import type { HabitCadence } from "@rakazo/contracts";

const DAY_MS = 86_400_000;

function utcMidnight(date: string): number {
  return Date.parse(`${date}T00:00:00Z`);
}

/** The calendar date at `now` in `timeZone`, e.g. "2026-10-03". */
export function localDateIn(timeZone: string, now: Date = new Date()): string {
  const parts = new Map(
    new Intl.DateTimeFormat("en-US", {
      timeZone,
      year: "numeric",
      month: "2-digit",
      day: "2-digit",
    })
      .formatToParts(now)
      .map((part) => [part.type, part.value]),
  );
  return `${parts.get("year")}-${parts.get("month")}-${parts.get("day")}`;
}

/** `date` moved by `days` calendar days. */
export function addDays(date: string, days: number): string {
  return new Date(utcMidnight(date) + days * DAY_MS).toISOString().slice(0, 10);
}

/** Day of the week, 0 = Sunday … 6 = Saturday. */
export function weekdayOf(date: string): number {
  return new Date(utcMidnight(date)).getUTCDay();
}

/** The Monday that starts the week containing `date`. */
export function weekStartOf(date: string): string {
  return addDays(date, -((weekdayOf(date) + 6) % 7));
}

function isWeekday(date: string): boolean {
  const day = weekdayOf(date);
  return day >= 1 && day <= 5;
}

/** The latest Monday–Friday date strictly before `date`. */
function previousWeekday(date: string): string {
  let cursor = addDays(date, -1);
  while (!isWeekday(cursor)) cursor = addDays(cursor, -1);
  return cursor;
}

/** Whether the habit is done in the period that contains `today`. */
export function habitDoneInPeriod(
  cadence: HabitCadence,
  doneDates: ReadonlySet<string>,
  today: string,
): boolean {
  if (cadence !== "weekly") return doneDates.has(today);
  const start = weekStartOf(today);
  for (let date = start; date <= today; date = addDays(date, 1)) {
    if (doneDates.has(date)) return true;
  }
  return false;
}

/**
 * Periods in a row the habit was done, ending today. The current period counts once it is done;
 * until then it stays open and does not break the streak. Weekday habits skip weekends, weekly
 * habits count Monday-to-Sunday weeks with at least one done day.
 */
export function habitStreak(
  cadence: HabitCadence,
  doneDates: ReadonlySet<string>,
  today: string,
): number {
  let streak = 0;
  if (cadence === "weekly") {
    const weeks = new Set([...doneDates].map(weekStartOf));
    const current = weekStartOf(today);
    for (let week = weeks.has(current) ? current : addDays(current, -7); weeks.has(week); ) {
      streak += 1;
      week = addDays(week, -7);
    }
    return streak;
  }
  if (cadence === "weekdays") {
    let cursor = isWeekday(today) && doneDates.has(today) ? today : previousWeekday(today);
    while (doneDates.has(cursor)) {
      streak += 1;
      cursor = previousWeekday(cursor);
    }
    return streak;
  }
  let cursor = doneDates.has(today) ? today : addDays(today, -1);
  while (doneDates.has(cursor)) {
    streak += 1;
    cursor = addDays(cursor, -1);
  }
  return streak;
}