[Object ID] Generate MongoDB object id. #Go

objectid.go

package main

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

func main() {
	fmt.Println(NewObjectId())
}

var machineId = readMachineId()
var processId = os.Getpid()
var objectIdCounter = readRandomUint32()

// readRandomUint32 returns a random objectIdCounter.
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))
}

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
}

func NewObjectId() string {
	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.
	b[7] = byte(processId >> 8)
	b[8] = byte(processId)
	// Increment, 3 bytes, big endian
	i := atomic.AddUint32(&objectIdCounter, 1)
	b[9] = byte(i >> 16)
	b[10] = byte(i >> 8)
	b[11] = byte(i)
	return hex.EncodeToString(b[:])
}