<?php

namespace App\Services\Crawl;

/**
 * 跨进程互斥 + 最小请求间隔，满足 arXiv「每 3 秒至多 1 次、单连接」要求。
 * 仅在「排队等待」时持锁，HTTP 请求在锁外执行，避免阻塞其它进程过久。
 */
class ArxivRequestGate
{
    private const LOCK_BASENAME = 'arxiv_request.lock';

    private const STATE_BASENAME = 'arxiv_last_request_at.txt';

    public function __construct(
        protected float $minIntervalSeconds = 4.0,
    ) {}

    public static function fromConfig(): self
    {
        return new self((float) config('crawl.arxiv.min_interval_seconds', 4));
    }

    /**
     * 等待轮到自己发起 arXiv 请求（持锁时间仅包含间隔 sleep）。
     */
    public function waitTurn(): void
    {
        $dir = storage_path('framework/cache');
        if (! is_dir($dir)) {
            mkdir($dir, 0755, true);
        }

        $lockPath = $dir.'/'.self::LOCK_BASENAME;
        $statePath = $dir.'/'.self::STATE_BASENAME;

        $handle = fopen($lockPath, 'c+');
        if ($handle === false) {
            throw new \RuntimeException('无法创建 arXiv 请求锁文件');
        }

        try {
            if (! flock($handle, LOCK_EX)) {
                throw new \RuntimeException('无法获取 arXiv 请求锁');
            }

            $last = is_readable($statePath) ? (float) trim((string) file_get_contents($statePath)) : 0.0;
            $remain = $this->minIntervalSeconds - (microtime(true) - $last);
            if ($remain > 0) {
                usleep((int) ceil($remain * 1_000_000));
            }
        } finally {
            flock($handle, LOCK_UN);
            fclose($handle);
        }
    }

    /**
     * 记录本次 arXiv 请求已发出（用于下一次间隔计算）。
     */
    public function markSent(): void
    {
        $dir = storage_path('framework/cache');
        $statePath = $dir.'/'.self::STATE_BASENAME;
        file_put_contents($statePath, (string) microtime(true));
    }

    /**
     * @template T
     *
     * @param  callable(): T  $callback
     * @return T
     */
    public function run(callable $callback): mixed
    {
        $this->waitTurn();
        try {
            return $callback();
        } finally {
            $this->markSent();
        }
    }
}