<?php
/**
 * Server-side storage for the expected answer of the question a user is
 * currently looking at.
 *
 * The generators store the expected-answer payload here and put only an
 * opaque single-use token in the form's hidden `checkerData` field, so a
 * student cannot edit the answer client-side.
 *
 * Table: tkPendingAnswer (username PK, token, payload, created) - see
 * timekeeper_sql.php. One pending row per user.
 */
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  $payload e.g. ['answer1' => 42, 'question' => [...]] or ['word' => 'because']
     * @return string
     */
    public static function issue($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 === '') {
            return null;
        }

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

        $row = $db->retrieve(self::TABLE, 'token, payload, created', "username = '{$u}'");
        if (!is_array($row) || !hash_equals((string) $row['token'], $token)) {
            return null; // wrong token: leave the real pending question intact
        }

        // Single use: delete() returns the affected-row count, so of two
        // simultaneous submits only the first gets 1 here.
        if (!$db->delete(self::TABLE, "username = '{$u}' AND token = '" . $db->escape($row['token']) . "'")) {
            return null;
        }

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

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