55 lines
1.3 KiB
Factor
55 lines
1.3 KiB
Factor
! Copyright (c) 2009 Guillaume Nargeot.
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! See http://factorcode.org/license.txt for BSD license.
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USING: arrays assocs hashtables kernel math math.functions
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project-euler.common sequences sorting ;
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IN: project-euler.062
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! http://projecteuler.net/index.php?section=problems&id=062
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! DESCRIPTION
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! -----------
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! The cube, 41063625 (345^3), can be permuted to produce two
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! other cubes: 56623104 (384^3) and 66430125 (405^3). In
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! fact, 41063625 is the smallest cube which has exactly three
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! permutations of its digits which are also cube.
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! Find the smallest cube for which exactly five permutations of
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! its digits are cube.
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! SOLUTION
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! --------
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<PRIVATE
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: cube ( n -- n^3 ) 3 ^ ; inline
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: >key ( n -- k ) cube number>digits natural-sort ; inline
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: has-entry? ( n assoc -- ? ) [ >key ] dip key? ; inline
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: (euler062) ( n assoc -- n )
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2dup has-entry? [
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2dup [ >key ] dip
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[ dup 0 swap [ 1 + ] change-nth ] change-at
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2dup [ >key ] dip at first 5 =
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[
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[ >key ] dip at second
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] [
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[ 1 + ] dip (euler062)
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] if
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] [
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2dup 1 pick cube 2array -rot
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[ >key ] dip set-at [ 1 + ] dip
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(euler062)
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] if ;
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PRIVATE>
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: euler062 ( -- answer )
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1 1 <hashtable> (euler062) ;
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! [ euler062 ] 100 ave-time
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! 78 ms ave run time - 0.9 SD (100 trials)
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SOLUTION: euler062
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