package entity

import (
	"regexp"
)

const (
	SparkIDMin = 8
	SparkIDMax = 16
	// sparkIDBodyLen is 13: int64 fits in 13 base36 digits (36^13 > 2^63).
	sparkIDBodyLen = 13
	sparkIDPrefix  = 's'
)

var sparkIDRe = regexp.MustCompile(`^[a-zA-Z][a-zA-Z0-9_]{7,15}$`)

// ValidSparkID checks a new or updated handle. Existing rows may still be 6-32.
func ValidSparkID(id string) bool {
	n := len(id)
	return n >= SparkIDMin && n <= SparkIDMax && sparkIDRe.MatchString(id)
}

// DefaultSparkID maps a Snowflake uid to a unique 14-character handle.
// Format: 's' + 13-digit zero-padded base36(uid). Same uid always yields the same id;
// different uids never collide because the body is a fixed-width encoding of the integer.
func DefaultSparkID(userID int64) string {
	if userID < 0 {
		userID = -userID
	}
	body := strconv36(userID)
	var b [1 + sparkIDBodyLen]byte
	b[0] = sparkIDPrefix
	pad := sparkIDBodyLen - len(body)
	for i := 1; i <= pad; i++ {
		b[i] = '0'
	}
	copy(b[1+pad:], body)
	return string(b[:])
}

func strconv36(n int64) string {
	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:])
}