/**
 * Date helpers that work in whole calendar days at local midnight, so day
 * counting is not affected by time-of-day or DST shifts.
 */

const MS_PER_DAY = 24 * 60 * 60 * 1000;

/** Strip time; return a new Date at local midnight. */
export function atMidnight(d: Date): Date {
  return new Date(d.getFullYear(), d.getMonth(), d.getDate());
}

/** Parse an ISO YYYY-MM-DD string as a local-midnight Date. Returns null if invalid. */
export function parseISODate(value: string): Date | null {
  const match = /^(\d{4})-(\d{2})-(\d{2})$/.exec(value.trim());
  if (!match) return null;
  const [, y, m, d] = match;
  const year = Number(y);
  const month = Number(m);
  const day = Number(d);
  const date = new Date(year, month - 1, day);
  // Reject overflow (e.g. 2026-02-30 rolling into March).
  if (
    date.getFullYear() !== year ||
    date.getMonth() !== month - 1 ||
    date.getDate() !== day
  ) {
    return null;
  }
  return date;
}

/** Format a Date as ISO YYYY-MM-DD (local). */
export function toISODate(d: Date): string {
  const y = d.getFullYear();
  const m = String(d.getMonth() + 1).padStart(2, "0");
  const day = String(d.getDate()).padStart(2, "0");
  return `${y}-${m}-${day}`;
}

/** Whole days from a to b (b - a). Negative if b is before a. */
export function daysBetween(a: Date, b: Date): number {
  const diff = atMidnight(b).getTime() - atMidnight(a).getTime();
  return Math.round(diff / MS_PER_DAY);
}

/**
 * Inclusive day count of [start, end]. Zero if end < start.
 * e.g. same day => 1.
 */
export function inclusiveDayCount(start: Date, end: Date): number {
  const n = daysBetween(start, end) + 1;
  return n > 0 ? n : 0;
}

/** Return the later of two dates. */
export function maxDate(a: Date, b: Date): Date {
  return a.getTime() >= b.getTime() ? a : b;
}

/** Return the earlier of two dates. */
export function minDate(a: Date, b: Date): Date {
  return a.getTime() <= b.getTime() ? a : b;
}

/** Clamp d into [lo, hi]. */
export function clampDate(d: Date, lo: Date, hi: Date): Date {
  return maxDate(lo, minDate(d, hi));
}

/**
 * Count days of the inclusive window [winStart, winEnd] that are covered by the
 * inclusive period [perStart, perEnd] — i.e. the size of the overlap.
 */
export function overlapDays(
  winStart: Date,
  winEnd: Date,
  perStart: Date,
  perEnd: Date
): number {
  const start = maxDate(winStart, perStart);
  const end = minDate(winEnd, perEnd);
  return inclusiveDayCount(start, end);
}