/**
 * Birthday helpers for the loyalty birthday gift.
 *
 * Brainerce stores a birthday as a MONTH and a DAY and never a year, so there
 * is no age on file and nothing here needs a date library or a date picker.
 * The platform mints a one-time coupon and emails it ahead of the day, which
 * only works if the storefront actually collects the two values.
 *
 * Both `updateMyProfile()` and `registerCustomer()` reject a month sent without
 * a day (and the reverse) with HTTP 400, and reject a day the month does not
 * have, so every form that collects a birthday validates with these helpers
 * before it submits.
 */

/**
 * Translation keys for the month names, index 0 = January. They live in the
 * `common` namespace so the account form and the signup form share one list
 * instead of each carrying its own twelve keys.
 */
export const BIRTH_MONTH_KEYS = [
  'monthJanuary',
  'monthFebruary',
  'monthMarch',
  'monthApril',
  'monthMay',
  'monthJune',
  'monthJuly',
  'monthAugust',
  'monthSeptember',
  'monthOctober',
  'monthNovember',
  'monthDecember',
] as const;

/**
 * How many days each month offers.
 *
 * February gets 29, not 28: the 29th IS a valid birthday and the platform
 * celebrates it on 28 February in years that do not have one. No year is ever
 * stored, so there is no leap-year arithmetic to do here.
 */
const DAYS_IN_BIRTH_MONTH = [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

/** Highest valid day for a birthday month (1-12). */
export function daysInBirthMonth(month: number): number {
  return DAYS_IN_BIRTH_MONTH[month - 1] ?? 31;
}

/**
 * Day numbers to render in the day grid. With no month chosen yet the full
 * 1-31 range is offered; picking a month narrows it right away.
 */
export function birthDayOptions(month: number | null): number[] {
  const count = month ? daysInBirthMonth(month) : 31;
  return Array.from({ length: count }, (_, index) => index + 1);
}

/**
 * Translation key for a stored month, or null when the value is outside 1-12.
 * Guards the display path so a bad value renders nothing rather than a raw
 * `common.` key path.
 */
export function birthMonthKey(month: number): string | null {
  return BIRTH_MONTH_KEYS[month - 1] ?? null;
}

/**
 * Read a stored form value into the number the API wants, or null when no
 * birthday is set.
 */
export function toBirthdayNumber(value: string): number | null {
  const parsed = Number(value);
  return Number.isInteger(parsed) && parsed > 0 ? parsed : null;
}