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// 日历实现中，对于 date 的 group 和 sort
// Group date by year, then by momth
// sort method here help make an array contian calandar object
// order by the :
// first year: 2017, 2016, 2015
// then monthe: 12, 11, 10, 9, 8, 7, 6, ... , 3, 2, 1
// then daY: 1, 2, 3, 4, 5, ... , 30, 31
sort(dates) {
  // _sort:
  // first level of date obj is:
  // {2015: {month: {...}}, 2016: {month: {...}}, 2017: {month: {...}}}
  // shoule be push into calendar array in order:
  // [2017.12, 2017.11, 2017.10, ..., 2017.1, 2016.12, 2016.11 ...]
  const _sort = (dateObj) => Object.keys(dateObj).sort((a, b) => b - a);

  return _sort(dates).reduce((yearArr, year) => {
    return _sort(dates[year]).reduce((monthObj, month) => {
      yearArr.push(this.make(year, month, dates[year][month]));
      return yearArr;
    }, {});
  }, []);
}

// turn Array: [2015-01-01, 2016-02-02, 2017-03-03] 
// to Object: { 2015: {01: [2015-01-01]}, 2016: { 02: [2016-02-02] }. { 2017: { 03: [2017-03-03] } }}
//
groupDate(dates) {
  return new Promise((resolve, reject) => {
    resolve(dates.reduce((l, r) => {
      // 2017-01-01 => [2017, 01, 01]
      let date = r.split('-').map(s => parseInt(s))

      // date[0] => year: 2017, date[1] => month: 1. date[2] => day: 1
      if (typeof l[date[0]] === 'undefined') l[date[0]] = {}
      if (typeof l[date[0]][date[1]] === 'undefined') l[date[0]][date[1]] = []

      l[date[0]][date[1]].push(r)
      return l;
    }, {}))
  })
}