Ulid generation

package main

import (
    "fmt"
    "log"
    "math/rand"
    "time"

    "github.com/oklog/ulid"
)

func main() {
    t := time.Now()
    // t := time.Unix(1000000, 0)
    entropy := ulid.Monotonic(rand.New(rand.NewSource(t.UnixNano())), 0)
    // entropy := rand.Reader
    // entropy := ulid.Monotonic(rand.Reader, 0)
    ids := make([]ulid.ULID, 10000)
    for i := range ids {
        ids[i] = ulid.MustNew(ulid.Timestamp(t), entropy)
    }
    seen := make(map[ulid.ULID]bool)
    for _, id := range ids {
        fmt.Println(id)
        if seen[id] {
            log.Fatal("dup")
        }
        seen[id] = true
    }
}