app/src/utils/czech-holidays.ts

/**
 * Czech public holidays & work fund calculator.
 * Port of PHP CzechHolidays class.
 */

import { localDateStr } from "./date";

const holidayCache = new Map<number, string[]>();

/** Easter Sunday using the Anonymous Gregorian algorithm */
function getEasterSunday(year: number): string {
  const a = year % 19;
  const b = Math.floor(year / 100);
  const c = year % 100;
  const d = Math.floor(b / 4);
  const e = b % 4;
  const f = Math.floor((b + 8) / 25);
  const g = Math.floor((b - f + 1) / 3);
  const h = (19 * a + b - d - g + 15) % 30;
  const i = Math.floor(c / 4);
  const k = c % 4;
  const l = (32 + 2 * e + 2 * i - h - k) % 7;
  const m = Math.floor((a + 11 * h + 22 * l) / 451);
  const month = Math.floor((h + l - 7 * m + 114) / 31);
  const day = ((h + l - 7 * m + 114) % 31) + 1;
  return `${year}-${String(month).padStart(2, "0")}-${String(day).padStart(2, "0")}`;
}

/** All Czech public holidays for a year (11 fixed + 2 Easter-based) */
export function getHolidays(year: number): string[] {
  if (holidayCache.has(year)) return holidayCache.get(year)!;

  const y = String(year);
  const holidays = [
    `${y}-01-01`, // New Year's / Restoration of Czech Independence
    `${y}-05-01`, // Labour Day
    `${y}-05-08`, // Victory Day
    `${y}-07-05`, // Saints Cyril and Methodius Day
    `${y}-07-06`, // Jan Hus Day
    `${y}-09-28`, // Czech Statehood Day
    `${y}-10-28`, // Czechoslovak Independence Day
    `${y}-11-17`, // Freedom and Democracy Day
    `${y}-12-24`, // Christmas Eve
    `${y}-12-25`, // Christmas Day
    `${y}-12-26`, // St. Stephen's Day
  ];

  // Easter-based.
  // TZ NOTE (audit finding addressed): `new Date("YYYY-MM-DD")` parses as UTC
  // midnight, but the ±2/±1 day shifts below mutate via the LOCAL-time setters
  // (setDate/getDate) and `localDateStr` reads back in local time. This is only
  // correct because the app forces `process.env.TZ = "Europe/Prague"` (a
  // positive UTC offset) in src/config/env.ts — UTC-midnight resolves to the
  // same local calendar day, so Good Friday/Easter Monday land on the right
  // dates. Under a NEGATIVE-offset timezone this would read one day early. Safe
  // given the forced Prague TZ; do not "fix" by mixing UTC and local accessors.
  const easterSunday = getEasterSunday(year);
  const easterDate = new Date(easterSunday);
  const goodFriday = new Date(easterDate);
  goodFriday.setDate(goodFriday.getDate() - 2);
  const easterMonday = new Date(easterDate);
  easterMonday.setDate(easterMonday.getDate() + 1);

  holidays.push(localDateStr(goodFriday)); // Good Friday
  holidays.push(localDateStr(easterMonday)); // Easter Monday

  holidays.sort();
  holidayCache.set(year, holidays);
  return holidays;
}

/** Check if a date string (YYYY-MM-DD) is a Czech public holiday */
export function isHoliday(dateStr: string): boolean {
  const year = parseInt(dateStr.substring(0, 4), 10);
  return getHolidays(year).includes(dateStr);
}

/** Business days in a month (Mon-Fri excluding public holidays) */
export function getBusinessDaysInMonth(
  year: number,
  month: number,
  upToDay?: number,
): number {
  const holidays = getHolidays(year);
  let count = 0;
  const daysInMonth = new Date(year, month + 1, 0).getDate();
  const maxDay = upToDay ? Math.min(upToDay, daysInMonth) : daysInMonth;

  for (let day = 1; day <= maxDay; day++) {
    const date = new Date(year, month, day);
    const dow = date.getDay();
    if (dow !== 0 && dow !== 6) {
      const dateStr = `${year}-${String(month + 1).padStart(2, "0")}-${String(day).padStart(2, "0")}`;
      if (!holidays.includes(dateStr)) {
        count++;
      }
    }
  }
  return count;
}

/** Monthly work fund in hours (business days × 8) */
export function getMonthlyWorkFund(
  year: number,
  month: number,
  upToDay?: number,
): number {
  return getBusinessDaysInMonth(year, month, upToDay) * 8;
}