Uses the randomness pool if it was enabled with EnableRandPool.
//
// A note about uniqueness derived from the UUID Wikipedia entry:
//
//  Randomly generated UUIDs have 122 random bits.  One's annual risk of being
//  hit by a meteorite is estimated to be one chance in 17 billion, that
//  means the probability is about 0.00000000006 (6 × 10−11),
//  equivalent to the odds of creating a few tens of trillions of UUIDs in a
//  year and having one duplicate.
func NewRandom() (UUID, error) {
	return newRandomFromPool()
}

func newRandomFromPool() (UUID, error) {
	var uuid UUID
	poolMu.Lock()
	if poolPos == randPoolSize {
		_, err := io.ReadFull(rander, pool[:])
		if err != nil {
			poolMu.Unlock()
			return UUID{}, err
		}
		poolPos = 0
	}
	copy(uuid[:], pool[poolPos:(poolPos+16)])
	poolPos += 16
	poolMu.Unlock()

	uuid[6] = (uuid[6] & 0x0f) | 0x40 // Version 4
	uuid[8] = (uuid[8] & 0x3f) | 0x80 // Variant is 10
	return uuid, nil
}

// A UUID is a 128 bit (16 byte) Universal Unique IDentifier as defined in RFC
// 4122.
type UUID [16]byte

// A Version represents a UUID's version.
type Version byte

// A Variant represents a UUID's variant.
type Variant byte

// Constants returned by Variant.
const (
	Invalid   = Variant(iota) // Invalid UUID
	RFC4122                   // The variant specified in RFC4122
	Reserved                  // Reserved, NCS backward compatibility.
	Microsoft                 // Reserved, Microsoft Corporation backward compatibility.
	Future                    // Reserved for future definition.
)

const randPoolSize = 16 * 16

var (
	rander  = rand.Reader // random function
	poolMu  sync.Mutex
	poolPos = randPoolSize     // protected with poolMu
	pool    [randPoolSize]byte // protected with poolMu
)

// Must returns uuid if err is nil and panics otherwise.
func Must(uuid UUID, err error) UUID {
	if err != nil {
		panic(err)
	}
	return uuid
}

// String returns the string form of uuid, xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
// , or "" if uuid is invalid.
func (uuid UUID) String() string {
	var buf [36]byte
	encodeHex(buf[:], uuid)
	return string(buf[:])
}

// URN returns the RFC 2141 URN form of uuid,
// urn:uuid:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx,  or "" if uuid is invalid.
func (uuid UUID) URN() string {
	var buf [36 + 9]byte
	copy(buf[:], "urn:uuid:")
	encodeHex(buf[9:], uuid)
	return string(buf[:])
}

func encodeHex(dst []byte, uuid UUID) {
	hex.Encode(dst, uuid[:4])
	dst[8] = '-'
	hex.Encode(dst[9:13], uuid[4:6])
	dst[13] = '-'
	hex.Encode(dst[14:18], uuid[6:8])
	dst[18] = '-'
	hex.Encode(dst[19:23], uuid[8:10])
	dst[23] = '-'
	hex.Encode(dst[24:], uuid[10:])
}

// Variant returns the variant encoded in uuid.
func (uuid UUID) Variant() Variant {
	switch {
	case (uuid[8] & 0xc0) == 0x80:
		return RFC4122
	case (uuid[8] & 0xe0) == 0xc0:
		return Microsoft
	case (uuid[8] & 0xe0) == 0xe0:
		return Future
	default:
		return Reserved
	}
}

// Version returns the version of uuid.
func (uuid UUID) Version() Version {
	return Version(uuid[6] >> 4)
}

func (v Version) String() string {
	if v > 15 {
		return fmt.Sprintf("BAD_VERSION_%d", v)
	}
	return fmt.Sprintf("VERSION_%d", v)
}

func (v Variant) String() string {
	switch v {
	case RFC4122:
		return "RFC4122"
	case Reserved:
		return "Reserved"
	case Microsoft:
		return "Microsoft"
	case Future:
		return "Future"
	case Invalid:
		return "Invalid"
	}
	return fmt.Sprintf("BadVariant%d", int(v))
}