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