package entity

import (
	"regexp"
	"strings"
)

const (
	SparkIDMin = 6
	SparkIDMax = 32
)

var sparkIDRe = regexp.MustCompile(`^[a-zA-Z][a-zA-Z0-9_]{5,31}$`)

func ValidSparkID(id string) bool {
	n := len(id)
	return n >= SparkIDMin && n <= SparkIDMax && sparkIDRe.MatchString(id)
}

// DefaultSparkID is a unique, letter-prefixed handle derived from the Snowflake id.
func DefaultSparkID(userID int64) string {
	s := "s" + strings.ToLower(strconv36(userID))
	for len(s) < SparkIDMin {
		s += "0"
	}
	if len(s) > SparkIDMax {
		s = s[:SparkIDMax]
	}
	return s
}

func strconv36(n int64) string {
	if n < 0 {
		n = -n
	}
	const digits = "0123456789abcdefghijklmnopqrstuvwxyz"
	if n == 0 {
		return "0"
	}
	var buf [16]byte
	i := len(buf)
	for n > 0 {
		i--
		buf[i] = digits[n%36]
		n /= 36
	}
	return string(buf[i:])
}