Add a solution to project-euler.164

extra/project-euler/164/164.factor

@@ -0,0 +1,33 @@
+! Copyright (c) 2008 Eric Mertens
+! See http://factorcode.org/license.txt for BSD license.
+USING: arrays assocs kernel math math.ranges sequences ;
+
+IN: project-euler.164
+
+! http://projecteuler.net/index.php?section=problems&id=164
+
+! DESCRIPTION
+! -----------
+
+! How many 20 digit numbers n (without any leading zero) exist such
+! that no three consecutive digits of n have a sum greater than 9?
+
+! SOLUTION
+! --------
+
+<PRIVATE
+
+: next-keys ( key -- keys )
+    [ peek ] [ 10 swap sum - ] bi [ 2array ] with map ;
+
+: next-table ( assoc -- assoc )
+    H{ } clone swap
+    [ swap next-keys [ pick at+ ] with each ] assoc-each ;
+
+: init-table ( -- assoc )
+    9 [1,b] [ 1array 1 ] H{ } map>assoc ;
+
+PRIVATE>
+
+: euler164 ( -- n )
+    init-table 19 [ next-table ] times values sum ;