Bucklescript: get week number of year

get-week-number.re

/* For a given date, get the ISO week number
 *
 * This is copied from
 *    https://stackoverflow.com/a/6117889/5550386
 *
 * ... which is based on information at:
 *
 *    http://www.merlyn.demon.co.uk/weekcalc.htm#WNR
 *
 * Algorithm is to find nearest Thursday, it's year
 * is the year of the week number. Then get weeks
 * between that date and the first day of that year.
 *
 * Note that dates in one year can be weeks of previous
 * or next year, overlap is up to 3 days.
 *
 * e.g. 2014/12/29 is Monday in week  1 of 2015
 *      2012/1/1   is Sunday in week 52 of 2011
 */
let getWeekNumber = (date: Js.Date.t) => {
  open Js.Date;
  /* Copy date so don't modify original */
  let date = date |> getTime |> fromFloat;

  /* Set to nearest Thursday: current date + 4 - current day number
     Make Sunday's day number 7 */
  let date =
    setUTCDate(date, getUTCDate(date) +. 4.0 -. getUTCDay(date))
    |> fromFloat;

  /* Get first day of year */
  let yearStart =
    fromFloat(
      utcWithYMD(~year=getUTCFullYear(date), ~month=0., ~date=1., ()),
    );

  let milliSecondsInADay = 86400000.0;

  /* Calculate full weeks to nearest Thursday */
  let weekNumber =
    ceil(
      ((getTime(date) -. getTime(yearStart)) /. milliSecondsInADay +. 1.)
      /. 7.,
    );

  weekNumber;
};