factor/extra/project-euler/052/052.factor

52 lines
1.2 KiB
Factor

! Copyright (c) 2008 Aaron Schaefer.
! See http://factorcode.org/license.txt for BSD license.
USING: combinators.short-circuit kernel math math.functions
project-euler.common sequences sorting grouping ;
IN: project-euler.052
! http://projecteuler.net/index.php?section=problems&id=52
! DESCRIPTION
! -----------
! It can be seen that the number, 125874, and its double, 251748, contain
! exactly the same digits, but in a different order.
! Find the smallest positive integer, x, such that 2x, 3x, 4x, 5x, and 6x,
! contain the same digits.
! SOLUTION
! --------
! Analysis shows the number must be odd, divisible by 3, and larger than 123456
<PRIVATE
: map-nx ( n x -- seq )
<iota> [ 1 + * ] with map ; inline
: all-same-digits? ( seq -- ? )
[ number>digits natural-sort ] map all-equal? ;
: candidate? ( n -- ? )
{ [ odd? ] [ 3 divisor? ] } 1&& ;
: next-all-same ( x n -- n )
dup candidate? [
2dup swap map-nx all-same-digits?
[ nip ] [ 1 + next-all-same ] if
] [
1 + next-all-same
] if ;
PRIVATE>
: euler052 ( -- answer )
6 123456 next-all-same ;
! [ euler052 ] 100 ave-time
! 92 ms ave run time - 6.29 SD (100 trials)
SOLUTION: euler052