(day-of-week) ( year month day -- n )

Vocabulary

calendar

Inputs

year	an integer
month	an integer
day	an integer

Outputs

n	an integer

Word description

An implementation of the Zeller's congruence algorithm that computes the day of the week given a date. Days are indexed starting from Sunday, which is index 0.

Notes

User code should use the day-of-week word, which takes a timestamp instead of integers.

Definition

USING: kernel math ;

IN: calendar

: (day-of-week) ( year month day -- n )
    [
        dup 2 <= [ [ 1 - ] [ 12 + ] bi* ] when
        [ dup [ 4 /i + ] [ 100 /i - ] [ 400 /i + ] tri ] dip
        [ 1 + 3 * 5 /i + ] keep 2 * +
    ] dip 1 + + 7 mod ;