// Single responsibility: the one rule deciding whether a STRING is an ISO-8601 date or date-time
// that names an instant on its own. Its own file because the validator, the HTTP coercion and
// `@ultimat3/time`'s `fromIso` have to answer identically — two copies is how they came to disagree.

/**
 * A clock time, and the zone it is stated in. `2026-08-19T10:00` carries the first and not the
 * second, so `new Date` resolves it through the HOST process's zone: the same wire value is
 * `14:00Z` on a `TZ=America/New_York` pod and `10:00Z` on a `TZ=UTC` one, from one request.
 * That is the framework's "no date without an explicit zone, no ambient default anywhere" rule
 * failing at the parse end rather than the format end.
 *
 * A date-only form carries no clock time and is UTC by specification, so it is not this.
 */
const CLOCK_TIME = /[t ]\d{1,2}:\d{2}/i;
const UTC_OFFSET = /(?:z|[+-]\d{2}:?\d{2})$/i;

/**
 * The ISO-8601 shape, and nothing else. `new Date` also parses `'March 14, 2026'`, `'3/14/2026'`
 * and `'12'` — each at the host's LOCAL midnight, so the instant depended on `TZ`. Case-insensitive
 * because RFC 3339 §5.6 permits a lowercase `t` and `z`.
 */
const ISO_SHAPE =
  /^(\d{4})-(\d{2})-(\d{2})(?:[T ]\d{2}:\d{2}(?::\d{2}(?:\.\d+)?)?(?:Z|[+-]\d{2}:?\d{2})?)?$/i;

/**
 * Days per month, January first, in a common year. **Twin: `daysInMonth` in
 * `packages/time/src/plain-date.ts`** — this package is tier 0 and cannot import `@ultimat3/time`,
 * so the Gregorian table is restated rather than shared. The two are held equal by the parity
 * table in `iso-date.test.ts` and `packages/time/src/plain-date.test.ts`; edit both or neither.
 */
const DAYS_IN_MONTH: readonly number[] = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

const isLeapYear = (year: number): boolean =>
  (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0;

/**
 * The half a regex cannot hold. `new Date('2026-02-30')` does not refuse — it answers March 2nd —
 * so a typo validated, and the instant that was stored is one the caller never wrote.
 */
function isCalendarDay(year: number, month: number, day: number): boolean {
  const days = DAYS_IN_MONTH[month - 1];
  if (days === undefined) return false;
  return day >= 1 && day <= (month === 2 && isLeapYear(year) ? 29 : days);
}

/** What a `t.date` string must not be: a clock time with no offset and no `Z`. */
export function isZonelessDateTime(value: string): boolean {
  return CLOCK_TIME.test(value) && !UTC_OFFSET.test(value);
}

/**
 * True when `value` is an ISO-8601 date, or date-time carrying `Z` or an offset, naming a day its
 * month has: the only strings whose instant is the same on every host and is the one written.
 * Check it before `new Date(value)`, every time.
 */
export function isIsoDateTime(value: string): boolean {
  const match = ISO_SHAPE.exec(value);
  if (match === null || isZonelessDateTime(value)) return false;
  return isCalendarDay(Number(match[1]), Number(match[2]), Number(match[3]));
}