1 ! Copyright (c) 2008 Aaron Schaefer.
2 ! See https://factorcode.org/license.txt for BSD license.
3 USING: kernel math ranges math.order project-euler.common
7 ! https://projecteuler.net/problem=44
12 ! Pentagonal numbers are generated by the formula, Pn=n(3nā1)/2.
13 ! The first ten pentagonal numbers are:
15 ! 1, 5, 12, 22, 35, 51, 70, 92, 117, 145, ...
17 ! It can be seen that P4 + P7 = 22 + 70 = 92 = P8. However,
18 ! their difference, 70 ā 22 = 48, is not pentagonal.
20 ! Find the pair of pentagonal numbers, Pj and Pk, for which
21 ! their sum and difference is pentagonal and D = |Pk ā Pj| is
22 ! minimised; what is the value of D?
28 ! Brute force using a cartesian product and an arbitrarily
33 : nth-pentagonal ( n -- seq )
34 dup 3 * 1 - * 2 /i ; inline
36 : sum-and-diff? ( m n -- ? )
37 [ + ] [ - ] 2bi [ pentagonal? ] both? ; inline
39 : euler044-step ( min m n -- min' )
40 [ nth-pentagonal ] bi@
41 2dup sum-and-diff? [ - abs min ] [ 2drop ] if ; inline
45 : euler044 ( -- answer )
53 ! [ euler044 ] 10 ave-time
54 ! 289 ms ave run time - 0.27 SD (10 trials)