Slow solution to Project Euler problem 92

extra/project-euler/092/092.factor

@@ -0,0 +1,55 @@
+! Copyright (c) 2008 Aaron Schaefer.
+! See http://factorcode.org/license.txt for BSD license.
+USING: combinators.lib kernel math math.ranges namespaces project-euler.common
+    sequences ;
+IN: project-euler.092
+
+! http://projecteuler.net/index.php?section=problems&id=92
+
+! DESCRIPTION
+! -----------
+
+! A number chain is created by continuously adding the square of the digits in
+! a number to form a new number until it has been seen before.
+
+! For example,
+
+!     44 -> 32 -> 13 -> 10 -> 1 -> 1
+!     85 -> 89 -> 145 -> 42 -> 20 -> 4 -> 16 -> 37 -> 58 -> 89
+
+! Therefore any chain that arrives at 1 or 89 will become stuck in an endless
+! loop. What is most amazing is that EVERY starting number will eventually
+! arrive at 1 or 89.
+
+! How many starting numbers below ten million will arrive at 89?
+
+
+! SOLUTION
+! --------
+
+<PRIVATE
+
+: next-link ( n -- m )
+    number>digits [ sq ] sigma ;
+
+: chain-ending ( n -- m )
+    dup 1 = over 89 = or [ next-link chain-ending ] unless ;
+
+: lower-endings ( -- seq )
+    567 [1,b] [ chain-ending ] map ;
+
+: fast-chain-ending ( seq n -- m )
+    dup 567 > [ next-link ] when 1- swap nth ;
+
+PRIVATE>
+
+: euler092 ( -- answer )
+    lower-endings 9999999 [1,b] [ fast-chain-ending 89 = ] with count ;
+
+! [ euler092 ] time
+! 68766 ms run / 372 ms GC time
+
+! TODO: solution is still too slow, maybe try using a 10000000-byte array that
+! keeps track of each number in the chain and their endings
+
+MAIN: euler092

extra/project-euler/common/common.factor

@@ -14,7 +14,7 @@ IN: project-euler.common
 ! log10 - #25, #134
 ! max-path - #18, #67
 ! nth-triangle - #12, #42
-! number>digits - #16, #20, #30, #34
+! number>digits - #16, #20, #30, #34, #35, #38, #43, #52, #55, #56, #92
 ! palindrome? - #4, #36, #55
 ! pandigital? - #32, #38
 ! pentagonal? - #44, #45
@@ -78,7 +78,7 @@ PRIVATE>
     ] if ;
 
 : number>digits ( n -- seq )
-    number>string string>digits ;
+    [ dup zero? not ] [ 10 /mod ] [ ] unfold reverse nip ;
 
 : nth-triangle ( n -- n )
     dup 1+ * 2 / ;

extra/project-euler/project-euler.factor

@@ -15,8 +15,8 @@ USING: definitions io io.files kernel math math.parser project-euler.ave-time
     project-euler.041 project-euler.042 project-euler.043 project-euler.044
     project-euler.045 project-euler.048 project-euler.052 project-euler.053
     project-euler.056 project-euler.067 project-euler.075 project-euler.079
-    project-euler.097 project-euler.134 project-euler.169 project-euler.173
-    project-euler.175 ;
+    project-euler.092 project-euler.097 project-euler.134 project-euler.169
+    project-euler.173 project-euler.175 ;
 IN: project-euler
 
 <PRIVATE