<?php
// SPDX-License-Identifier: GPL-3.0-or-later
declare(strict_types=1);

namespace Aiwend\Chat;

/**
 * A small fixed-window counter on top of any key/value store with an expiry (WordPress transients in production, an array in tests).
 * It is an abuse throttle, not an identity or authorization control. The read-then-write is made atomic per key by an optional
 * lock (production: a MySQL/MariaDB named lock, so simultaneous requests are counted one after the other); without a lock a burst of
 * simultaneous requests can overshoot the limit by about the number of PHP workers. A limit of 0 or less switches the throttle off.
 */
final class FixedWindowLimiter
{
    /**
     * @param callable(string): mixed $get returns the stored value or false
     * @param callable(string, mixed, int): mixed $set stores a value for $ttl seconds
     * @param (callable(): int)|null $now seconds clock (injectable for tests)
     * @param (callable(string): bool)|null $lock takes an exclusive lock for a key; false = it could not be had (the request is refused)
     * @param (callable(string): void)|null $unlock releases it
     */
    public function __construct(private $get, private $set, private $now = null, private $lock = null, private $unlock = null)
    {
    }

    public function allow(string $key, int $limit, int $windowSeconds): bool
    {
        if ($limit <= 0) {
            return true;
        }
        if ($this->lock !== null && !($this->lock)($key)) {
            return false;
        }
        try {
            return $this->count($key, $limit, $windowSeconds);
        } finally {
            if ($this->unlock !== null) {
                ($this->unlock)($key);
            }
        }
    }

    private function count(string $key, int $limit, int $windowSeconds): bool
    {
        $now = $this->now !== null ? ($this->now)() : time();
        $state = ($this->get)($key);
        if (!is_array($state) || !isset($state['n'], $state['t']) || $now - (int) $state['t'] >= $windowSeconds) {
            $state = ['n' => 0, 't' => $now];
        }
        if ((int) $state['n'] >= $limit) {
            return false;
        }
        $state['n'] = (int) $state['n'] + 1;
        ($this->set)($key, $state, max(1, $windowSeconds - ($now - (int) $state['t'])));
        return true;
    }
}