package shorturl

import (
	"crypto/sha256"
	"math/big"

	"github.com/google/uuid"
)

const alphabet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"

// Takes UUID string and returns 12-char slug
func SlugFromUUID(uuid uuid.UUID) string {
	sum := sha256.Sum256([]byte(uuid.String()))

	// 9 bytes = 72 bits → enough for 12 base62 chars
	return base62Fixed(sum[:9], 12)
}

func base62Fixed(data []byte, length int) string {
	n := new(big.Int).SetBytes(data)
	base := big.NewInt(62)
	mod := new(big.Int)

	out := make([]byte, length)

	for i := length - 1; i >= 0; i-- {
		if n.Sign() == 0 {
			out[i] = alphabet[0]
			continue
		}
		n.DivMod(n, base, mod)
		out[i] = alphabet[mod.Int64()]
	}

	return string(out)
}