Perl DateTime 模块计算第一、第二、第三、第四、最后一个星期日、星期一、...每个月的星期六

Question

我正在使用 Perl DateTime Module ，我想知道如何计算 First、Second、Third、Fourth 和 最后 星期日，星期一，...指定月份的星期六。

我的方法:

从 i= 1...DateTime->last_day_of_month( ... ) 开始循环。

将日期 i 分配给 date($dt) 并使用 $dt->day_of_week() 获取星期几。

使用计数器跟踪第一、第二、第三、第四、最后。

如果星期几与所需的日期匹配并且计数器与所需的时间间隔匹配，则中断循环。

您能否提出比上述更好(或更短)的方法？感谢任何帮助。

Answer 1

这是对 my answer for finding the previous Monday (or any specified day of the week) 的直接修改.唯一的困难是弄清楚您的开始日期。

use DateTime;

# Here $nth is 1, 2, 3... for first, second, third, etc.
# Or -1, -2, -3... for last, next-to-last, etc.
# $dow is 1-7 for Monday-Sunday
# $month is 1-12
sub nth_day_of_month {
  my ($nth, $dow, $year, $month) = @_;

  my $date = ($nth > 0
              # For 1st etc. we want the last day of that week
              # (i.e. 7, 14, 21, 28 ...).  We have to use add because
              # the last week may extend into next month.
              ? DateTime->new(year => $year, month => $month, day => 1)
                        ->add( days => $nth * 7 - 1)
              # For last etc. we want the last day of the month
              # (minus a week if next-to-last, etc)
              : DateTime->last_day_of_month(year => $year, month => $month)
                        ->add( weeks => $nth + 1)); # $nth is negative

  # Back up to the first $dow on or before $date
  $date->subtract(days => ($date->day_of_week - $dow) % 7);

  # If we're not in the right month, then that month doesn't have the
  # specified date (e.g. there's no 5th Tuesday in Sept. 2013).
  return (($date->month == $month) ? $date : undef);
}

更新:这是一个稍微更高效的版本。它使用相同的算法，但它结合了对 add 和 subtract 的调用，因此它只需执行一次日期数学运算。

sub nth_day_of_month {
  my ($nth, $dow, $year, $month) = @_;

  my ($date, $delta);
  if ($nth > 0) {
    # For 1st etc. we want the last day of that week (i.e. 7, 14, 21, 28, "35")
    $date  = DateTime->new(year => $year, month => $month, day => 1);
    $delta = $nth * 7 - 1;
  } else {
    # For last etc. we want the last day of the month
    # (minus a week if next-to-last, etc)
    $date  = DateTime->last_day_of_month(year => $year, month => $month);
    $delta = 7 * ($nth + 1); # $nth is negative
  }

  # Back up to the first $dow on or before $date + $delta
  $date->add(days => $delta - ($date->day_of_week + $delta - $dow) % 7);

  # If we're not in the right month, then that month doesn't have the
  # specified date (e.g. there's no 5th Tuesday in Sept. 2013).
  return (($date->month == $month) ? $date : undef);
}