objectid.go

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

package tokenauth

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

// ObjectId is a unique ID identifying a BSON value. It must be exactly 12 bytes long.
// clone from https://github.com/go-mgo/mgo/blob/v2-unstable/bson/bson.go.
// http://www.mongodb.org/display/DOCS/Object+IDs.
type ObjectId string

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

var objectIdCounter uint32 = 0

// readMachineId generates machine id and puts it into the machineId global
// variable. 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
}

// NewObjectId returns a new unique ObjectId.
func NewObjectId() ObjectId {

	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(&objectIdCounter, 1)
	b[9] = byte(i >> 16)
	b[10] = byte(i >> 8)
	b[11] = byte(i)
	return ObjectId(b[:])
}

// byteSlice returns byte slice of id from start to end.
// Calling this function with an invalid id will cause a runtime panic.
func (id ObjectId) byteSlice(start, end int) []byte {
	if len(id) != 12 {
		panic(fmt.Sprintf("Invalid ObjectId: %q", string(id)))
	}
	return []byte(string(id)[start:end])
}

// Time returns the timestamp part of the id.
// It's a runtime error to call this method with an invalid id.
func (id ObjectId) Time() time.Time {
	// First 4 bytes of ObjectId is 32-bit big-endian seconds from epoch.
	secs := int64(binary.BigEndian.Uint32(id.byteSlice(0, 4)))
	return time.Unix(secs, 0)
}

// Machine returns the 3-byte machine id part of the id.
// It's a runtime error to call this method with an invalid id.
func (id ObjectId) Machine() []byte {
	return id.byteSlice(4, 7)
}

// Pid returns the process id part of the id.
// It's a runtime error to call this method with an invalid id.
func (id ObjectId) Pid() uint16 {
	return binary.BigEndian.Uint16(id.byteSlice(7, 9))
}

// Counter returns the incrementing value part of the id.
// It's a runtime error to call this method with an invalid id.
func (id ObjectId) Counter() int32 {
	b := id.byteSlice(9, 12)
	// Counter is stored as big-endian 3-byte value
	return int32(uint32(b[0])<<16 | uint32(b[1])<<8 | uint32(b[2]))
}

// Hex returns a hex representation of the ObjectId.
func (id ObjectId) Hex() string {
	return hex.EncodeToString([]byte(id))
}