type GreetingPeriod = "morning" | "noon" | "afternoon" | "evening" | "late-night";

/**
 * One line, two to five words, no follow-up question. Since 2026-10-01 the voice
 * is a warm older sister's (知心姐姐): it greets and cares, never comments on the
 * user (no 夜猫子 / 还没睡呀).
 *
 * The homepage opening used to be a salutation plus a question ("这么晚还醒着，
 * {名字}。直接说说最在意的问题吧。") — an office voice that answered for the user
 * before they had said anything. Asking is the composer placeholder's job now;
 * the greeting only greets. No metaphors (星 / 月 / 陪), no emoji, no period.
 */
const NAME_TOKEN = "{名字}";

const periodGreetings: Record<GreetingPeriod, readonly string[]> = {
  morning: ["早安", "早安，{名字}", "早呀，今天慢慢来"],
  noon: ["中午好，记得吃饭", "午安，{名字}"],
  afternoon: ["下午好，歇口气吧", "下午好，{名字}"],
  evening: ["今天辛苦了", "晚上好，{名字}", "晚上好，歇一歇"],
  // Late night never remarks on being awake (产品 2026-10-01: 「你管别人睡不睡」).
  "late-night": ["这会儿很安静", "夜里好，{名字}", "慢慢说，{名字}"],
};

/** Keyed by `Date#getDay()`. Days with nothing worth saying stay out of the pool. */
const weekdayGreetings: Readonly<Record<number, readonly string[]>> = {
  0: ["周日，慢一点"],
  1: ["周一了，别太累"],
  5: ["周五了，松口气"],
  6: ["周末好好歇歇"],
};

/** Only reachable once the account has at least one session behind it. */
const returningGreetings: readonly string[] = [
  "又见面了，{名字}",
  "你来啦，{名字}",
  "欢迎回来",
  "最近还好吗，{名字}？",
];

function greetingPeriod(hour: number): GreetingPeriod {
  if (hour >= 5 && hour < 11) return "morning";
  if (hour >= 11 && hour < 14) return "noon";
  if (hour >= 14 && hour < 18) return "afternoon";
  if (hour >= 18 && hour < 23) return "evening";
  return "late-night";
}

/**
 * Every line this moment can produce, in pool order: daypart, weekday, returning.
 * A blank name drops the lines that would print an empty one; the daypart pool
 * always keeps a nameless line, so the result is never empty.
 */
function starterGreetingPool(name: string, now: Date, hasSessions: boolean): readonly string[] {
  const templates = [
    ...periodGreetings[greetingPeriod(now.getHours())],
    ...(weekdayGreetings[now.getDay()] ?? []),
    ...(hasSessions ? returningGreetings : []),
  ];
  return name ? templates : templates.filter((template) => !template.includes(NAME_TOKEN));
}

export type StartGreetingParts = {
  readonly text: string;
};

export function createStartGreetingParts(
  name: string,
  now = new Date(),
  variantSelection = Math.random(),
  hasSessions = false,
): StartGreetingParts {
  const displayName = name.trim();
  const pool = starterGreetingPool(displayName, now, hasSessions);
  const index = Math.min(Math.max(Math.floor(variantSelection * pool.length), 0), pool.length - 1);
  return { text: pool[index]!.replaceAll(NAME_TOKEN, displayName) };
}

export function createStartGreeting(
  name: string,
  now = new Date(),
  variantSelection = Math.random(),
  hasSessions = false,
): string {
  return createStartGreetingParts(name, now, variantSelection, hasSessions).text;
}

export function createOnboardingFallbackGreeting(name: string): string {
  return `欢迎，${name.trim()}`;
}