Solution to Project Euler problem 19

extra/project-euler/019/019.factor

@@ -0,0 +1,54 @@
+! Copyright (c) 2007 Aaron Schaefer.
+! See http://factorcode.org/license.txt for BSD license.
+USING: calendar combinators.lib kernel math namespaces ;
+IN: project-euler.019
+
+! http://projecteuler.net/index.php?section=problems&id=19
+
+! DESCRIPTION
+! -----------
+
+! You are given the following information, but you may prefer to do some
+! research for yourself.
+
+!     * 1 Jan 1900 was a Monday.
+!     * Thirty days has September, April, June and November.  All the rest have
+!       thirty-one, Saving February alone, Which has twenty-eight, rain or
+!       shine.  And on leap years, twenty-nine.
+!     * A leap year occurs on any year evenly divisible by 4, but not on a
+!       century unless it is divisible by 400.
+
+! How many Sundays fell on the first of the month during the twentieth century
+! (1 Jan 1901 to 31 Dec 2000)?
+
+
+! SOLUTION
+! --------
+
+<PRIVATE
+
+: start-date ( -- timestamp )
+    1901 1 1 0 0 0 0 make-timestamp ;
+
+: end-date ( -- timestamp )
+    2000 12 31 0 0 0 0 make-timestamp ;
+
+: (first-days) ( end-date start-date -- )
+    2dup timestamp- 0 >= [
+        dup day-of-week , 1 +month (first-days)
+    ] [
+        2drop
+    ] if ;
+
+: first-days ( start-date end-date -- seq )
+    [ swap (first-days) ] { } make ;
+
+PRIVATE>
+
+: euler019 ( -- answer )
+    start-date end-date first-days [ zero? ] count ;
+
+! [ euler019 ] 100 ave-time
+! 131 ms run / 3 ms GC ave time - 100 trials
+
+MAIN: euler019

extra/project-euler/project-euler.factor

@@ -6,7 +6,7 @@ USING: io io.files kernel math.parser namespaces sequences strings
     project-euler.005 project-euler.006 project-euler.007 project-euler.008
     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.017 project-euler.018 project-euler.019 project-euler.067 ;
 IN: project-euler
 
 <PRIVATE