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