generating "sequential" random IDs

id.go

package main

import (
	"crypto/rand"
	"fmt"
	"log"
	"math/big"
	"time"
)

func main() {
	seq, err := GenerateSequence(1001)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println(seq)
}

// GenerateSequence is a take on on http://instagram-engineering.tumblr.com/post/10853187575/sharding-ids-at-instagram
// that uses random integers instead of an auto-incrementing sequence.
func GenerateSequence(shardID int64) (int64, error) {
	epoch := time.Date(2015, time.July, 1, 0, 0, 0, 0, time.UTC)
	ms := time.Since(epoch).Nanoseconds() / 1000000
	// Fill the first 41 bits with our time (in ms)
	id := ms << (64 - 41)

	// Generate a random int between 1 & 100000
	num, err := rand.Int(rand.Reader, big.NewInt(100000))
	if err != nil {
		return id, err
	}

	// Fill the next 13 bits
	id |= num.Int64() % 2000 << (64 - 41)

	// Fill the remaining 10 bits
	id |= (shardID % 1024)

	return id, nil
}