Solution to Project Euler problem 24

extra/project-euler/023/023.factor

@@ -31,25 +31,29 @@ IN: project-euler.023
 ! SOLUTION
 ! --------
 
-<PRIVATE
-
 ! The upper limit can be dropped to 20161 which reduces our search space
 ! and every even number > 46 can be expressed as a sum of two abundants
+
+<PRIVATE
+
 : source-023 ( -- seq )
     46 [1,b] 47 20161 2 <range> append ;
 
-: abundants-below ( n -- seq )
+: abundants-upto ( n -- seq )
     [1,b] [ abundant? ] subset ;
 
 : possible-sums ( seq -- seq )
     dup { } -rot [
-        dupd [ + ] curry map rot append prune swap 1 tail
+        dupd [ + ] curry map
+        rot append prune swap 1 tail
     ] each drop natural-sort ;
 
 PRIVATE>
 
 : euler023 ( -- answer )
-    20161 abundants-below possible-sums source-023 seq-diff sum ;
+    20161 abundants-upto possible-sums source-023 seq-diff sum ;
+
+! TODO: solution is still too slow, although it takes under 1 minute
 
 ! [ euler023 ] time
 ! 52780 ms run / 3839 ms GC

extra/project-euler/024/024.factor

@@ -0,0 +1,48 @@
+! Copyright (c) 2007 Aaron Schaefer.
+! See http://factorcode.org/license.txt for BSD license.
+USING: kernel math math.parser math.ranges namespaces sequences ;
+IN: project-euler.024
+
+! http://projecteuler.net/index.php?section=problems&id=24
+
+! DESCRIPTION
+! -----------
+
+! A permutation is an ordered arrangement of objects. For example, 3124 is one
+! possible permutation of the digits 1, 2, 3 and 4. If all of the permutations
+! are listed numerically or alphabetically, we call it lexicographic order. The
+! lexicographic permutations of 0, 1 and 2 are:
+
+!     012   021   102   120   201   210
+
+! What is the millionth lexicographic permutation of the digits 0, 1, 2, 3, 4,
+! 5, 6, 7, 8 and 9?
+
+
+! SOLUTION
+! --------
+
+<PRIVATE
+
+: (>permutation) ( seq n -- seq )
+    [ [ dupd >= [ 1+ ] when ] curry map ] keep add* ;
+
+: >permutation ( factoradic -- permutation )
+    reverse 1 cut [ (>permutation) ] each ;
+
+PRIVATE>
+
+: factoradic ( k order -- factoradic )
+    [ [1,b] [ 2dup mod , /i ] each ] { } make reverse nip ;
+
+: permutation ( k seq -- seq )
+    dup length swapd factoradic >permutation
+    [ [ dupd swap nth , ] each drop ] { } make ;
+
+: euler024 ( -- answer )
+    999999 10 permutation 10 swap digits>integer ;
+
+! [ euler024 ] 100 ave-time
+! 0 ms run / 0 ms GC ave time - 100 trials
+
+MAIN: euler024

extra/project-euler/project-euler.factor

@@ -7,7 +7,8 @@ USING: definitions io io.files kernel math.parser sequences vocabs
     project-euler.009 project-euler.010 project-euler.011 project-euler.012
     project-euler.013 project-euler.014 project-euler.015 project-euler.016
     project-euler.017 project-euler.018 project-euler.019 project-euler.020
-    project-euler.021 project-euler.022 project-euler.067 project-euler.134 ;
+    project-euler.021 project-euler.022 project-euler.023 project-euler.024
+    project-euler.067 project-euler.134 ;
 IN: project-euler
 
 <PRIVATE