计算某一天属于一年中的第几周

详细介绍见 Calculating the ISO week number. Jan Stout. http://www.proesite.com/timex/wkcalc.htm 

 

Simple week number的定义如下：

1. 第一周开始于该年的一月一号

2. 第n+1周开始于第n周后的7天

 

SWN( y, m, d ) = 1 + ( DP( y, m ) + d-1 ) / 7

where DP ("Days Passed") is given by:

DP( y, 1 ) = 0
DP( y, m+1 ) = DP( y, m ) + ML( y, m )

and ML ("Month Length") is defined as:

ML( y, 1 ) = 31
ML( y, 2 ) = 28 + LEAP( y )
ML( y, 3 ) = 31
ML( y, 4 ) = 30
ML( y, 5 ) = 31
ML( y, 6 ) = 30
ML( y, 7 ) = 31
ML( y, 8 ) = 31
ML( y, 9 ) = 30
ML( y, 10 ) = 31
ML( y, 11 ) = 30
ML( y, 12 ) = 31


and LEAP( y ) is defined as:

LEAP( y ) = ( y % 4 == 0 ) && ( ( y % 100 != 0 ) || ( y % 400 == 0 ) )

ISO week number[ISO8601]的定义如下：

1. 每周从星期一开始

2.一年的第一周是该calendar年中第一个包含星期四的星期，也就是说至少包含4天的第一个周。比如说2004年1月1号是星期四，则2004的第一周开始于2003.12.29.

 

ISO_WN( y, m, d )

{

    dow     = DOW( y, m, d ); // Day of the week

    dow0101 = DOW( y, 1, 1 );

 

    if      ( m == 1  &&  3 < dow0101 < 7 - (d-1) )

    {

        // days before week 1 of the current year have the same week number as

        // the last day of the last week of the previous year

 

        dow     = dow0101 - 1;

        dow0101 = DOW( y-1, 1, 1 );

        m       = 12;

        d       = 31;

    }

    else if ( m == 12  &&  30 - (d-1) < DOW( y+1, 1, 1 ) < 4 )

    {

        // days after the last week of the current year have the same week number as

        // the first day of the next year, (i.e. 1)

 

        return 1;

    }

 

    return ( DOW( y, 1, 1 ) < 4 ) + 4 * (m-1) + ( 2 * (m-1) + (d-1) + dow0101 - dow + 6 ) * 36 / 256;

 

}