Solution to Project Euler problem 37

extra/project-euler/037/037.factor

@@ -0,0 +1,52 @@
+! Copyright (c) 2008 Aaron Schaefer.
+! See http://factorcode.org/license.txt for BSD license.
+USING: kernel math math.parser math.primes sequences ;
+IN: project-euler.037
+
+! http://projecteuler.net/index.php?section=problems&id=37
+
+! DESCRIPTION
+! -----------
+
+! The number 3797 has an interesting property. Being prime itself, it is
+! possible to continuously remove digits from left to right, and remain prime
+! at each stage: 3797, 797, 97, and 7. Similarly we can work from right to
+! left: 3797, 379, 37, and 3.
+
+! Find the sum of the only eleven primes that are both truncatable from left to
+! right and right to left.
+
+! NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.
+
+
+! SOLUTION
+! --------
+
+<PRIVATE
+
+: r-trunc? ( n -- ? )
+    10 /i dup 0 > [
+        dup prime? [ r-trunc? ] [ drop f ] if
+    ] [
+        drop t
+    ] if ;
+
+: reverse-digits ( n -- m )
+    number>string reverse 10 string>integer ;
+
+: l-trunc? ( n -- ? )
+    reverse-digits 10 /i reverse-digits dup 0 > [
+        dup prime? [ l-trunc? ] [ drop f ] if
+    ] [
+        drop t
+    ] if ;
+
+PRIVATE>
+
+: euler037 ( -- answer )
+    23 1000000 primes-between [ r-trunc? ] subset [ l-trunc? ] subset sum ;
+
+! [ euler037 ] 100 ave-time
+! 768 ms run / 9 ms GC ave time - 100 trials
+
+MAIN: euler037

extra/project-euler/project-euler.factor

@@ -11,8 +11,8 @@ USING: definitions io io.files kernel math.parser sequences vocabs
     project-euler.025 project-euler.026 project-euler.027 project-euler.028
     project-euler.029 project-euler.030 project-euler.031 project-euler.032
     project-euler.033 project-euler.034 project-euler.035 project-euler.036
-    project-euler.067 project-euler.134 project-euler.169 project-euler.173
-    project-euler.175 ;
+    project-euler.037 project-euler.067 project-euler.134 project-euler.169
+    project-euler.173 project-euler.175 ;
 IN: project-euler
 
 <PRIVATE