// Copyright 2016 Google Inc.  All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package uuid

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

// 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.
)

var rander = rand.Reader // random function

// Parse decodes s into a UUID or returns an error.  Both the UUID form of
// xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx and
// urn:uuid:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx are decoded.
func Parse(s string) (UUID, error) {
	var uuid UUID
	if len(s) != 36 {
		if len(s) != 36+9 {
			return uuid, fmt.Errorf("invalid UUID length: %d", len(s))
		}
		if strings.ToLower(s[:9]) != "urn:uuid:" {
			return uuid, fmt.Errorf("invalid urn prefix: %q", s[:9])
		}
		s = s[9:]
	}
	if s[8] != '-' || s[13] != '-' || s[18] != '-' || s[23] != '-' {
		return uuid, errors.New("invalid UUID format")
	}
	for i, x := range [16]int{
		0, 2, 4, 6,
		9, 11,
		14, 16,
		19, 21,
		24, 26, 28, 30, 32, 34} {
		if v, ok := xtob(s[x:]); !ok {
			return uuid, errors.New("invalid UUID format")
		} else {
			uuid[i] = v
		}
	}
	return uuid, nil
}

// ParseBytes is like Parse, exect it parses a byte slice instead of a string.
func ParseBytes(b []byte) (UUID, error) {
	// Parsing a string is actually faster than parsing a byte slice as it
	// is cheaper to slice a string.  Further, it is not safe to convert
	// a string into a byte slice but the opposite direction is.  These
	// stem from the fact that a byte slice is 3 words while a string
	// is only 2 words.
	return Parse(*(*string)(unsafe.Pointer(&b)))
}

// 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.
// valid.
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))
}

// SetRand sets the random number generator to r, which implents io.Reader.
// If r.Read returns an error when the package requests random data then
// a panic will be issued.
//
// Calling SetRand with nil sets the random number generator to the default
// generator.
func SetRand(r io.Reader) {
	if r == nil {
		rander = rand.Reader
		return
	}
	rander = r
}