"use client";

/**
 * The daily reminder's text, composed **in the browser**.
 *
 * The server never sees the notes — it only knows which push endpoints exist —
 * so the personal part of the nudge has to be written here and stored with the
 * subscription (see lib/push-store.ts). Personal means: how many of the user's
 * cards are due and which deck they belong to; the notification then opens the
 * review screen. When there is nothing to name — no decks yet, or nothing due —
 * the message is the plain nudge in the user's own language (English or Czech,
 * like every other string in the app).
 *
 * Always resolves; a failure to read the local stores only costs the personal
 * words, never the reminder.
 */

import type { SrsRecord } from "./history";
import { ensureLearningHydrated, learningStore } from "./history";
import { plural, t } from "./i18n";
import { dueItems } from "./learning";
import { deckStore } from "./storage";

export type ReminderMessage = { title: string; body: string; url: string };

/** The generic nudge, used whenever the notes cannot carry the message. */
export function genericReminder(): ReminderMessage {
  return { title: t("push.title"), body: t("push.generic"), url: "/" };
}

/** Which deck has the most due cards (ties keep the first, i.e. the oldest). */
function busiestDeck(schedules: SrsRecord[]): { title: string; count: number } | null {
  const byDeck = new Map<string, { title: string; count: number }>();
  for (const item of dueItems(schedules)) {
    const entry = byDeck.get(item.deck.id) ?? { title: item.deck.title, count: 0 };
    entry.count += 1;
    byDeck.set(item.deck.id, entry);
  }
  let top: { title: string; count: number } | null = null;
  for (const entry of byDeck.values()) {
    if (!top || entry.count > top.count) top = entry;
  }
  return top;
}

export async function buildReminderMessage(): Promise<ReminderMessage> {
  try {
    await ensureLearningHydrated();
    const top = busiestDeck(learningStore.getSrsSnapshot());
    if (!top || deckStore.getSnapshot().length === 0) return genericReminder();
    return {
      title: t("push.title"),
      body: plural(top.count, "push.due", { deck: top.title }),
      /* The review screen holds the due queue the nudge talks about. */
      url: "/review",
    };
  } catch {
    return genericReminder();
  }
}