// Plain UTC calendar-date math, shared by the scheduled-todo server logic
// and its client components. Deliberately UTC-based rather than local-time
// -- Prisma's `@db.Date` columns round-trip as UTC-midnight `Date`s, and
// `rrule`'s occurrence math is documented to want the same, so keeping
// every date here anchored to UTC midnight avoids DST/timezone drift
// between "today" on the server and an occurrence date from the database.

/** Truncates a Date to UTC midnight of the same calendar day. */
export function startOfUTCDate(date: Date): Date {
  return new Date(Date.UTC(date.getUTCFullYear(), date.getUTCMonth(), date.getUTCDate()));
}

/** Adds (or subtracts, if negative) whole days, staying UTC-midnight-aligned. */
export function addUTCDays(date: Date, days: number): Date {
  const d = startOfUTCDate(date);
  d.setUTCDate(d.getUTCDate() + days);
  return d;
}

/** Monday of the calendar week containing `date` (Mon–Sun weeks). */
export function getWeekStart(date: Date): Date {
  const d = startOfUTCDate(date);
  const day = d.getUTCDay(); // 0 = Sun .. 6 = Sat
  const diffToMonday = day === 0 ? -6 : 1 - day;
  return addUTCDays(d, diffToMonday);
}

/** "YYYY-MM-DD", used as both a display value and a stable map/comparison key. */
export function toDateKey(date: Date): string {
  return date.toISOString().slice(0, 10);
}

export function isSameUTCDate(a: Date, b: Date): boolean {
  return toDateKey(a) === toDateKey(b);
}