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.
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:])
	}
}

// PostID generates a post ID.
func PostID() string {
	return ID("P", 16)
}

// StoryID generates a post ID.
func StoryID() string {
	return ID("S", 16)
}

// ChapterID generates a post ID.
func ChapterID() string {
	return ID("SC", 24)
}