package keygen

import "crypto/subtle"

// Reconstructed from legacy/keygen.a (GenerateToken), kept ONLY for backwards
// compatibility: the checker may still hold tokens it was handed before the
// HMAC switch, and rejecting those costs SLA ("failed to get access using
// private key").
//
//	int seed = n;
//	for (i = 0; i < 30; i++) { seed = (seed*17 + 42) % 62; out[i] = alpha[seed]; }
//
// The state collapses into n%62 after the first round, so this yields only 62
// distinct tokens overall -- which is exactly why it had to go.
func legacyKey(vid int) string {
	seed := vid
	out := make([]byte, tokenLen)
	for i := 0; i < tokenLen; i++ {
		seed = (seed*17 + 42) % 62
		out[i] = alphabet[seed]
	}
	return string(out)
}

// legacyCutoff is the highest video id that existed when the HMAC patch was
// deployed. Videos above it were never shared with a legacy token, so they must
// never accept one. 0 disables legacy acceptance entirely.
var legacyCutoff int

func SetLegacyCutoff(id int) { legacyCutoff = id }

func legacyAccepted(token string, vid int) bool {
	if legacyCutoff <= 0 || vid <= 0 || vid > legacyCutoff {
		return false
	}
	return subtle.ConstantTimeCompare([]byte(token), []byte(legacyKey(vid))) == 1
}