package generate

import (
	"crypto/rand"
	"encoding/binary"
	mathRand "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 {
		mathRand.Read(data[n:])
	}
}

// InternalErrorID generates a long string
func InternalErrorID() string {
	return ID("ISE", 32)
}

// TagID generates a location ID. len=8
func TagID(longName string) string {
	return friendlyID('T', longName, 8)
}

// CharacterID generates a character ID. len=8
func CharacterID(longName string) string {
	return friendlyID('C', longName, 8)
}

func friendlyID(prefix byte, name string, length int) string {
	b := strings.Builder{}
	b.Grow(4)
	b.WriteByte(prefix)
	for _, ch := range strings.ToLower(name) {
		if ch >= 'a' && ch <= 'z' {
			b.WriteRune(ch)

			if b.Len() > 3 {
				break
			}
		}
	}

	return ID(b.String(), length)
}

// StoryID generates a story ID: len=8
func StoryID() string {
	return ID("S", 12)
}

// PostID generates a post ID. len=12
func PostID() string {
	return ID("P", 16)
}

// AnnotationID generates an annotation.sql ID. len=12
func AnnotationID() string {
	return ID("A", 16)
}