package generate

import (
	"crypto/rand"
	"encoding/binary"
	mrand "math/rand"
	"strconv"
	"strings"
)

// ID generates an ID using crypto-random, falling back to math random when that fails
// to avoid disrupting operation because of a faulty RNG. Database implementations does
// not need to use this if some other scheme improves storage.
func ID(prefix string, length int) string {
	var data [32]byte

	result := strings.Builder{}
	result.Grow(length + 32)
	result.WriteString(prefix)

	pos := 0
	for result.Len() < length {
		if pos == 0 {
			randRead(data[:])
		}

		result.WriteString(strconv.FormatUint(binary.BigEndian.Uint64(data[pos:pos+8]), 36))

		pos = (pos + 8) % 32
	}

	return result.String()[:length]
}

func randRead(data []byte) {
	n, err := rand.Read(data)
	if err != nil {
		mrand.Read(data[n:])
	}
}