1 ! Copyright (c) 2008 Aaron Schaefer.
2 ! See https://factorcode.org/license.txt for BSD license.
3 USING: kernel math math.parser math.primes sequences
7 ! https://projecteuler.net/problem=37
12 ! The number 3797 has an interesting property. Being prime
13 ! itself, it is possible to continuously remove digits from left
14 ! to right, and remain prime at each stage: 3797, 797, 97, and
15 ! 7. Similarly we can work from right to left: 3797, 379, 37,
18 ! Find the sum of the only eleven primes that are both
19 ! truncatable from left to right and right to left.
21 ! NOTE: 2, 3, 5, and 7 are not considered to be truncatable
32 dup prime? [ r-trunc? ] [ drop f ] if
37 : reverse-digits ( n -- m )
38 number>string reverse string>number ;
41 reverse-digits 10 /i reverse-digits dup 0 > [
42 dup prime? [ l-trunc? ] [ drop f ] if
49 : euler037 ( -- answer )
50 23 1000000 primes-between [ r-trunc? ] filter [ l-trunc? ] filter sum ;
52 ! [ euler037 ] 100 ave-time
53 ! 130 ms ave run time - 6.27 SD (100 trials)