Solution to Project Euler problem 79

extra/project-euler/079/079.factor

@@ -0,0 +1,65 @@
+! Copyright (c) 2008 Aaron Schaefer.
+! See http://factorcode.org/license.txt for BSD license.
+USING: assocs hashtables io.files kernel math math.parser namespaces sequences ;
+IN: project-euler.079
+
+! http://projecteuler.net/index.php?section=problems&id=79
+
+! DESCRIPTION
+! -----------
+
+! A common security method used for online banking is to ask the user for three
+! random characters from a passcode. For example, if the passcode was 531278,
+! they may asked for the 2nd, 3rd, and 5th characters; the expected reply would
+! be: 317.
+
+! The text file, keylog.txt, contains fifty successful login attempts.
+
+! Given that the three characters are always asked for in order, analyse the
+! file so as to determine the shortest possible secret passcode of unknown
+! length.
+
+
+! SOLUTION
+! --------
+
+<PRIVATE
+
+: source-079 ( -- seq )
+    "extra/project-euler/079/keylog.txt" resource-path file-lines ;
+
+: >edges ( seq -- seq )
+    [
+        [ string>digits [ 2 head , ] keep 2 tail* , ] each
+    ] { } make ;
+
+: find-source ( seq -- elt )
+    dup values swap keys [ prune ] 2apply seq-diff
+    dup empty? [ "Topological sort failed" throw ] [ first ] if ;
+
+: remove-source ( seq elt -- seq )
+    [ swap member? not ] curry subset ;
+
+: (topological-sort) ( seq -- )
+    dup length 1 > [
+        dup find-source dup , remove-source (topological-sort)
+    ] [
+        dup empty? [ drop ] [ first [ , ] each ] if
+    ] if ;
+
+PRIVATE>
+
+: topological-sort ( seq -- seq )
+    [ [ (topological-sort) ] { } make ] keep
+    concat prune dupd seq-diff append ;
+
+: euler079 ( -- answer )
+    source-079 >edges topological-sort 10 swap digits>integer ;
+
+! [ euler079 ] 100 ave-time
+! 2 ms run / 0 ms GC ave time - 100 trials
+
+! TODO: prune and seq-diff are relatively slow; topological sort could be
+! cleaned up and generalized much better, but it works for this problem
+
+MAIN: euler079

extra/project-euler/079/keylog.txt

@@ -0,0 +1,50 @@
+319
+680
+180
+690
+129
+620
+762
+689
+762
+318
+368
+710
+720
+710
+629
+168
+160
+689
+716
+731
+736
+729
+316
+729
+729
+710
+769
+290
+719
+680
+318
+389
+162
+289
+162
+718
+729
+319
+790
+680
+890
+362
+319
+760
+316
+729
+380
+319
+728
+716

extra/project-euler/project-euler.factor

@@ -14,8 +14,8 @@ USING: definitions io io.files kernel math math.parser project-euler.ave-time
     project-euler.037 project-euler.038 project-euler.039 project-euler.040
     project-euler.041 project-euler.042 project-euler.043 project-euler.044
     project-euler.048 project-euler.052 project-euler.067 project-euler.075
-    project-euler.097 project-euler.134 project-euler.169 project-euler.173
+    project-euler.079 project-euler.097 project-euler.134 project-euler.169
-    project-euler.175 ;
+    project-euler.173 project-euler.175 ;
 IN: project-euler
 
 <PRIVATE