Factor solution to project Euler problem 17

extra/project-euler/017/017.factor

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+! Copyright (c) 2007 Samuel Tardieu.
+! See http://factorcode.org/license.txt for BSD license.
+USING: kernel math namespaces sequences strings ;
+IN: project-euler.017
+
+! http://projecteuler.net/index.php?section=problems&id=17
+
+! DESCRIPTION
+! -----------
+
+! If the numbers 1 to 5 are written out in words: one, two, three, four, five;
+! there are 3 + 3 + 5 + 4 + 4 = 19 letters used in total.
+
+! If all the numbers from 1 to 1000 (one thousand) inclusive were written out
+! in words, how many letters would be used?
+
+! NOTE: Do not count spaces or hyphens. For example, 342 (three hundred and
+! forty-two) contains 23 letters and 115 (one hundred and fifteen) contains
+! 20 letters.
+
+! SOLUTION
+! --------
+
+<PRIVATE
+
+: units ( n -- )
+  {
+    "zero" "one" "two" "three" "four" "five" "six" "seven" "eight" "nine"
+    "ten" "eleven" "twelve" "thirteen" "fourteen" "fifteen" "sixteen"
+    "seventeen" "eighteen" "nineteen"
+  } nth % ;
+
+: tenths ( n -- )
+  {
+    f f "twenty" "thirty" "fourty" "fifty" "sixty" "seventy" "eighty" "ninety"
+  } nth % ;
+
+DEFER: make-english
+
+: maybe-add ( n sep -- )
+  over 0 = [ 2drop ] [ % make-english ] if ;
+
+: 0-99 ( n -- )
+  dup 20 < [ units ] [ 10 /mod swap tenths "-" maybe-add ] if ;
+
+: 0-999 ( n -- )
+  100 /mod swap
+  dup 0 = [ drop 0-99 ] [ units " hundred" % " and " maybe-add ] if ;
+
+: make-english ( n -- )
+  1000 /mod swap
+  dup 0 = [ drop 0-999 ] [ 0-999 " thousand" % " and " maybe-add ] if ;
+
+PRIVATE>
+
+: >english ( n -- str )
+  [ make-english ] "" make ;
+
+: euler017 ( -- answer )
+  1000 [ 1 + >english [ letter? ] subset length ] map sum ;
+
+! [ euler017 ] 100 ave-time
+! 9 ms run / 0 ms GC ave time - 100 trials
+
+MAIN: euler017