"use client";

import { useMemo, useState, useSyncExternalStore } from "react";
import { useRouter } from "next/navigation";

import { SessionFeed, type SessionItem } from "@/components/SessionFeed";
import { isReported } from "@/lib/question-reports";
import { Button } from "@/components/ui";
import { haptic } from "@/lib/haptics";
import { recordAnswer } from "@/lib/history";
import { useI18n } from "@/lib/i18n";
import { deckStore } from "@/lib/storage";
import { cn } from "@/lib/utils";

const COUNTS = [5, 10, 15, 20];
const TIMERS = [0, 300, 600, 900];

/**
 * Exam mode: pick one or more decks, a question count and an optional timer,
 * then answer without hints. Results show the score plus a per-deck and
 * per-topic breakdown; every wrong answer lands in the mistakes set (the
 * normal history does that automatically).
 */
export function ExamScreen() {
  const { t } = useI18n();
  const router = useRouter();
  const decks = useSyncExternalStore(deckStore.subscribe, deckStore.getSnapshot, deckStore.getServerSnapshot);
  const [selected, setSelected] = useState<Set<string>>(new Set());
  const [count, setCount] = useState(10);
  const [timer, setTimer] = useState(0);
  const [running, setRunning] = useState<SessionItem[] | null>(null);

  const pool = useMemo(
    () =>
      decks
        .filter((deck) => selected.has(deck.id))
        .flatMap((deck) => deck.activities.map((activity) => ({ deck, activity })))
        // Reported questions are out of rotation (see lib/question-reports).
        .filter(({ deck, activity }) => !isReported(deck.id, activity.id)),
    [decks, selected],
  );
  const effectiveCount = Math.min(count, pool.length);

  const toggle = (id: string) => {
    haptic("toggle");
    setSelected((current) => {
      const next = new Set(current);
      if (next.has(id)) next.delete(id);
      else next.add(id);
      return next;
    });
  };

  const start = () => {
    const shuffled = [...pool];
    for (let index = shuffled.length - 1; index > 0; index--) {
      const swap = Math.floor(Math.random() * (index + 1));
      [shuffled[index], shuffled[swap]] = [shuffled[swap], shuffled[index]];
    }
    setRunning(shuffled.slice(0, effectiveCount));
  };

  if (running) {
    return (
      <SessionFeed
        title={t("exam.title")}
        items={running}
        timerSeconds={timer || undefined}
        noHints
        testId="exam-feed"
        onExit={() => setRunning(null)}
        onAnswered={(item, correct) => {
          haptic(correct ? "success" : "error");
          recordAnswer({
            deckId: item.deck.id,
            deckTitle: item.deck.title,
            activityId: item.activity.id,
            type: item.activity.type,
            topic: item.activity.topic,
            correct,
          });
        }}
        renderResults={({ results, items, correct, total, score }) => {
          const byDeck = new Map<string, { title: string; answered: number; correct: number }>();
          const byTopic = new Map<string, { answered: number; correct: number }>();
          for (const item of items) {
            const result = results[item.activity.id];
            if (!result) continue;
            const deck = byDeck.get(item.deck.id) ?? { title: item.deck.title, answered: 0, correct: 0 };
            deck.answered += 1;
            if (result.correct) deck.correct += 1;
            byDeck.set(item.deck.id, deck);
            if (item.activity.topic) {
              const topic = byTopic.get(item.activity.topic) ?? { answered: 0, correct: 0 };
              topic.answered += 1;
              if (result.correct) topic.correct += 1;
              byTopic.set(item.activity.topic, topic);
            }
          }
          return (
            <div className="card space-y-4 p-5">
              <div className="text-center">
                <p className="text-[11px] font-semibold uppercase tracking-wider text-[var(--text-subtle)]">
                  {t("exam.results")}
                </p>
                <p className="mt-2 text-[42px] font-extrabold leading-none text-[var(--text)]">{score}%</p>
                <p className="mt-1 text-sm font-medium text-[var(--text-muted)]">
                  {t("results.gotOf", { correct, total, retry: "" })}
                </p>
              </div>

              <div className="space-y-1.5">
                <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">
                  {t("exam.byDeck")}
                </p>
                {[...byDeck.entries()].map(([id, entry]) => (
                  <div key={id} className="flex items-center justify-between gap-3 text-[13px]">
                    <span className="min-w-0 truncate font-medium text-[var(--text)]">{entry.title}</span>
                    <span className="shrink-0 tabular-nums text-[var(--text-muted)]">
                      {entry.correct}/{entry.answered}
                    </span>
                  </div>
                ))}
              </div>

              {byTopic.size > 0 ? (
                <div className="space-y-1.5">
                  <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">
                    {t("exam.byTopic")}
                  </p>
                  {[...byTopic.entries()].map(([topic, entry]) => (
                    <div key={topic} className="flex items-center justify-between gap-3 text-[13px]">
                      <span className="min-w-0 truncate font-medium text-[var(--text)]">{topic}</span>
                      <span className="shrink-0 tabular-nums text-[var(--text-muted)]">
                        {entry.correct}/{entry.answered}
                      </span>
                    </div>
                  ))}
                </div>
              ) : null}

              <div className="flex flex-wrap justify-center gap-2">
                <Button size="lg" className="touch-target" onClick={() => router.push("/")}>
                  {t("review.done")}
                </Button>
                {correct < total ? (
                  <Button size="lg" variant="secondary" className="touch-target" onClick={() => router.push("/review?mistakes=1")}>
                    {t("exam.studyMistakes")}
                  </Button>
                ) : null}
              </div>
            </div>
          );
        }}
      />
    );
  }

  return (
    <div className="mx-auto max-w-2xl space-y-5">
      <section className="space-y-1.5">
        <h1 className="text-[26px] font-semibold leading-tight tracking-[-0.02em] text-[var(--text)]">
          {t("exam.title")}
        </h1>
        <p className="text-[14px] text-frost-600 dark:text-frost-300">{t("exam.subtitle")}</p>
      </section>

      <section className="card space-y-2 p-4">
        <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">{t("exam.pickDecks")}</p>
        {decks.length === 0 ? (
          <p className="text-[13px] text-[var(--text-muted)]">{t("exam.noDecks")}</p>
        ) : (
          <ul className="space-y-1.5">
            {decks.map((deck) => {
              const active = selected.has(deck.id);
              return (
                <li key={deck.id}>
                  <button
                    type="button"
                    aria-pressed={active}
                    onClick={() => toggle(deck.id)}
                    className={cn(
                      "touch-target flex w-full items-center gap-3 rounded-xl border px-3 py-2 text-left transition",
                      active
                        ? "border-accent-400 bg-[var(--surface-strong)]"
                        : "border-[var(--hairline)] bg-[var(--surface)] hover:bg-[var(--hover)]",
                    )}
                  >
                    <span
                      aria-hidden
                      className={cn(
                        "grid size-5 shrink-0 place-items-center rounded-md border text-[11px] font-bold",
                        active ? "grad-ice border-transparent text-[#06111f]" : "border-[var(--hairline)]",
                      )}
                    >
                      {active ? "✓" : ""}
                    </span>
                    <span className="min-w-0 flex-1">
                      <span className="block truncate text-[14px] font-semibold text-[var(--text)]">{deck.title}</span>
                      <span className="block text-[11.5px] text-[var(--text-subtle)]">
                        {t("deck.activities", { n: deck.activities.length })}
                      </span>
                    </span>
                  </button>
                </li>
              );
            })}
          </ul>
        )}
      </section>

      <section className="card space-y-3 p-4">
        <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">{t("exam.questions")}</p>
        <div className="segmented" role="group" aria-label={t("exam.questions")}>
          {COUNTS.map((option) => (
            <button
              key={option}
              type="button"
              aria-pressed={count === option}
              onClick={() => {
                haptic("toggle");
                setCount(option);
              }}
            >
              <span className="seg-label">{option}</span>
            </button>
          ))}
        </div>
        <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">{t("exam.timer")}</p>
        <div className="segmented" role="group" aria-label={t("exam.timer")}>
          {TIMERS.map((option) => (
            <button
              key={option}
              type="button"
              aria-pressed={timer === option}
              onClick={() => {
                haptic("toggle");
                setTimer(option);
              }}
            >
              <span className="seg-label">{option === 0 ? t("exam.timerOff") : `${option / 60} min`}</span>
            </button>
          ))}
        </div>
      </section>

      <Button
        size="lg"
        className="touch-target w-full"
        disabled={pool.length === 0}
        onClick={start}
      >
        {t("exam.start", { n: effectiveCount })}
      </Button>
    </div>
  );
}