factor/extra/project-euler/062/062.factor

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1.3 KiB
Factor

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