cinny/src/app/hooks/useUserNotes.ts

import { useCallback, useEffect, useState } from 'react';
import { ClientEvent, ClientEventHandlerMap, MatrixClient } from 'matrix-js-sdk';
import { useMatrixClient } from './useMatrixClient';
import { getAccountData, setAccountData } from '../utils/accountData';

const NOTES_KEY = 'io.lotus.user_notes';
export const USER_NOTE_MAX_LENGTH = 500;

type UserNotesContent = Record<string, string>;

function readNotes(mx: MatrixClient): UserNotesContent {
  return getAccountData<UserNotesContent>(mx, NOTES_KEY) ?? {};
}

// Module-scoped serialization state.
//
// useUserNotes() can be mounted by many components at once, so a per-instance
// latest/queue would only serialize writes within one instance. Notes for
// different users saved from different instances (before the server echo lands)
// would each compute from a stale snapshot and clobber each other, since
// setAccountData replaces the whole record with no server merge. We therefore
// keep a single shared latest record + write queue, keyed off the active client.
type UserNotesModuleState = {
  mx: MatrixClient;
  latest: UserNotesContent;
  writeQueue: Promise<unknown>;
  listeners: Set<(record: UserNotesContent) => void>;
  onAccountData: ClientEventHandlerMap[ClientEvent.AccountData];
};

let moduleState: UserNotesModuleState | null = null;

// Lazily initialize the shared state for the given client. On a client change
// (login/logout swaps the MatrixClient) we tear down the old subscription and
// re-initialize against the new client so we never leak or double-subscribe.
function ensureModuleState(mx: MatrixClient): UserNotesModuleState {
  if (moduleState && moduleState.mx === mx) {
    return moduleState;
  }

  if (moduleState) {
    moduleState.mx.removeListener(ClientEvent.AccountData, moduleState.onAccountData);
  }

  const state: UserNotesModuleState = {
    mx,
    latest: readNotes(mx),
    writeQueue: Promise.resolve(),
    listeners: new Set(),
    // Reassigned below once `state` is captured.
    onAccountData: () => undefined,
  };

  state.onAccountData = (evt) => {
    if (evt.getType() === NOTES_KEY) {
      const record = evt.getContent<UserNotesContent>() ?? {};
      state.latest = record;
      state.listeners.forEach((listener) => listener(record));
    }
  };

  mx.on(ClientEvent.AccountData, state.onAccountData);
  moduleState = state;
  return state;
}

function enqueueNotesWrite(
  mx: MatrixClient,
  compute: (current: UserNotesContent) => UserNotesContent,
): Promise<void> {
  const state = ensureModuleState(mx);
  const run = state.writeQueue.then(async () => {
    const next = compute(state.latest);
    state.latest = next;
    state.listeners.forEach((listener) => listener(next));
    await setAccountData(mx, NOTES_KEY, next);
  });
  // Keep the chain alive even if one write rejects, but propagate the
  // rejection to this caller so it can react (e.g. retry).
  state.writeQueue = run.catch(() => undefined);
  return run;
}

export function useUserNotes(): {
  getNote: (userId: string) => string;
  setNote: (userId: string, note: string) => Promise<void>;
} {
  const mx = useMatrixClient();
  const [notes, setNotes] = useState<UserNotesContent>(() => ensureModuleState(mx).latest);

  // Subscribe to the shared module state. A single AccountData listener is
  // installed per client (in ensureModuleState); each hook instance only
  // registers a local setter and unregisters it on unmount / client change.
  useEffect(() => {
    const state = ensureModuleState(mx);
    setNotes(state.latest);
    state.listeners.add(setNotes);
    return () => {
      state.listeners.delete(setNotes);
    };
  }, [mx]);

  const getNote = useCallback((userId: string) => notes[userId] ?? '', [notes]);

  const setNote = useCallback(
    (userId: string, note: string) => {
      const trimmed = note.trim().slice(0, USER_NOTE_MAX_LENGTH);
      return enqueueNotesWrite(mx, (current) => {
        const updated = { ...current };
        if (trimmed) {
          updated[userId] = trimmed;
        } else {
          delete updated[userId];
        }
        return updated;
      });
    },
    [mx],
  );

  return { getNote, setNote };
}