"use client";

/**
 * Answer history and the spaced-repetition schedule, kept in IndexedDB
 * (unbounded event log — localStorage would fill up). The in-memory mirror
 * makes reads synchronous for React (`useSyncExternalStore`); writes update
 * the mirror immediately and persist asynchronously.
 *
 * Versioned DB `freezmo-learning` v1 with stores:
 *   answers  keyPath id     (indexes: at, deckId)
 *   srs      keyPath key    (index: dueAt)  key = `${deckId}:${activityId}`
 */

import { type AnswerRecord, type SrsRecord } from "./schemas";
import { markSyncDirty } from "./sync-state";

export type { AnswerRecord, SrsRecord };

const DB_NAME = "freezmo-learning";
const DB_VERSION = 1;
const ANSWERS = "answers";
const SRS = "srs";
/** The log is pruned beyond this many answers (oldest first). */
export const MAX_ANSWERS = 20_000;
/** A wrong answer comes back after this long, even inside a session. */
export const WRONG_RETRY_MS = 10 * 60_000;

const EMPTY_ANSWERS: AnswerRecord[] = [];
const EMPTY_SRS: SrsRecord[] = [];

type Listener = () => void;

let answers: AnswerRecord[] = [];
let schedules = new Map<string, SrsRecord>();
let cachedAnswers: AnswerRecord[] = EMPTY_ANSWERS;
let cachedSrs: SrsRecord[] = EMPTY_SRS;
const listeners = new Set<Listener>();
let hydrated = false;
let hydration: Promise<void> | null = null;

function notify(): void {
  cachedAnswers = answers;
  cachedSrs = [...schedules.values()];
  listeners.forEach((listener) => listener());
}

function openDb(): Promise<IDBDatabase> {
  return new Promise((resolve, reject) => {
    const request = indexedDB.open(DB_NAME, DB_VERSION);
    request.onupgradeneeded = () => {
      const db = request.result;
      if (!db.objectStoreNames.contains(ANSWERS)) {
        const store = db.createObjectStore(ANSWERS, { keyPath: "id" });
        store.createIndex("at", "at");
        store.createIndex("deckId", "deckId");
      }
      if (!db.objectStoreNames.contains(SRS)) {
        const store = db.createObjectStore(SRS, { keyPath: "key" });
        store.createIndex("dueAt", "dueAt");
      }
    };
    request.onsuccess = () => resolve(request.result);
    request.onerror = () => reject(request.error);
  });
}

async function withDb<T>(_mode: IDBTransactionMode, run: (db: IDBDatabase) => T | Promise<T>): Promise<T> {
  const db = await openDb();
  try {
    return await run(db);
  } finally {
    // Transactions keep the connection alive until they settle; close on the
    // next task so pending requests finish first.
    window.setTimeout(() => db.close(), 1000);
  }
}

function requestToPromise<T>(request: IDBRequest<T>): Promise<T> {
  return new Promise((resolve, reject) => {
    request.onsuccess = () => resolve(request.result);
    request.onerror = () => reject(request.error);
  });
}

/** Loads IndexedDB into the mirror exactly once. Safe to call repeatedly. */
export function ensureLearningHydrated(): Promise<void> {
  if (hydrated) return Promise.resolve();
  if (hydration) return hydration;
  hydration = (async () => {
    if (typeof indexedDB === "undefined") {
      hydrated = true;
      return;
    }
    try {
      await withDb("readonly", async (db) => {
        const transaction = db.transaction([ANSWERS, SRS], "readonly");
        const [loadedAnswers, loadedSrs] = await Promise.all([
          requestToPromise(transaction.objectStore(ANSWERS).getAll() as IDBRequest<AnswerRecord[]>),
          requestToPromise(transaction.objectStore(SRS).getAll() as IDBRequest<SrsRecord[]>),
        ]);
        // Writes that happened before hydration win (dedupe by id/key).
        const known = new Set(answers.map((entry) => entry.id));
        const merged = [...answers, ...loadedAnswers.filter((entry) => !known.has(entry.id))];
        merged.sort((a, b) => b.at - a.at);
        answers = merged.slice(0, MAX_ANSWERS);
        for (const entry of loadedSrs) {
          const current = schedules.get(entry.key);
          if (!current || current.lastAt < entry.lastAt) schedules.set(entry.key, entry);
        }
      });
    } catch (error) {
      console.warn("[freezmo] could not read the learning history", error);
    } finally {
      hydrated = true;
      notify();
    }
  })();
  return hydration;
}

export const learningStore = {
  subscribe(listener: Listener): () => void {
    listeners.add(listener);
    void ensureLearningHydrated();
    return () => listeners.delete(listener);
  },
  getAnswersSnapshot(): AnswerRecord[] {
    return cachedAnswers;
  },
  getSrsSnapshot(): SrsRecord[] {
    return cachedSrs;
  },
  getServerAnswers(): AnswerRecord[] {
    return EMPTY_ANSWERS;
  },
  getServerSrs(): SrsRecord[] {
    return EMPTY_SRS;
  },
};

export type AnswerInput = {
  deckId: string;
  deckTitle: string;
  activityId: string;
  type: string;
  topic?: string;
  correct: boolean;
};

/**
 * Records one answer and updates its schedule. The schedule is a light SM-2:
 * correct → 1 day, then 3 days, then interval × ease; wrong → back in ten
 * minutes with the ease nudged down.
 */
export function recordAnswer(input: AnswerInput, at = Date.now()): void {
  const record: AnswerRecord = {
    id: `${at.toString(36)}-${Math.random().toString(36).slice(2, 8)}`,
    deckId: input.deckId,
    deckTitle: input.deckTitle,
    activityId: input.activityId,
    type: input.type,
    topic: input.topic,
    correct: input.correct,
    at,
  };
  answers = [record, ...answers].slice(0, MAX_ANSWERS);

  const key = `${input.deckId}:${input.activityId}`;
  const previous = schedules.get(key);
  let ease = previous?.ease ?? 2.5;
  let reps = previous?.reps ?? 0;
  let intervalDays = previous?.intervalDays ?? 0;
  let lapses = previous?.lapses ?? 0;

  if (input.correct) {
    ease = Math.min(3, ease + 0.1);
    reps += 1;
    intervalDays = reps === 1 ? 1 : reps === 2 ? 3 : Math.min(365, Math.round(Math.max(1, intervalDays) * ease));
  } else {
    ease = Math.max(1.3, ease - 0.2);
    reps = 0;
    intervalDays = 0;
    lapses += 1;
  }

  const schedule: SrsRecord = {
    key,
    deckId: input.deckId,
    activityId: input.activityId,
    ease,
    reps,
    intervalDays,
    lapses,
    dueAt: at + (input.correct ? intervalDays * 86_400_000 : WRONG_RETRY_MS),
    lastAt: at,
    lastCorrect: input.correct,
  };
  schedules = new Map(schedules);
  schedules.set(key, schedule);

  notify();
  markSyncDirty();

  void (async () => {
    try {
      await ensureLearningHydrated();
      await withDb("readwrite", (db) => {
        const transaction = db.transaction([ANSWERS, SRS], "readwrite");
        transaction.objectStore(ANSWERS).put(record);
        transaction.objectStore(SRS).put(schedule);
      });
      // Prune the log beyond the cap (oldest first) in a separate pass.
      const overflow = answers.length - MAX_ANSWERS;
      if (overflow > 0) {
        const oldest = answers.slice(MAX_ANSWERS).map((entry) => entry.id);
        await withDb("readwrite", (db) => {
          const transaction = db.transaction(ANSWERS, "readwrite");
          const store = transaction.objectStore(ANSWERS);
          for (const id of oldest) store.delete(id);
        });
      }
    } catch (error) {
      console.warn("[freezmo] could not save the answer history", error);
    }
  })();
}

/** Used by backup import and sync pulls. */
/**
 * Puts specific cards on the review schedule, due right now — Teach it back's
 * "add the missed cards to review". A card that was already scheduled keeps
 * its ease and lapse count; only the due date is pulled forward. Returns how
 * many cards were scheduled.
 */
export function scheduleForReview(deckId: string, activityIds: string[], at = Date.now()): number {
  const changed: SrsRecord[] = [];
  for (const activityId of activityIds) {
    const key = `${deckId}:${activityId}`;
    const previous = schedules.get(key);
    changed.push({
      key,
      deckId,
      activityId,
      ease: previous?.ease ?? 2.5,
      reps: previous?.reps ?? 0,
      intervalDays: 0,
      lapses: previous?.lapses ?? 0,
      dueAt: at,
      lastAt: at,
      lastCorrect: previous?.lastCorrect ?? false,
    });
  }
  if (changed.length === 0) return 0;

  schedules = new Map(schedules);
  for (const schedule of changed) schedules.set(schedule.key, schedule);
  notify();
  markSyncDirty();

  void (async () => {
    try {
      await ensureLearningHydrated();
      await withDb("readwrite", (db) => {
        const transaction = db.transaction(SRS, "readwrite");
        const store = transaction.objectStore(SRS);
        for (const schedule of changed) store.put(schedule);
      });
    } catch (error) {
      console.warn("[freezmo] could not schedule the review cards", error);
    }
  })();

  return changed.length;
}

export async function replaceLearning(nextAnswers: AnswerRecord[], nextSrs: SrsRecord[]): Promise<void> {
  const sorted = [...nextAnswers].sort((a, b) => b.at - a.at).slice(0, MAX_ANSWERS);
  answers = sorted;
  schedules = new Map(nextSrs.map((entry) => [entry.key, entry]));
  notify();
  try {
    await withDb("readwrite", (db) => {
      const transaction = db.transaction([ANSWERS, SRS], "readwrite");
      const answersStore = transaction.objectStore(ANSWERS);
      const srsStore = transaction.objectStore(SRS);
      answersStore.clear();
      srsStore.clear();
      for (const entry of sorted) answersStore.put(entry);
      for (const entry of nextSrs) srsStore.put(entry);
    });
  } catch (error) {
    console.warn("[freezmo] could not replace the learning history", error);
  }
}

/** Wipes the log and every schedule (used by the stats screen). */
export async function clearLearning(): Promise<void> {
  await replaceLearning([], []);
}