factor/extra/project-euler/019/019.factor

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1.7 KiB
Factor

! Copyright (c) 2007 Samuel Tardieu, Aaron Schaefer.
! See http://factorcode.org/license.txt for BSD license.
USING: calendar combinators kernel math math.ranges namespaces sequences
math.order project-euler.common ;
IN: project-euler.019
! http://projecteuler.net/index.php?section=problems&id=19
! DESCRIPTION
! -----------
! You are given the following information, but you may prefer to do some
! research for yourself.
! * 1 Jan 1900 was a Monday.
! * Thirty days has September, April, June and November. All the rest have
! thirty-one, Saving February alone, Which has twenty-eight, rain or
! shine. And on leap years, twenty-nine.
! * A leap year occurs on any year evenly divisible by 4, but not on a
! century unless it is divisible by 400.
! How many Sundays fell on the first of the month during the twentieth century
! (1 Jan 1901 to 31 Dec 2000)?
! SOLUTION
! --------
! Use Zeller congruence, which is implemented in the "calendar" module
! already, as "zeller-congruence ( year month day -- n )" where n is
! the day of the week (Sunday is 0).
: euler019 ( -- answer )
1901 2000 [a,b] [
12 [1,b] [ 1 zeller-congruence ] with map
] map concat [ 0 = ] count ;
! [ euler019 ] 100 ave-time
! 1 ms ave run time - 0.51 SD (100 trials)
! ALTERNATE SOLUTIONS
! -------------------
<PRIVATE
: start-date ( -- timestamp )
1901 1 1 <date> ;
: end-date ( -- timestamp )
2000 12 31 <date> ;
: first-days ( end-date start-date -- days )
[ 2dup after=? ]
[ dup 1 months time+ swap day-of-week ]
produce 2nip ;
PRIVATE>
: euler019a ( -- answer )
end-date start-date first-days [ 0 = ] count ;
! [ euler019a ] 100 ave-time
! 17 ms ave run time - 2.13 SD (100 trials)
SOLUTION: euler019