A package to create a unique identifier (UUID)

uuid.go

// Creates a unique identifier
package uuid

import (
	"crypto/md5"
	"crypto/rand"
	"encoding/binary"
	"fmt"
	"io"
	"os"
	"sync/atomic"
	"time"
)

type UniqueId string

// NewUniqueId returns a new unique UniqueId.
func NewUniqueId() UniqueId {

	var b [12]byte

	// Timestamp, 4 bytes, big endian
	binary.BigEndian.PutUint32(b[:], uint32(time.Now().Unix()))

	// Machine, first 3 bytes of md5(hostname)
	b[4] = machineId[0]
	b[5] = machineId[1]
	b[6] = machineId[2]

	// Pid, 2 bytes, specs don't specify endianness, but we use big endian.
	pid := os.Getpid()
	b[7] = byte(pid >> 8)
	b[8] = byte(pid)

	// Increment, 3 bytes, big endian
	i := atomic.AddUint32(&UniqueIdCounter, 1)
	b[9] = byte(i >> 16)
	b[10] = byte(i >> 8)
	b[11] = byte(i)

	return UniqueId(b[:])
}

// String returns a string representation of the unique id
func (id UniqueId) String() string {
	return fmt.Sprintf("%x", string(id))
}

// UniqueIdCounter is atomically incremented when generating a new UniqueId
// using NewUniqueId() function. It's used as a counter part of an id.
var UniqueIdCounter uint32 = readRandomUint32()

// readRandomUint32 returns a random UniqueIdCounter.
func readRandomUint32() uint32 {

	var b [4]byte

	_, err := io.ReadFull(rand.Reader, b[:])
	if err != nil {
		panic(fmt.Errorf("cannot read random object id: %v", err))
	}

	return uint32((uint32(b[0]) << 0) | (uint32(b[1]) << 8) | (uint32(b[2]) << 16) | (uint32(b[3]) << 24))
}

// machineId stores machine id generated once and used in subsequent calls
// to NewUniqueId function.
var machineId = readMachineId()

// readMachineId generates and returns a machine id.
// If this function fails to get the hostname it will cause a runtime error.
func readMachineId() []byte {

	var sum [3]byte

	id := sum[:]
	hostname, err1 := os.Hostname()
	if err1 != nil {
		_, err2 := io.ReadFull(rand.Reader, id)
		if err2 != nil {
			panic(fmt.Errorf("cannot get hostname: %v; %v", err1, err2))
		}
		return id
	}

	hw := md5.New()
	hw.Write([]byte(hostname))
	copy(id, hw.Sum(nil))

	return id
}