Deterministic ULID generation from int64 in #golang

example.go

package main

import (
	"fmt"
	"time"

	"intulid"
	"github.com/oklog/ulid/v2"
)

type Event struct {
	id        int64
	createdAt time.Time
	Foo       string
	Bar       string
}

func main() {
	t := time.Unix(123, 567)

	uniq := make(map[string]struct{})

	for i := 0; i < 1000000000; i++ {
		e := Event{
			id:        int64(i),
			createdAt: t,
			Foo:       "foo",
			Bar:       "bar",
		}

		id, err := intulid.New(ulid.Timestamp(e.createdAt), e.id, []byte(`foo`))
		if err != nil {
			panic(err)
		}

		fmt.Printf("%d -> %s\n", e.id, id.String())

		if _, ok := uniq[id.String()]; ok {
			fmt.Printf("%d -> %s\n", e.id, id.String())
			panic("duplicate id")
		} else {
			uniq[id.String()] = struct{}{}
		}
	}
}

int.go

package intulid

import (
	"encoding/binary"
	"fmt"

	"github.com/oklog/ulid/v2"
	"golang.org/x/crypto/sha3"
)

type entropy []byte

func New(ms uint64, num int64, salt []byte) (ulid.ULID, error) {
	if len(salt) > 120 {
		return ulid.ULID{}, fmt.Errorf("salt len could not be longer than 120 bytes")
	}

	e := make(entropy, 8, 128)
	binary.LittleEndian.PutUint64(e[:8], uint64(num))
	e = append(e, salt...)

	return ulid.New(ms, e)
}

func (e entropy) Read(p []byte) (n int, err error) {
	sha3.ShakeSum256(p, e)
	return len(p), nil
}