import { useState, useCallback } from "react";

export function useDownloadQueue(maxConcurrent = 3) {
    const [queue, setQueue] = useState([]); // queued tasks
    const [activeCount, setActiveCount] = useState(0);
    const [statuses, setStatuses] = useState({}); // { id: "idle"|"queued"|"downloading"|"done"|"failed" }

    const scheduleNext = useCallback(() => {
        setQueue((queueCurr) => {
            if (activeCount >= maxConcurrent || queueCurr.length === 0)
                return queueCurr;

            const [task, ...rest] = queueCurr;

            setStatuses((s) => ({ ...s, [task.id]: "downloading" }));
            setActiveCount((c) => c + 1);

            // Run the download fully in async IIFE
            (async () => {
                try {
                    await task.downloadFn(); // triggers fetch + blob + <a> click
                    setStatuses((s) => ({ ...s, [task.id]: "done" }));
                } catch (err) {
                    console.error(err);
                    setStatuses((s) => ({ ...s, [task.id]: "failed" }));
                } finally {
                    setActiveCount((c) => c - 1);
                    scheduleNext(); // promote next in queue
                }
            })();

            return rest;
        });
    }, [activeCount, maxConcurrent]);

    const startDownload = useCallback(
        (id, downloadFn) => {
            setStatuses((s) => ({ ...s, [id]: "queued" }));
            setQueue((curr) => [...curr, { id, downloadFn }]);
            scheduleNext();
        },
        [scheduleNext]
    );

    const getStatus = useCallback((id) => statuses[id] || "idle", [statuses]);

    return { startDownload, getStatus };
}