42 lines
1.0 KiB
Factor
42 lines
1.0 KiB
Factor
! Copyright (c) 2007 Aaron Schaefer, Daniel Ehrenberg.
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! See http://factorcode.org/license.txt for BSD license.
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USING: arrays combinators.lib hashtables kernel math math.parser math.ranges
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sequences sorting ;
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IN: project-euler.004
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! http://projecteuler.net/index.php?section=problems&id=4
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! DESCRIPTION
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! -----------
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! A palindromic number reads the same both ways. The largest palindrome made
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! from the product of two 2-digit numbers is 9009 = 91 * 99.
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! Find the largest palindrome made from the product of two 3-digit numbers.
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! SOLUTION
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! --------
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: palindrome? ( n -- ? )
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number>string dup reverse = ;
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: cartesian-product ( seq1 seq2 -- seq1xseq2 )
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swap [ swap [ 2array ] map-with ] map-with concat ;
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<PRIVATE
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: max-palindrome ( seq -- palindrome )
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natural-sort [ palindrome? ] find-last nip ;
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PRIVATE>
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: euler004 ( -- answer )
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100 999 [a,b] [ 10 mod zero? not ] subset dup
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cartesian-product [ product ] map prune max-palindrome ;
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! [ euler004 ] 100 ave-time
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! 1608 ms run / 102 ms GC ave time - 100 trials
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MAIN: euler004
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