1 ! Copyright (c) 2008 Aaron Schaefer.
2 ! See https://factorcode.org/license.txt for BSD license.
3 USING: kernel math ranges project-euler.common sequences ;
6 ! https://projecteuler.net/problem=38
11 ! Take the number 192 and multiply it by each of 1, 2, and 3:
17 ! By concatenating each product we get the 1 to 9 pandigital,
18 ! 192384576. We will call 192384576 the concatenated product of
21 ! The same can be achieved by starting with 9 and multiplying by
22 ! 1, 2, 3, 4, and 5, giving the pandigital, 918273645, which is
23 ! the concatenated product of 9 and (1,2,3,4,5).
25 ! What is the largest 1 to 9 pandigital 9-digit number that can
26 ! be formed as the concatenated product of an integer with
27 ! (1,2, ..., n) where n > 1?
33 ! Only need to search 4-digit numbers starting with 9 since a
34 ! 2-digit number starting with 9 would produce 8 or 11 digits,
35 ! and a 3-digit number starting with 9 would produce 7 or 11
40 : (concat-product) ( accum n multiplier -- m )
44 [ * number>digits append! ] 2keep 1 + (concat-product)
47 : concat-product ( n -- m )
48 V{ } clone swap 1 (concat-product) ;
52 : euler038 ( -- answer )
53 9123 9876 [a..b] [ concat-product ] map [ pandigital? ] filter supremum ;
55 ! [ euler038 ] 100 ave-time
56 ! 11 ms ave run time - 1.5 SD (100 trials)