## Thursday, 30 April 2015

### Get the day of the week from grub4dos

The next E2B 1.67 Beta c will include the grub4dos batch file dow.g4b

dow.g4b takes 3 parameters:   year    month    day-of-month
and returns the grub4dos variable dow which is a number from 0-6 (0-Sunday, 1-Monday, etc.).
This means you can call it to test what day of the week it is.
For instance, you can call dow.g4b like this:

call /%grub%/dow.g4b   %@date:~0,4%    %@date:~5,2%   %@date:~8,2%

and test dow like this:

# only allow use at weekends
if not "%dow%"=="0" if not "%dow%"=="6" halt

# don't allow at weekends, just shutdown!
if "%dow%"=="0" halt
if "%dow%"=="6" halt

The grub4dos code below uses the Sakamoto algorithm and is not fully optimised so that you can see how it works. As it will only be executed once, it does not need to be efficient.

!BAT
#    dayofweek(y, m, d) /* 1 <= m <= 12,  y > 1752 (in the U.K.) */
#    {
#        static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
#        if (mm < 3) y = y - 1;
#        return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;
#    }

set dow=
setlocal

set y=%1
set m=%2
set d=%3
set /a y=%y%+0 > nul
set /a m=%m%+0 > nul
set /a d=%d%+0 > nul
if "%y%"=="0" exit
if "%m%"=="0" exit
if "%d%"=="0" exit
if %m%==1 set t=0
if %m%==2 set t=3
if %m%==3 set t=2
if %m%==4 set t=5
if %m%==5 set t=0
if %m%==6 set t=3
if %m%==7 set t=5
if %m%==8 set t=1
if %m%==9 set t=4
if %m%==10 set t=6
if %m%==11 set t=2
if %m%==12 set t=4

set y1=%y%
if %m%<=2 set /a y1=%y1%-1 > nul
set /a x=%y1%/4 > nul
set /a xx=%y1%/100 > nul
set /a xxx=%y1%/400 > nul
# 2015 4 30
#echo   1983 + 495 - 19 + 4 + 5 + 30
#echo %y1% + %x% - %xx% + %xxx% + %t% + %d%
set /a xxxx=%y1% + %x% - %xx% + %xxx% + %t% + %d% > nul
set /a dow = %xxxx% % 7 > nul
#echo dow=%dow%

endlocal && set dow=%dow%

### checktime

We could also use the built-in grub4dos command checktime

# get day of week
checktime | set A=
set dow=%A:~20,1%
set A=
echo Day of Week=%dow%