import { DateTime } from "luxon";

// A "date-only" value here always means UTC midnight — Prisma's `@db.Date`
// columns (`Planning.startDate`/`finishDate`, see apps/api's schema.prisma)
// carry no time-of-day, so every comparison/computation on them needs to be
// anchored the same way to stay meaningful. `DateTime` (Luxon) is the
// in-memory representation everywhere in this package; plain `Date`/ISO
// `string` only ever appear at the two boundaries that require them —
// Prisma (`Date`) and URLs/query strings (`string`).

/**
 * Parses a strict `YYYY-MM-DD` string into a UTC-midnight {@link DateTime}.
 * Returns `null` for anything that isn't a real calendar date — including a
 * value that's merely shaped right but impossible (e.g. `2026-02-30`),
 * unlike native `Date` which would silently roll it over to March 2nd.
 */
export function parseDateOnly(iso: string): DateTime | null {
  const parsed = DateTime.fromISO(iso, { zone: "utc" });
  return parsed.isValid ? parsed.startOf("day") : null;
}

/**
 * Formats a {@link DateTime} back to `YYYY-MM-DD`, the inverse of
 * {@link parseDateOnly}.
 *
 * @throws if `date` is an invalid `DateTime` — every `DateTime` produced by
 *   this package's own functions is always valid, so this only fires if a
 *   caller constructs one by hand incorrectly.
 */
export function formatDateOnly(date: DateTime): string {
  const iso = date.toISODate();
  if (iso === null) {
    throw new Error("Cannot format an invalid DateTime as a date-only string");
  }
  return iso;
}

/**
 * Normalizes a `Date` (e.g. a value read back from Prisma) or a
 * `DateTime` in any zone/with any time-of-day to a UTC-midnight
 * {@link DateTime} — the common representation every other function in
 * this package expects and returns.
 */
export function toDateOnly(date: Date | DateTime): DateTime {
  const dateTime = date instanceof DateTime ? date : DateTime.fromJSDate(date, { zone: "utc" });
  return dateTime.toUTC().startOf("day");
}