/*
This package provides implementation of Universally Unique Identifier (UUID).
Supported versions are 1, 3, 4 and 5 (as specified in RFC 4122) and
version 2 (as specified in DCE 1.1).
*/
package uuid

import (
	"bytes"
	"crypto/md5"
	"crypto/rand"
	"crypto/sha1"
	"encoding/binary"
	"errors"
	"fmt"
	"hash"
	"net"
	"os"
	"time"
)

// UUID layout variants.
const (
	VariantNCS = iota
	VariantRFC4122
	VariantMicrosoft
	VariantFuture
)

// UUID DCE domains.
const (
	DomainPerson = iota
	DomainGroup
	DomainOrg
)

// Difference in 100-nanosecond intervals between
// UUID epoch (October 15, 1582) and Unix epoch (January 1, 1970).
const epochStart = 122192928000000000

// UUID v1/v2 storage.
var (
	clockSequence uint16
	lastTime      uint64
	hardwareAddr  [6]byte
	posixUID      = uint32(os.Getuid())
	posixGID      = uint32(os.Getgid())
)

// Epoch calculation function
var epochFunc func() uint64

// Initialize storage
func init() {
	buf := make([]byte, 2)
	rand.Read(buf)
	clockSequence = binary.BigEndian.Uint16(buf)

	// Initialize hardwareAddr randomly in case
	// of real network interfaces absence
	rand.Read(hardwareAddr[:])

	interfaces, err := net.Interfaces()
	if err == nil {
		for _, iface := range interfaces {
			if len(iface.HardwareAddr) >= 6 {
				copy(hardwareAddr[:], iface.HardwareAddr)
				break
			}
		}
	}
	epochFunc = unixTimeFunc
}

// Returns difference in 100-nanosecond intervals between
// UUID epoch (October 15, 1582) and current time.
// This is default epoch calculation function.
func unixTimeFunc() uint64 {
	return epochStart + uint64(time.Now().UnixNano()/100)
}

// Returns current epoch calculation function.
func GetEpochFunc() func() uint64 {
	return epochFunc
}

// Sets epoch calculation function.
func SetEpochFunc(newEpochFunc func() uint64) {
	epochFunc = newEpochFunc
}

// UUID representation compliant with specification
// described in RFC 4122.
type UUID [16]byte

// Predefined namespace UUIDs.
var (
	NamespaceDNS  = &UUID{0x6b, 0xa7, 0xb8, 0x10, 0x9d, 0xad, 0x11, 0xd1, 0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8}
	NamespaceURL  = &UUID{0x6b, 0xa7, 0xb8, 0x11, 0x9d, 0xad, 0x11, 0xd1, 0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8}
	NamespaceOID  = &UUID{0x6b, 0xa7, 0xb8, 0x12, 0x9d, 0xad, 0x11, 0xd1, 0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8}
	NamespaceX500 = &UUID{0x6b, 0xa7, 0xb8, 0x14, 0x9d, 0xad, 0x11, 0xd1, 0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8}
)

// Returns true if u1 and u2 equals, otherwise returns false.
func Equal(u1 *UUID, u2 *UUID) bool {
	return bytes.Equal(u1[:], u2[:])
}

// Returns algorithm version used to generate UUID.
func (u *UUID) Version() uint {
	return uint(u[6] >> 4)
}

// Returns UUID layout variant.
func (u *UUID) Variant() uint {
	switch {
	case (u[8] & 0x80) == 0x00:
		return VariantNCS
	case (u[8]&0xc0)|0x80 == 0x80:
		return VariantRFC4122
	case (u[8]&0xe0)|0xc0 == 0xc0:
		return VariantMicrosoft
	}
	return VariantFuture
}

// Returns canonical string representation of UUID:
// xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx.
func (u *UUID) String() string {
	return fmt.Sprintf("%x-%x-%x-%x-%x",
		u[:4], u[4:6], u[6:8], u[8:10], u[10:])
}

// Sets version bits.
func (u *UUID) SetVersion(v byte) {
	u[6] = (u[6] & 0x0f) | (v << 4)
}

// Sets variant bits as described in RFC 4122.
func (u *UUID) SetVariant() {
	u[8] = (u[8] & 0xbf) | 0x80
}

// Returns UUID epoch timestamp
func getTimestamp() uint64 {
	timeNow := epochStart + epochFunc()
	// Clock changed backwards since last UUID generation.
	// Should increase clock sequence.
	if timeNow <= lastTime {
		clockSequence++
	}
	lastTime = timeNow
	return timeNow
}

// Returns UUID based on current timestamp and MAC address.
func NewV1() (*UUID, error) {
	u := new(UUID)

	timeNow := getTimestamp()

	binary.BigEndian.PutUint32(u[0:], uint32(timeNow))
	binary.BigEndian.PutUint16(u[4:], uint16(timeNow>>32))
	binary.BigEndian.PutUint16(u[6:], uint16(timeNow>>48))
	binary.BigEndian.PutUint16(u[8:], clockSequence)

	copy(u[10:], hardwareAddr[:])

	u.SetVersion(1)
	u.SetVariant()

	return u, nil
}

// Returns DCE Security UUID based on POSIX UID/GID.
func NewV2(domain byte) (*UUID, error) {
	u := new(UUID)

	switch domain {
	case DomainPerson:
		binary.BigEndian.PutUint32(u[0:], posixUID)
	case DomainGroup:
		binary.BigEndian.PutUint32(u[0:], posixGID)
	default:
		return nil, errors.New("Unsupported domain")
	}

	timeNow := getTimestamp()

	binary.BigEndian.PutUint16(u[4:], uint16(timeNow>>32))
	binary.BigEndian.PutUint16(u[6:], uint16(timeNow>>48))
	binary.BigEndian.PutUint16(u[8:], clockSequence)
	u[9] = domain
	copy(u[10:], hardwareAddr[:])
	u.SetVersion(2)
	u.SetVariant()

	return u, nil
}

// Returns UUID based on MD5 hash of namespace UUID and name.
func NewV3(ns *UUID, name string) (*UUID, error) {
	u := newFromHash(md5.New(), ns, name)
	u.SetVersion(3)
	u.SetVariant()

	return u, nil
}

// Returns random generated UUID.
func NewV4() (*UUID, error) {
	u := new(UUID)
	_, err := rand.Read(u[:])
	if err != nil {
		return nil, err
	}
	u.SetVersion(4)
	u.SetVariant()

	return u, nil
}

// Returns UUID based on SHA-1 hash of namespace UUID and name.
func NewV5(ns *UUID, name string) (*UUID, error) {
	u := newFromHash(sha1.New(), ns, name)
	u.SetVersion(5)
	u.SetVariant()

	return u, nil
}

// Returns UUID based on hashing of namespace UUID and name.
func newFromHash(h hash.Hash, ns *UUID, name string) *UUID {
	u := new(UUID)
	h.Write(ns[:])
	h.Write([]byte(name))
	copy(u[:], h.Sum(nil))

	return u
}