"use client";

import Link from "next/link";
import { useEffect, useMemo, useState, useSyncExternalStore } from "react";

import { Button, ProgressBar } from "@/components/ui";
import { ensureLearningHydrated, learningStore } from "@/lib/history";
import { useI18n } from "@/lib/i18n";
import { computeStats, dayKey } from "@/lib/learning";
import { settingsStore } from "@/lib/settings";
import { cn } from "@/lib/utils";

/**
 * Progress: day streak, daily-goal progress, a five-week calendar, accuracy
 * per subject and per deck, and the weakest topics (at least three answers).
 */
export function StatsScreen() {
  const { t, plural, locale } = useI18n();
  const answers = useSyncExternalStore(
    learningStore.subscribe,
    learningStore.getAnswersSnapshot,
    learningStore.getServerAnswers,
  );
  const dailyGoal = useSyncExternalStore(
    settingsStore.subscribe,
    () => settingsStore.getSnapshot().dailyGoal,
    () => 20,
  );
  const [ready, setReady] = useState(false);

  useEffect(() => {
    void ensureLearningHydrated().then(() => setReady(true));
  }, []);

  const stats = useMemo(() => computeStats(answers), [answers]);

  if (!ready) {
    return (
      <div className="space-y-4" aria-busy="true">
        <div className="skeleton h-24 rounded-3xl" />
        <div className="skeleton h-40 rounded-3xl" />
      </div>
    );
  }

  if (stats.total === 0) {
    return (
      <div className="card mx-auto max-w-md space-y-3 p-6 text-center">
        <p className="text-3xl" aria-hidden>
          📈
        </p>
        <h1 className="text-xl font-extrabold text-[var(--text)]">{t("stats.empty")}</h1>
        <p className="text-sm leading-relaxed text-[var(--text-muted)]">{t("stats.emptyBody")}</p>
        <Button className="touch-target" onClick={() => window.history.back()}>
          {t("stats.start")}
        </Button>
      </div>
    );
  }

  // Calendar: five weeks, columns aligned to weekdays (Monday first).
  const today = new Date();
  const weekdayIndex = (today.getDay() + 6) % 7;
  const start = new Date(today);
  start.setDate(today.getDate() - weekdayIndex - 28);
  const byDay = new Map(stats.days.map((entry) => [entry.day, entry]));
  const cells = Array.from({ length: 35 }, (_, index) => {
    const date = new Date(start);
    date.setDate(start.getDate() + index);
    const key = dayKey(date.getTime());
    return { key, date, future: date.getTime() > today.getTime() + 86_400_000, entry: byDay.get(key) };
  });
  const weekdayLabels = Array.from({ length: 7 }, (_, index) => {
    const date = new Date(start);
    date.setDate(start.getDate() + index);
    return new Intl.DateTimeFormat(locale, { weekday: "narrow" }).format(date);
  });
  const intensity = (count: number) =>
    count === 0
      ? "bg-[var(--surface-sunken)]"
      : count <= 2
        ? "bg-sky-200/80 dark:bg-sky-400/20"
        : count <= 5
          ? "bg-sky-400/70 dark:bg-sky-400/40"
          : "grad-ice";

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

      <section className="grid gap-3 sm:grid-cols-3">
        <div className="card space-y-1 p-4">
          <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">{t("stats.streak")}</p>
          <p className="text-[28px] font-extrabold leading-none text-[var(--text)]">{stats.streak}</p>
          <p className="text-[11.5px] text-[var(--text-muted)]">{plural(stats.total, "stats.answers")}</p>
        </div>
        <div className="card space-y-2 p-4">
          <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">{t("stats.today")}</p>
          <p className="text-[15px] font-bold text-[var(--text)]">
            {t("stats.todayOf", { n: stats.todayCount, goal: dailyGoal })}
          </p>
          <ProgressBar value={Math.min(stats.todayCount, dailyGoal)} max={dailyGoal} label={t("stats.today")} />
          {stats.todayCount >= dailyGoal ? (
            <p className="text-[11.5px] font-semibold text-emerald-700 dark:text-emerald-300">{t("stats.goalDone")}</p>
          ) : null}
        </div>
        <div className="card space-y-1 p-4">
          <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">{t("stats.accuracy")}</p>
          <p className="text-[28px] font-extrabold leading-none text-[var(--text)]">
            {Math.round(stats.accuracy * 100)}%
          </p>
          <p className="text-[11.5px] text-[var(--text-muted)]">
            {t("stats.correctOf", { correct: stats.correct, total: stats.total })}
          </p>
        </div>
      </section>

      <section className="card space-y-3 p-4">
        <div className="flex items-center justify-between gap-2">
          <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">{t("stats.calendar")}</p>
          <p className="flex items-center gap-1.5 text-[10.5px] text-[var(--text-subtle)]">
            {t("stats.legendLess")}
            <span className="size-3 rounded-[4px] bg-[var(--surface-sunken)]" />
            <span className="size-3 rounded-[4px] bg-sky-200/80 dark:bg-sky-400/20" />
            <span className="size-3 rounded-[4px] bg-sky-400/70 dark:bg-sky-400/40" />
            <span className="size-3 rounded-[4px] grad-ice" />
            {t("stats.legendMore")}
          </p>
        </div>
        <div className="grid grid-cols-7 gap-1.5">
          {weekdayLabels.map((label, index) => (
            <span key={index} className="text-center text-[10px] font-semibold text-[var(--text-subtle)]">
              {label}
            </span>
          ))}
          {cells.map((cell) => (
            <span
              key={cell.key}
              title={`${cell.key}: ${cell.entry?.count ?? 0}`}
              aria-label={`${cell.key}: ${cell.entry?.count ?? 0}`}
              className={cn(
                "aspect-square rounded-[6px]",
                cell.future ? "opacity-30" : "",
                intensity(cell.entry?.count ?? 0),
              )}
            />
          ))}
        </div>
      </section>

      {stats.subjects.length > 0 ? (
        <section className="card space-y-2 p-4">
          <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">{t("stats.bySubject")}</p>
          {stats.subjects.map((entry) => (
            <div key={entry.label} className="flex items-center justify-between gap-3 text-[13px]">
              <span className="min-w-0 truncate font-medium text-[var(--text)]">{entry.label}</span>
              <span className="shrink-0 tabular-nums text-[var(--text-muted)]">
                {Math.round((entry.correct / Math.max(1, entry.answered)) * 100)}% ·{" "}
                {t("stats.correctOf", { correct: entry.correct, total: entry.answered })}
              </span>
            </div>
          ))}
        </section>
      ) : null}

      <section className="card space-y-2 p-4">
        <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">{t("stats.byDeck")}</p>
        {stats.decks.map((entry) => (
          <div key={entry.label} className="flex items-center justify-between gap-3 text-[13px]">
            <span className="min-w-0 truncate font-medium text-[var(--text)]">{entry.label}</span>
            <span className="shrink-0 tabular-nums text-[var(--text-muted)]">
              {Math.round((entry.correct / Math.max(1, entry.answered)) * 100)}% ·{" "}
              {t("stats.correctOf", { correct: entry.correct, total: entry.answered })}
            </span>
          </div>
        ))}
      </section>

      {stats.weakestTopics.length > 0 ? (
        <section className="card space-y-2 p-4">
          <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">{t("stats.weakest")}</p>
          {stats.weakestTopics.map((entry) => (
            <div key={entry.label} className="flex items-center justify-between gap-3 text-[13px]">
              <span className="min-w-0 truncate font-medium text-[var(--text)]">{entry.label}</span>
              <span className="shrink-0 tabular-nums text-[var(--text-muted)]">
                {Math.round((entry.correct / Math.max(1, entry.answered)) * 100)}%
              </span>
            </div>
          ))}
        </section>
      ) : null}

      <div className="flex flex-wrap gap-2">
        <Link
          href="/review"
          className="inline-flex h-11 items-center justify-center rounded-[14px] btn-secondary px-5 text-[15px] font-medium touch-target"
        >
          {t("review.title")}
        </Link>
        <Link
          href="/exam"
          className="inline-flex h-11 items-center justify-center rounded-[14px] btn-ghost px-5 text-[15px] font-medium touch-target"
        >
          {t("exam.title")}
        </Link>
      </div>
    </div>
  );
}