import { useEffect, useRef } from 'react';

/** How often an open list reads itself again while it is seen. */
export const REREAD_WHILE_SEEN_MS = 30_000;

/**
 * Reads a list again by itself while the page showing it is seen, and once when it is seen again.
 *
 * FOR WHAT NOTHING ANNOUNCES. A patient ending a helper's access tells the helper's open page nothing, and the
 * page kept the person (and "Switch into their account") for as long as it stayed open - measured over five
 * minutes on staging. The server refused the switch, so this is about the list saying what is true, not the
 * boundary.
 *
 * One read at a time: a read still in the air when the next is due is not doubled (`reread` bounds its own
 * read). Nothing while hidden; a page seen again reads once, however many events say so. Its read is aborted
 * when the page goes.
 */
export const useRereadWhileSeen = (enabled: boolean, reread: (signal: AbortSignal) => Promise<unknown>): void => {
    const rereadRef = useRef(reread);
    rereadRef.current = reread;

    useEffect(() => {
        if (!enabled) return undefined;
        let inFlight: AbortController | null = null;
        const run = () => {
            if (inFlight) return;
            const controller = new AbortController();
            inFlight = controller;
            void rereadRef.current(controller.signal)
                .catch(() => undefined)
                .finally(() => {
                    controller.abort();
                    if (inFlight === controller) inFlight = null;
                });
        };
        let wasHidden = document.visibilityState === 'hidden';
        const onSeen = () => {
            const hidden = document.visibilityState === 'hidden';
            if (!hidden && wasHidden) run();
            wasHidden = hidden;
        };
        // Hidden is noticed on the timer too: a page that went away without an event still reads once on return.
        const tick = () => {
            if (document.visibilityState === 'hidden') { wasHidden = true; return; }
            run();
        };
        const timer = window.setInterval(tick, REREAD_WHILE_SEEN_MS);
        document.addEventListener('visibilitychange', onSeen);
        window.addEventListener('focus', onSeen);
        return () => {
            window.clearInterval(timer);
            document.removeEventListener('visibilitychange', onSeen);
            window.removeEventListener('focus', onSeen);
            inFlight?.abort();
            inFlight = null;
        };
    }, [enabled]);
};