// Package ulid provides thread-safe ULID (Universally Unique Lexicographically Sortable Identifier) generation.
//
// ULIDs are 128-bit identifiers that are lexicographically sortable and contain
// a timestamp component. This package uses a pool-based approach with
// cryptographically secure random seeding and monotonic ordering for optimal
// performance in concurrent environments.
package ulid

import (
	cryptorand "crypto/rand"
	"encoding/binary"
	"io"
	mathrand "math/rand"
	"os"
	"sync"
	"time"

	oklid "github.com/oklog/ulid/v2"
	"go.ntppool.org/common/logger"
)

// monotonicPool is a pool of monotonic ULID readers for performance optimization.
// Each reader is initialized with a cryptographically secure random seed
// and random increment value to ensure uniqueness across concurrent usage.
var monotonicPool = sync.Pool{
	New: func() any {
		log := logger.Setup()

		var seed int64
		err := binary.Read(cryptorand.Reader, binary.BigEndian, &seed)
		if err != nil {
			log.Error("crypto/rand error", "err", err)
			os.Exit(10)
		}

		rand := mathrand.New(mathrand.NewSource(seed))

		inc := uint64(mathrand.Int63())

		// log.Printf("seed: %d", seed)
		// log.Printf("inc:  %d", inc)

		// inc = inc & ^uint64(1<<63) // only want 63 bits
		mono := oklid.Monotonic(rand, inc)
		return mono
	},
}

// MakeULID generates a new ULID with the specified timestamp using a pooled monotonic reader.
// The function is thread-safe and optimized for high-concurrency environments.
//
// Each call retrieves a monotonic reader from the pool, generates a ULID with the
// given timestamp, and returns it. The reader is not returned to the pool as it
// maintains internal state for monotonic ordering.
//
// Returns a pointer to the generated ULID or an error if generation fails.
// Generation should only fail under extreme circumstances (entropy exhaustion).
func MakeULID(t time.Time) (*oklid.ULID, error) {
	mono := monotonicPool.Get().(io.Reader)

	id, err := oklid.New(oklid.Timestamp(t), mono)
	if err != nil {
		return nil, err
	}

	return &id, nil
}