import type { WeekDay } from './calendar.types';

/** Parse a Date or parseable string into a Date (throws on invalid). */
export function toDate(value: Date | string): Date {
  if (value instanceof Date) {
    const copy = new Date(value.getTime());
    if (Number.isNaN(copy.getTime())) {
      throw new Error(`[42/calendar] Invalid date: ${String(value)}`);
    }
    return copy;
  }
  // Treat a bare `YYYY-MM-DD` as a *local* date (not UTC) to avoid day drift.
  const match = /^(\d{4})-(\d{2})-(\d{2})$/.exec(value);
  const date = match
    ? new Date(Number(match[1]), Number(match[2]) - 1, Number(match[3]))
    : new Date(value);
  if (Number.isNaN(date.getTime())) {
    throw new Error(`[42/calendar] Invalid date: ${String(value)}`);
  }
  return date;
}

/** Midnight (local) of the given date. */
export function startOfDay(date: Date): Date {
  const out = new Date(date.getTime());
  out.setHours(0, 0, 0, 0);
  return out;
}

/** `YYYY-MM-DD` key in local time. */
export function dateKey(date: Date): string {
  const y = date.getFullYear();
  const m = String(date.getMonth() + 1).padStart(2, '0');
  const d = String(date.getDate()).padStart(2, '0');
  return `${y}-${m}-${d}`;
}

export function addDays(date: Date, days: number): Date {
  const out = new Date(date.getTime());
  out.setDate(out.getDate() + days);
  return out;
}

export function addMonths(date: Date, months: number): Date {
  const out = new Date(date.getTime());
  const targetDay = out.getDate();
  out.setDate(1);
  out.setMonth(out.getMonth() + months);
  // Clamp to the last valid day of the resulting month.
  const lastDay = new Date(out.getFullYear(), out.getMonth() + 1, 0).getDate();
  out.setDate(Math.min(targetDay, lastDay));
  return out;
}

export function startOfMonth(date: Date): Date {
  return new Date(date.getFullYear(), date.getMonth(), 1);
}

export function startOfWeek(date: Date, weekStartsOn: WeekDay): Date {
  const out = startOfDay(date);
  const diff = (out.getDay() - weekStartsOn + 7) % 7;
  return addDays(out, -diff);
}

export function isSameDay(a: Date, b: Date): boolean {
  return (
    a.getFullYear() === b.getFullYear() &&
    a.getMonth() === b.getMonth() &&
    a.getDate() === b.getDate()
  );
}

export function isSameMonth(a: Date, b: Date): boolean {
  return a.getFullYear() === b.getFullYear() && a.getMonth() === b.getMonth();
}

/** Fraction (0..1) of the day represented by the time portion of `date`. */
export function dayFraction(date: Date): number {
  return (date.getHours() * 60 + date.getMinutes()) / 1440;
}