factor/extra/project-euler/012/012.factor

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Factor

! Copyright (c) 2007 Aaron Schaefer.
! See http://factorcode.org/license.txt for BSD license.
USING: kernel math project-euler.common ;
IN: project-euler.012
! http://projecteuler.net/index.php?section=problems&id=12
! DESCRIPTION
! -----------
! The sequence of triangle numbers is generated by adding the natural numbers.
! So the 7th triangle number would be 1 + 2 + 3 + 4 + 5 + 6 + 7 = 28. The first
! ten terms would be:
! 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, ...
! Let us list the factors of the first seven triangle numbers:
! 1: 1
! 3: 1,3
! 6: 1,2,3,6
! 10: 1,2,5,10
! 15: 1,3,5,15
! 21: 1,3,7,21
! 28: 1,2,4,7,14,28
! We can see that the 7th triangle number, 28, is the first triangle number to
! have over five divisors.
! Which is the first triangle number to have over five-hundred divisors?
! SOLUTION
! --------
: euler012 ( -- answer )
8 [ dup nth-triangle tau* 500 < ] [ 1 + ] while nth-triangle ;
! [ euler012 ] 10 ave-time
! 6573 ms ave run time - 346.27 SD (10 trials)
SOLUTION: euler012