"use client";

/**
 * Pure selectors over the learning history: due items (SRS), mistakes,
 * streaks and accuracy breakdowns. No React, no storage side effects —
 * easy to reason about and to test.
 */

import type { AnswerRecord, SrsRecord } from "./history";
import type { Activity, Deck } from "./schemas";
import { deckStore } from "./storage";

export type ReviewItem = { deck: Deck; activity: Activity; dueAt?: number };

function deckMap(): Map<string, Deck> {
  return new Map(deckStore.getSnapshot().map((deck) => [deck.id, deck]));
}

export function itemKey(deckId: string, activityId: string): string {
  return `${deckId}:${activityId}`;
}

/* ------------------------------------------------------------------ */
/* Review (SRS)                                                        */
/* ------------------------------------------------------------------ */

export function dueCount(schedules: SrsRecord[], now = Date.now()): number {
  return schedules.filter((entry) => entry.dueAt <= now).length;
}

/** Activities whose schedule is due, soonest first, across every deck. */
export function dueItems(schedules: SrsRecord[], now = Date.now()): ReviewItem[] {
  const decks = deckMap();
  const items: ReviewItem[] = [];
  for (const schedule of schedules) {
    if (schedule.dueAt > now) continue;
    const deck = decks.get(schedule.deckId);
    const activity = deck?.activities.find((entry) => entry.id === schedule.activityId);
    if (deck && activity) items.push({ deck, activity, dueAt: schedule.dueAt });
  }
  return items.sort((a, b) => (a.dueAt ?? 0) - (b.dueAt ?? 0));
}

/* ------------------------------------------------------------------ */
/* Mistakes                                                            */
/* ------------------------------------------------------------------ */

/**
 * Activities whose **latest** answer was wrong (optionally within one deck).
 * Answering one correctly later removes it — no separate store to maintain.
 */
export function mistakeItems(answers: AnswerRecord[], deckId?: string): ReviewItem[] {
  const latest = new Map<string, AnswerRecord>();
  for (const answer of answers) {
    if (deckId && answer.deckId !== deckId) continue;
    const key = itemKey(answer.deckId, answer.activityId);
    if (!latest.has(key)) latest.set(key, answer);
  }
  const decks = deckMap();
  const items: ReviewItem[] = [];
  for (const answer of latest.values()) {
    if (answer.correct) continue;
    const deck = decks.get(answer.deckId);
    const activity = deck?.activities.find((entry) => entry.id === answer.activityId);
    if (deck && activity) items.push({ deck, activity });
  }
  return items;
}

export function mistakeCount(answers: AnswerRecord[], deckId?: string): number {
  return mistakeItems(answers, deckId).length;
}

/* ------------------------------------------------------------------ */
/* Stats                                                               */
/* ------------------------------------------------------------------ */

export type DayStat = { day: string; count: number; correct: number };
export type BucketStat = { label: string; answered: number; correct: number };
export type LearningStats = {
  total: number;
  correct: number;
  accuracy: number;
  todayCount: number;
  streak: number;
  days: DayStat[];
  decks: BucketStat[];
  subjects: BucketStat[];
  weakestTopics: BucketStat[];
};

export function dayKey(at: number): string {
  const date = new Date(at);
  return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, "0")}-${String(date.getDate()).padStart(2, "0")}`;
}

export function computeStats(answers: AnswerRecord[], now = Date.now()): LearningStats {
  const total = answers.length;
  const correct = answers.filter((entry) => entry.correct).length;

  const byDay = new Map<string, DayStat>();
  for (const answer of answers) {
    const key = dayKey(answer.at);
    const entry = byDay.get(key) ?? { day: key, count: 0, correct: 0 };
    entry.count += 1;
    if (answer.correct) entry.correct += 1;
    byDay.set(key, entry);
  }

  // Streak: consecutive days with at least one answer, ending today or
  // yesterday (a day that hasn't started yet shouldn't break it).
  let streak = 0;
  const cursor = new Date(now);
  if (!byDay.has(dayKey(now))) cursor.setDate(cursor.getDate() - 1);
  for (;;) {
    const key = dayKey(cursor.getTime());
    if (!byDay.has(key)) break;
    streak += 1;
    cursor.setDate(cursor.getDate() - 1);
  }

  const days: DayStat[] = [];
  const dayCursor = new Date(now);
  dayCursor.setDate(dayCursor.getDate() - 34);
  for (let index = 0; index < 35; index++) {
    const key = dayKey(dayCursor.getTime());
    days.push(byDay.get(key) ?? { day: key, count: 0, correct: 0 });
    dayCursor.setDate(dayCursor.getDate() + 1);
  }

  const decks = new Map<string, BucketStat>();
  const subjects = new Map<string, BucketStat>();
  const topics = new Map<string, BucketStat>();
  const lookup = deckMap();
  for (const answer of answers) {
    const deck = decks.get(answer.deckId) ?? { label: answer.deckTitle, answered: 0, correct: 0 };
    deck.answered += 1;
    if (answer.correct) deck.correct += 1;
    decks.set(answer.deckId, deck);

    const subject = lookup.get(answer.deckId)?.subject?.trim();
    if (subject) {
      const bucket = subjects.get(subject) ?? { label: subject, answered: 0, correct: 0 };
      bucket.answered += 1;
      if (answer.correct) bucket.correct += 1;
      subjects.set(subject, bucket);
    }

    if (answer.topic) {
      const bucket = topics.get(answer.topic) ?? { label: answer.topic, answered: 0, correct: 0 };
      bucket.answered += 1;
      if (answer.correct) bucket.correct += 1;
      topics.set(answer.topic, bucket);
    }
  }

  const accuracyOf = (bucket: BucketStat) => bucket.correct / Math.max(1, bucket.answered);

  return {
    total,
    correct,
    accuracy: total > 0 ? correct / total : 0,
    todayCount: byDay.get(dayKey(now))?.count ?? 0,
    streak,
    days,
    decks: [...decks.values()].sort((a, b) => b.answered - a.answered),
    subjects: [...subjects.values()].sort((a, b) => b.answered - a.answered),
    weakestTopics: [...topics.values()]
      .filter((bucket) => bucket.answered >= 3)
      .sort((a, b) => accuracyOf(a) - accuracyOf(b))
      .slice(0, 5),
  };
}