budget2.0/src/store/snapshot.ts

import { createSeedDataset } from "../domain/seed.ts";
import type { Dataset, FxRate, MonthId } from "../domain/types.ts";

export interface Persistence {
  read(): Promise<Dataset | null>;
  write(dataset: Dataset): Promise<void>;
}

export type Listener = () => void;

export interface HistoryState {
  canUndo: boolean;
  canRedo: boolean;
}

export interface SnapshotStore {
  load(): Promise<void>;
  get(): Dataset;
  mutate(fn: (draft: Dataset) => void): Promise<void>;
  replace(dataset: Dataset): Promise<void>;
  reset(): Promise<void>;
  getHistory(): HistoryState;
  undo(): Promise<boolean>;
  redo(): Promise<boolean>;
  subscribe(listener: Listener): () => void;
}

export function createSnapshotStore(
  persistence: Persistence,
  currentMonth: MonthId,
  /**
   * Rates for a FIRST RUN only. Passed in rather than read here so this module
   * stays free of the ambient environment; src/store/index.ts supplies
   * BAKED_FX_RATES. Loading an existing dataset never touches its rates — the
   * user may have cleared one deliberately.
   */
  seedRates: readonly FxRate[] = [],
): SnapshotStore {
  let snapshot: Dataset | null = null;
  const listeners = new Set<Listener>();
  const undoStack: Dataset[] = [];
  const redoStack: Dataset[] = [];
  let historyState: HistoryState = { canUndo: false, canRedo: false };
  const historyLimit = 50;

  // All mutations run through this chain, one at a time, so a mutation
  // always clones from the snapshot the previous mutation committed rather
  // than from a stale snapshot captured before it. Without this, two
  // mutations issued before either's write resolves both clone the same
  // starting point and the one that commits last silently discards the
  // other's change.
  let queue: Promise<unknown> = Promise.resolve();

  function enqueue<T>(work: () => Promise<T>): Promise<T> {
    const run = queue.then(work);
    // Keep the chain alive after a failure, but the caller still sees the
    // rejection via `run`, which is returned unmodified below.
    queue = run.catch(() => undefined);
    return run;
  }

  function notify() {
    for (const listener of listeners) listener();
  }

  function updateHistoryState() {
    historyState = { canUndo: undoStack.length > 0, canRedo: redoStack.length > 0 };
  }

  function pushHistory(stack: Dataset[], dataset: Dataset) {
    stack.push(dataset);
    if (stack.length > historyLimit) stack.shift();
  }

  function clearHistory() {
    undoStack.length = 0;
    redoStack.length = 0;
    updateHistoryState();
  }

  function get(): Dataset {
    if (!snapshot) throw new Error("Snapshot store is not loaded yet");
    return snapshot;
  }

  async function commit(next: Dataset, afterWrite?: () => void) {
    // Write first: if persistence fails, the in-memory snapshot must not have
    // moved, or the UI would show data that was never saved.
    await persistence.write(next);
    snapshot = next;
    afterWrite?.();
    notify();
  }

  function travel(from: Dataset[], to: Dataset[]): Promise<boolean> {
    return enqueue(async () => {
      const target = from.at(-1);
      if (!target) return false;
      const previous = get();
      await commit(structuredClone(target), () => {
        from.pop();
        pushHistory(to, previous);
        updateHistoryState();
      });
      return true;
    });
  }

  return {
    load() {
      return enqueue(async () => {
        const stored = await persistence.read();
        if (stored) {
          snapshot = stored;
          clearHistory();
          notify();
          return;
        }
        await commit(createSeedDataset(currentMonth, seedRates), clearHistory);
      });
    },

    get,

    mutate(fn) {
      return enqueue(async () => {
        const previous = get();
        const draft = structuredClone(previous);
        fn(draft);
        if (JSON.stringify(draft) === JSON.stringify(previous)) return;
        await commit(draft, () => {
          pushHistory(undoStack, previous);
          redoStack.length = 0;
          updateHistoryState();
        });
      });
    },

    replace(dataset) {
      return enqueue(() => commit(structuredClone(dataset), clearHistory));
    },

    /**
     * Back to first-run state: the seed posts, the seed currency table and the
     * seed rates this store was built with. Seeding lives here rather than in
     * the UI so "reset" and "a brand-new browser" cannot drift apart — they
     * are the same call.
     *
     * Goes through the queue like every other write, so a reset issued while a
     * mutation is still in flight cannot be overtaken by it.
     */
    reset() {
      return enqueue(() => commit(createSeedDataset(currentMonth, seedRates), clearHistory));
    },

    getHistory() {
      return historyState;
    },

    undo() {
      return travel(undoStack, redoStack);
    },

    redo() {
      return travel(redoStack, undoStack);
    },

    subscribe(listener) {
      listeners.add(listener);
      return () => listeners.delete(listener);
    },
  };
}