package shorturl

import (
	"crypto/rand"
	"fmt"
	"math/big"
	"regexp"
)

// CodeLength is the length of generated short codes. 10 base36 characters
// give 36^10 ≈ 3.6 × 10^15 combinations.
const CodeLength = 10

// codeAlphabet is the base36 alphabet (lowercase letters + digits).
const codeAlphabet = "abcdefghijklmnopqrstuvwxyz0123456789"

var codeAlphabetBig = big.NewInt(int64(len(codeAlphabet)))

// RandomCode returns a cryptographically random base36 code of CodeLength chars.
func RandomCode() (string, error) {
	buf := make([]byte, CodeLength)
	for i := range buf {
		n, err := rand.Int(rand.Reader, codeAlphabetBig)
		if err != nil {
			return "", fmt.Errorf("random code: %w", err)
		}
		buf[i] = codeAlphabet[n.Int64()]
	}
	return string(buf), nil
}

var validCodeRe = regexp.MustCompile(`^[a-z0-9]{4,16}$`)

// ValidCode returns true if s could plausibly be a short code we issued.
// Lenient on length (4-16) to allow future tuning without breaking old links.
func ValidCode(s string) bool {
	return validCodeRe.MatchString(s)
}