cinny/src/app/hooks/useRoomReadPositions.ts

import { Room, RoomEvent, RoomMember, RoomMemberEvent, MatrixEvent } from 'matrix-js-sdk';
import { ReceiptContent } from 'matrix-js-sdk/lib/@types/read_receipts';
import { useEffect, useState } from 'react';
import { useMatrixClient } from './useMatrixClient';
import { reactionOrEditEvent } from '../utils/room';

// Receipts can land on reaction/edit events which RoomTimeline skips (renders null).
// Walk backwards from the receipt event to find the nearest event that IS rendered.
function nearestRenderableId(
  liveEvents: MatrixEvent[],
  eventIndex: Map<string, number>,
  evtId: string,
): string | null {
  const idx = eventIndex.get(evtId) ?? -1;
  if (idx === -1) return null;
  for (let i = idx; i >= 0; i--) {
    const e = liveEvents[i];
    if (!reactionOrEditEvent(e)) return e.getId() ?? null;
  }
  return null;
}

// A `m.receipt` event's content names every user whose receipt moved, keyed by
// eventId -> receiptType -> userId. Exported for testing.
export function getReceiptUserIds(content: ReceiptContent): string[] {
  const userIds = new Set<string>();
  Object.values(content).forEach((receiptsByType) => {
    Object.values(receiptsByType).forEach((receiptsByUser) => {
      Object.keys(receiptsByUser).forEach((userId) => userIds.add(userId));
    });
  });
  return Array.from(userIds);
}

function computePositions(room: Room, myUserId: string): Map<string, string[]> {
  const map = new Map<string, string[]>();
  const liveEvents = room.getLiveTimeline().getEvents();
  // Build O(1) index once instead of O(T) findIndex per member
  const eventIndex = new Map<string, number>(liveEvents.map((e, i) => [e.getId() ?? '', i]));
  for (const member of room.getJoinedMembers()) {
    if (member.userId === myUserId) continue;
    const evtId = room.getEventReadUpTo(member.userId);
    if (!evtId) continue;
    const targetId = nearestRenderableId(liveEvents, eventIndex, evtId);
    if (!targetId) continue;
    const arr = map.get(targetId);
    if (arr) arr.push(member.userId);
    else map.set(targetId, [member.userId]);
  }
  return map;
}

// Recompute positions for only the given users, reusing the previous Map/arrays for
// everyone else. Returns the SAME `prevMap` reference when nothing actually changed,
// so unaffected `Message`s (the overwhelming majority on every receipt) skip re-render
// (Gitea #40).
export function computeUpdatedPositions(
  room: Room,
  myUserId: string,
  prevMap: Map<string, string[]>,
  changedUserIds: Iterable<string>,
): Map<string, string[]> {
  const usersToUpdate = new Set(changedUserIds);
  usersToUpdate.delete(myUserId);
  if (usersToUpdate.size === 0) return prevMap;

  const liveEvents = room.getLiveTimeline().getEvents();
  const eventIndex = new Map<string, number>(liveEvents.map((e, i) => [e.getId() ?? '', i]));

  // Each changed user's current target, so a receipt that didn't actually move
  // them past the previously-computed nearest renderable event is a true no-op.
  const oldTargetByUser = new Map<string, string>();
  prevMap.forEach((users, targetId) => {
    users.forEach((u) => {
      if (usersToUpdate.has(u)) oldTargetByUser.set(u, targetId);
    });
  });

  // Copy lazily: stay on `prevMap` (and its untouched per-event arrays) unless a
  // user actually moved, so unrelated Messages keep the same array/Map reference.
  let nextMap = prevMap;
  const ensureCopy = (): Map<string, string[]> => {
    if (nextMap === prevMap) nextMap = new Map(prevMap);
    return nextMap;
  };

  usersToUpdate.forEach((userId) => {
    const evtId = room.getEventReadUpTo(userId);
    const newTargetId = evtId ? nearestRenderableId(liveEvents, eventIndex, evtId) : null;
    const oldTargetId = oldTargetByUser.get(userId) ?? null;
    if (newTargetId === oldTargetId) return;

    const map = ensureCopy();
    if (oldTargetId) {
      const users = map.get(oldTargetId);
      if (users) {
        const filtered = users.filter((u) => u !== userId);
        if (filtered.length === 0) map.delete(oldTargetId);
        else map.set(oldTargetId, filtered);
      }
    }
    if (newTargetId) {
      const users = map.get(newTargetId);
      map.set(newTargetId, users ? [...users, userId] : [userId]);
    }
  });

  return nextMap;
}

export function useRoomReadPositions(room: Room): Map<string, string[]> {
  const mx = useMatrixClient();
  const myUserId = mx.getUserId() ?? '';
  const [positions, setPositions] = useState(() => computePositions(room, myUserId));

  useEffect(() => {
    setPositions(computePositions(room, myUserId));
    let debounceTimer: ReturnType<typeof setTimeout> | null = null;
    // Accumulated across every receipt event that lands during the debounce window so
    // a burst of receipts still only touches the users actually named in them.
    const pendingUserIds = new Set<string>();
    const onReceipt = (event: MatrixEvent): void => {
      getReceiptUserIds(event.getContent<ReceiptContent>()).forEach((userId) =>
        pendingUserIds.add(userId),
      );
      if (debounceTimer !== null) clearTimeout(debounceTimer);
      debounceTimer = setTimeout(() => {
        const userIds = Array.from(pendingUserIds);
        pendingUserIds.clear();
        debounceTimer = null;
        setPositions((prev) => computeUpdatedPositions(room, myUserId, prev, userIds));
      }, 150);
    };
    // RoomMemberEvent.Membership is emitted on the RoomMember (and re-emitted on the
    // MatrixClient), never on Room — listening on `room` never fired. Subscribe on
    // the client and filter to this room (Gitea #61).
    const onMembership = (_ev: MatrixEvent, member: RoomMember): void => {
      if (member.roomId !== room.roomId) return;
      setPositions(computePositions(room, myUserId));
    };
    room.on(RoomEvent.Receipt, onReceipt);
    mx.on(RoomMemberEvent.Membership, onMembership);
    return () => {
      if (debounceTimer !== null) clearTimeout(debounceTimer);
      room.removeListener(RoomEvent.Receipt, onReceipt);
      mx.removeListener(RoomMemberEvent.Membership, onMembership);
    };
  }, [room, myUserId, mx]);

  return positions;
}