interval.js

const t0 = new Date, t1 = new Date;

export function timeInterval(floori, offseti, count, field) {

  function interval(date) {
    return floori(date = arguments.length === 0 ? new Date : new Date(+date)), date;
  }

  interval.floor = (date) => {
    return floori(date = new Date(+date)), date;
  };

  interval.ceil = (date) => {
    return floori(date = new Date(date - 1)), offseti(date, 1), floori(date), date;
  };

  interval.round = (date) => {
    const d0 = interval(date), d1 = interval.ceil(date);
    return date - d0 < d1 - date ? d0 : d1;
  };

  interval.offset = (date, step) => {
    return offseti(date = new Date(+date), step == null ? 1 : Math.floor(step)), date;
  };

  interval.range = (start, stop, step) => {
    const range = [];
    start = interval.ceil(start);
    step = step == null ? 1 : Math.floor(step);
    if (!(start < stop) || !(step > 0)) return range; // also handles Invalid Date
    let previous;
    do range.push(previous = new Date(+start)), offseti(start, step), floori(start);
    while (previous < start && start < stop);
    return range;
  };

  interval.filter = (test) => {
    return timeInterval((date) => {
      if (date >= date) while (floori(date), !test(date)) date.setTime(date - 1);
    }, (date, step) => {
      if (date >= date) {
        if (step < 0) while (++step <= 0) {
          while (offseti(date, -1), !test(date)) {} // eslint-disable-line no-empty
        } else while (--step >= 0) {
          while (offseti(date, +1), !test(date)) {} // eslint-disable-line no-empty
        }
      }
    });
  };

  if (count) {
    interval.count = (start, end) => {
      t0.setTime(+start), t1.setTime(+end);
      floori(t0), floori(t1);
      return Math.floor(count(t0, t1));
    };

    interval.every = (step) => {
      step = Math.floor(step);
      return !isFinite(step) || !(step > 0) ? null
          : !(step > 1) ? interval
          : interval.filter(field
              ? (d) => field(d) % step === 0
              : (d) => interval.count(0, d) % step === 0);
    };
  }

  return interval;
}