import { useEffect, useMemo, useRef } from 'react';
import { viewerEpoch } from '../utils/writeThroughHold';

/**
 * Who a reread is for and which notice it answers. A reread may publish only while the same opening (the viewer or
 * the open item, as `key` names it) and the signed-in viewer stand, and no later event about the same resource has
 * arrived since: a newer notice, a removal, or a whole resource. Anything else is retired, success and refusal alike.
 */
export const useNoticeFence = (key: unknown) => {
    const opening = useRef(0);
    const generations = useRef(new Map<string, number>());

    // A change of viewer or item, a close and an unmount all end the opening.
    useEffect(() => () => {
        opening.current += 1;
    }, [key]);

    return useMemo(() => ({
        /** Marks a new event about `id`; the returned check says whether that event still stands. */
        claim(id: string): () => boolean {
            const at = opening.current;
            // Read at the answer too: a viewer change counts even before React has drawn it.
            const viewer = viewerEpoch();
            const mine = (generations.current.get(id) ?? 0) + 1;
            generations.current.set(id, mine);
            return () => opening.current === at && viewerEpoch() === viewer && generations.current.get(id) === mine;
        },
    }), []);
};