valor-software/ng2-bootstrap

/**
 *
 * @param {number} year
 * @param {number} dow - start-of-first-week
 * @param {number} doy - start-of-year
 * @returns {number}
 */
import { createUTCDate } from '../create/date-from-array';
import { daysInYear } from './year';
import { getDayOfYear } from './day-of-year';
import { getFullYear } from '../utils/date-getters';

function firstWeekOffset(year: number, dow: number, doy: number): number {
  // first-week day -- which january is always in the first week (4 for iso, 1 for other)
  const fwd = dow - doy + 7;
  // first-week day local weekday -- which local weekday is fwd
  const fwdlw = (createUTCDate(year, 0, fwd).getUTCDay() - dow + 7) % 7;

  return -fwdlw + fwd - 1;
}

// https://en.wikipedia.org/wiki/ISO_week_date#Calculating_a_date_given_the_year.2C_week_number_and_weekday
export function dayOfYearFromWeeks(
  year: number,
  week: number,
  weekday: number,
  dow: number,
  doy: number
): { year: number; dayOfYear: number } {
  const localWeekday = (7 + weekday - dow) % 7;
  const weekOffset = firstWeekOffset(year, dow, doy);
  const dayOfYear = 1 + 7 * (week - 1) + localWeekday + weekOffset;
  let resYear: number;
  let resDayOfYear: number;

  if (dayOfYear <= 0) {
    resYear = year - 1;
    resDayOfYear = daysInYear(resYear) + dayOfYear;
  } else if (dayOfYear > daysInYear(year)) {
    resYear = year + 1;
    resDayOfYear = dayOfYear - daysInYear(year);
  } else {
    resYear = year;
    resDayOfYear = dayOfYear;
  }

  return {
    year: resYear,
    dayOfYear: resDayOfYear
  };
}

export function weekOfYear(
  date: Date,
  dow: number,
  doy: number
): { week: number; year: number } {
  const weekOffset = firstWeekOffset(getFullYear(date), dow, doy);
  const week = Math.floor((getDayOfYear(date) - weekOffset - 1) / 7) + 1;
  let resWeek: number;
  let resYear: number;

  if (week < 1) {
    resYear = getFullYear(date) - 1;
    resWeek = week + weeksInYear(resYear, dow, doy);
  } else if (week > weeksInYear(getFullYear(date), dow, doy)) {
    resWeek = week - weeksInYear(getFullYear(date), dow, doy);
    resYear = getFullYear(date) + 1;
  } else {
    resYear = getFullYear(date);
    resWeek = week;
  }

  return {
    week: resWeek,
    year: resYear
  };
}

export function weeksInYear(year: number, dow: number, doy: number): number {
  const weekOffset = firstWeekOffset(year, dow, doy);
  const weekOffsetNext = firstWeekOffset(year + 1, dow, doy);

  return (daysInYear(year) - weekOffset + weekOffsetNext) / 7;
}