flashcards/packages/backend/src/services/sessions.ts

import { and, eq, inArray, sql } from 'drizzle-orm';
import type { Db } from '../db/client.js';
import { attempts, cardProgress, cards, lessons, sessions } from '../db/schema.js';
import { ApiError } from '../lib/errors.js';
import type {
  AttemptCreateInput, QueueItem, SessionRow, SessionStartInput,
} from '@flashcard/shared';
import { applyResult } from './leitner.js';
import { getDescendantLessonIds } from './lessons.js';
import { canReadLesson } from './permissions.js';

const REINSERT_OFFSET = 3;

function nowSec() { return Math.floor(Date.now() / 1000); }

function shuffleInPlace<T>(arr: T[]): T[] {
  for (let i = arr.length - 1; i > 0; i--) {
    const j = Math.floor(Math.random() * (i + 1));
    [arr[i], arr[j]] = [arr[j]!, arr[i]!];
  }
  return arr;
}

export interface StartedSession {
  session: SessionRow;
  queue: QueueItem[];
}

export async function startSession(
  db: Db, userId: number, input: SessionStartInput
): Promise<StartedSession> {
  const lesson = db.select().from(lessons).where(eq(lessons.id, input.lessonId)).get();
  if (!lesson) throw ApiError.notFound('Lesson');
  if (!(await canReadLesson(db, userId, input.lessonId))) {
    throw new ApiError(403, 'FORBIDDEN_LESSON', 'Cannot start a session for this lesson');
  }

  const lessonIds = await getDescendantLessonIds(db, input.lessonId);
  const lessonRows = db.select().from(lessons).where(inArray(lessons.id, lessonIds)).all();
  const bidirById = new Map(lessonRows.map((l) => [l.id, l.bidirectional]));

  const allCards = db.select().from(cards).where(inArray(cards.lessonId, lessonIds)).all();
  const direction = input.direction ?? 'forward';

  const candidates: QueueItem[] = [];
  for (const c of allCards) {
    const isBidi = bidirById.get(c.lessonId) === true;
    if (direction === 'forward' || direction === 'both') {
      candidates.push({ cardId: c.id, direction: 'forward' });
    }
    if ((direction === 'backward' || direction === 'both') && isBidi) {
      candidates.push({ cardId: c.id, direction: 'backward' });
    }
  }

  // Ensure per-user progress rows for each candidate
  for (const item of candidates) {
    const existing = db.select().from(cardProgress).where(and(
      eq(cardProgress.cardId, item.cardId),
      eq(cardProgress.direction, item.direction),
      eq(cardProgress.userId, userId),
    )).get();
    if (!existing) {
      db.insert(cardProgress).values({
        cardId: item.cardId, direction: item.direction,
        userId, box: 1, nextDueAt: 0,
      }).run();
    }
  }

  const progressRows = allCards.length === 0
    ? []
    : db.select().from(cardProgress)
        .where(and(
          inArray(cardProgress.cardId, allCards.map((c) => c.id)),
          eq(cardProgress.userId, userId),
        ))
        .all();
  const progByKey = new Map<string, typeof progressRows[number]>();
  for (const p of progressRows) progByKey.set(`${p.cardId}:${p.direction}`, p);

  const now = nowSec();
  const due: QueueItem[] = [];
  const future: QueueItem[] = [];
  for (const item of candidates) {
    const p = progByKey.get(`${item.cardId}:${item.direction}`)!;
    (p.nextDueAt <= now ? due : future).push(item);
  }

  const shuffle = input.shuffle ?? true;
  const sortByBox = (a: QueueItem, b: QueueItem) => {
    const pa = progByKey.get(`${a.cardId}:${a.direction}`)!;
    const pb = progByKey.get(`${b.cardId}:${b.direction}`)!;
    return pa.box - pb.box;
  };
  if (shuffle) {
    shuffleInPlace(due);
    shuffleInPlace(future);
  }
  due.sort(sortByBox);
  future.sort(sortByBox);

  let queue: QueueItem[] = [...due, ...future];
  const max = input.maxCards ?? null;
  if (max !== null) queue = queue.slice(0, max);

  const [row] = db.insert(sessions).values({
    lessonId: input.lessonId,
    userId,
    queueSnapshot: JSON.stringify({ remaining: queue, index: 0 }),
  }).returning().all();

  return { session: rowToSession(row!), queue };
}

function rowToSession(r: typeof sessions.$inferSelect): SessionRow {
  return {
    id: r.id,
    lessonId: r.lessonId,
    startedAt: r.startedAt,
    endedAt: r.endedAt ?? null,
    durationSeconds: r.durationSeconds ?? null,
    cardsShown: r.cardsShown,
    cardsCorrect: r.cardsCorrect,
    cardsIncorrect: r.cardsIncorrect,
    status: r.status,
  };
}

interface QueueState { remaining: QueueItem[]; index: number; }
function readQueue(snapshot: string | null | undefined): QueueState {
  if (!snapshot) return { remaining: [], index: 0 };
  return JSON.parse(snapshot) as QueueState;
}

function assertSessionOwnership(sess: { userId: number | null } | undefined, userId: number) {
  if (!sess) throw ApiError.notFound('Session');
  if (sess.userId !== userId) throw new ApiError(403, 'FORBIDDEN_LESSON', 'Not your session');
}

export async function getNextItem(db: Db, userId: number, sessionId: number): Promise<QueueItem | null> {
  const row = db.select().from(sessions).where(eq(sessions.id, sessionId)).get();
  assertSessionOwnership(row, userId);
  if (row!.status !== 'active') return null;
  const state = readQueue(row!.queueSnapshot);
  return state.remaining[state.index] ?? null;
}

export async function recordAttempt(
  db: Db, userId: number, sessionId: number, input: AttemptCreateInput
): Promise<void> {
  const sess = db.select().from(sessions).where(eq(sessions.id, sessionId)).get();
  assertSessionOwnership(sess, userId);
  if (sess!.status !== 'active') throw ApiError.validation('Session is not active');

  const now = nowSec();
  db.insert(attempts).values({
    sessionId,
    cardId: input.cardId,
    direction: input.direction,
    result: input.result,
    timeToAnswerMs: input.timeToAnswerMs ?? null,
  }).run();

  const prog = db.select().from(cardProgress).where(and(
    eq(cardProgress.cardId, input.cardId),
    eq(cardProgress.direction, input.direction),
    eq(cardProgress.userId, userId),
  )).get();
  if (prog) {
    const delta = applyResult(
      { box: prog.box, correctCount: prog.correctCount, incorrectCount: prog.incorrectCount },
      input.result,
      now
    );
    db.update(cardProgress).set({
      box: delta.box,
      correctCount: delta.correctCount,
      incorrectCount: delta.incorrectCount,
      nextDueAt: delta.nextDueAt,
      lastShownAt: delta.lastShownAt,
    }).where(and(
      eq(cardProgress.cardId, input.cardId),
      eq(cardProgress.direction, input.direction),
      eq(cardProgress.userId, userId),
    )).run();
  }

  const state = readQueue(sess!.queueSnapshot);
  state.index += 1;
  if (input.result === 'incorrect') {
    const insertAt = Math.min(state.remaining.length, state.index + REINSERT_OFFSET);
    state.remaining.splice(insertAt, 0, { cardId: input.cardId, direction: input.direction });
  }

  db.update(sessions).set({
    queueSnapshot: JSON.stringify(state),
    cardsShown: sess!.cardsShown + 1,
    cardsCorrect: sess!.cardsCorrect + (input.result === 'correct' ? 1 : 0),
    cardsIncorrect: sess!.cardsIncorrect + (input.result === 'incorrect' ? 1 : 0),
  }).where(eq(sessions.id, sessionId)).run();
}

export async function endSession(db: Db, userId: number, sessionId: number): Promise<SessionRow> {
  const sess = db.select().from(sessions).where(eq(sessions.id, sessionId)).get();
  assertSessionOwnership(sess, userId);
  const endedAt = nowSec();
  const duration = endedAt - sess!.startedAt;
  const [row] = db.update(sessions).set({
    status: 'completed', endedAt, durationSeconds: duration, queueSnapshot: null,
  }).where(eq(sessions.id, sessionId)).returning().all();
  return rowToSession(row!);
}

export async function abandonSession(db: Db, userId: number, sessionId: number): Promise<SessionRow> {
  const sess = db.select().from(sessions).where(eq(sessions.id, sessionId)).get();
  assertSessionOwnership(sess, userId);
  const endedAt = nowSec();
  const duration = endedAt - sess!.startedAt;
  const [row] = db.update(sessions).set({
    status: 'abandoned', endedAt, durationSeconds: duration,
  }).where(eq(sessions.id, sessionId)).returning().all();
  return rowToSession(row!);
}

export async function getActiveSession(db: Db, userId: number): Promise<SessionRow | null> {
  const row = db.select().from(sessions)
    .where(and(eq(sessions.status, 'active'), eq(sessions.userId, userId)))
    .orderBy(sql`${sessions.startedAt} DESC`).get();
  return row ? rowToSession(row) : null;
}

export async function getSessionState(
  db: Db, userId: number, sessionId: number
): Promise<{ session: SessionRow; queue: QueueItem[]; index: number } | null> {
  const row = db.select().from(sessions).where(eq(sessions.id, sessionId)).get();
  if (!row) return null;
  if (row.userId !== userId) return null;
  const state = readQueue(row.queueSnapshot);
  return { session: rowToSession(row), queue: state.remaining, index: state.index };
}