Friday the 13th

In honor of January 13, 2012, a Friday the 13th, I thought it might be fun to use Factor to explore similar dates in past and future history. According to Wikipedia, such a day "occurs at least once, but at most three times a year".

friday-13th?

A day is "Friday the 13th" if it is both (a) Friday and (b) the 13th:

: friday-13th? ( timestamp -- ? )
    [ day>> 13 = ] [ friday? ] bi and ;

Trying it for today and tomorrow, to make sure it works:

IN: scratchpad now friday-13th? .
t

IN: scratchpad : tomorrow ( -- timestamp )
                   now 1 days time+ ;

               tomorrow friday-13th? .
f

friday-13ths

Getting all Friday the 13th's for a given year:

: friday-13ths ( year -- seq )
    12 [0,b) [
        13 <date> dup friday? [ drop f ] unless
    ] with map sift ;

Or, for a range of years:

: all-friday-13ths ( start-year end-year -- seq )
    [a,b] [ friday-13ths ] map concat ;

Trying it for 2012:

IN: scratchpad 2012 friday-13ths .
{
    T{ timestamp
        { year 2012 }
        { month 1 }
        { day 13 }
    }
    T{ timestamp
        { year 2012 }
        { month 4 }
        { day 13 }
    }
    T{ timestamp
        { year 2012 }
        { month 7 }
        { day 13 }
    }
}

next-friday-13th

We can iterate, looking for the next Friday the 13th:

: next-friday-13th ( timestamp -- date )
    dup day>> 13 >= [ 1 months time+ ] when 13 >>day
    [ dup friday? not ] [ 1 months time+ ] while ;

Trying it for today, shows the next Friday the 13th is April, 13, 2012:

IN: scratchpad now next-friday-13th .
T{ timestamp
    { year 2012 }
    { month 4 }
    { day 13 }
}

The code (and some tests) for this is on my Github.