<?php
/**
 * Server-side storage for the expected answer of the question a user is
 * currently looking at.
 *
 * Before: the generators put the correct answer in a hidden `checkerData`
 * field and the checker trusted whatever came back, so a student could edit
 * it. Now the payload stays on the server and the form only carries an opaque
 * single-use token (still posted as `checkerData`, so form markup is unchanged).
 *
 * Schema (add to timekeeper_sql.php in e107's format, or run once by hand with
 * the e107_ prefix on the table name):
 *
 *   CREATE TABLE tkPendingAnswer (
 *     username varchar(100) NOT NULL,
 *     token char(32) NOT NULL,
 *     payload text NOT NULL,
 *     created int(10) unsigned NOT NULL,
 *     PRIMARY KEY (username)
 *   ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
 *
 * One pending row per user, matching the existing one-open-question model.
 */
class TkAnswerStore
{
    const TABLE = 'tkPendingAnswer';
    const TTL = 7200; // seconds a question stays answerable

    /**
     * Store the expected-answer payload for $user and return the token to
     * embed in the form's hidden `checkerData` field.
     *
     * @param string $user
     * @param array  $payload e.g. ['answer1' => 42, 'question' => '6 x 7'] or ['word' => 'because']
     * @return string
     */
    public static function issue($user, array $payload)
    {
        $db = e107::getDb();
        $u = $db->escape($user);

        // Replace this user's pending row and sweep expired ones.
        $db->delete(self::TABLE, "username = '{$u}' OR created < " . (time() - self::TTL));

        $token = bin2hex(random_bytes(16));
        $db->insert(self::TABLE, [
            'username' => $user,
            'token'    => $token,
            'payload'  => json_encode($payload),
            'created'  => time(),
        ]);

        return $token;
    }

    /**
     * Validate the posted token and return the payload, consuming it so the
     * same question cannot be scored twice. Returns null if the token is
     * missing, wrong (e.g. stale browser tab), expired, or already used.
     *
     * @param string      $user
     * @param string|null $token value of POST `checkerData`
     * @return array|null
     */
   public static function consume($user, $token)
{
    if (!is_string($token) || $token === '') {
        error_log("TK consume: no token posted (user=$user)");
        return null;
    }

    $db = e107::getDb();
    $u = $db->escape($user);

    $row = $db->retrieve(self::TABLE, 'token, payload, created', "username = '{$u}'");
    if (!is_array($row)) {
        error_log("TK consume: no pending row for user=$user");
        return null;
    }
    if (!hash_equals((string) $row['token'], $token)) {
        error_log("TK consume: token mismatch user=$user posted=" . substr($token, 0, 40));
        return null;
    }

    $deleted = $db->delete(self::TABLE, "username = '{$u}' AND token = '" . $db->escape($row['token']) . "'");
    error_log("TK consume: delete returned " . var_export($deleted, true));
    // Temporarily ignore the delete result while diagnosing:
    // if (!$deleted) { return null; }

    if (time() - (int) $row['created'] > self::TTL) {
        error_log("TK consume: expired");
        return null;
    }

    $payload = json_decode($row['payload'], true);
    return is_array($payload) ? $payload : null;
}
}