Solution to Project Euler problem 46

extra/project-euler/046/046.factor

@@ -0,0 +1,52 @@
+! Copyright (c) 2008 Aaron Schaefer.
+! See http://factorcode.org/license.txt for BSD license.
+USING: kernel math math.functions math.primes math.ranges sequences ;
+IN: project-euler.046
+
+! http://projecteuler.net/index.php?section=problems&id=46
+
+! DESCRIPTION
+! -----------
+
+! It was proposed by Christian Goldbach that every odd composite number can be
+! written as the sum of a prime and twice a square.
+
+!     9  =  7 + 2 * 1^2
+!     15 =  7 + 2 * 2^2
+!     21 =  3 + 2 * 3^2
+!     25 =  7 + 2 * 3^2
+!     27 = 19 + 2 * 2^2
+!     33 = 31 + 2 * 1^2
+
+! It turns out that the conjecture was false.
+
+! What is the smallest odd composite that cannot be written as the sum of a
+! prime and twice a square?
+
+
+! SOLUTION
+! --------
+
+<PRIVATE
+
+: perfect-squares ( n -- seq )
+    2 /i sqrt >integer [1,b] [ sq ] map ;
+
+: fits-conjecture? ( n -- ? )
+    dup perfect-squares [ 2 * - ] with map [ prime? ] contains? ;
+
+: next-odd-composite ( n -- m )
+    dup odd? [ 2 + ] [ 1+ ] if dup prime? [ next-odd-composite ] when ;
+
+: disprove-conjecture ( n -- m )
+    dup fits-conjecture? [ next-odd-composite disprove-conjecture ] when ;
+
+PRIVATE>
+
+: euler046 ( -- answer )
+    9 disprove-conjecture ;
+
+! [ euler046 ] 100 ave-time
+! 150 ms run / 2 ms GC ave time - 100 trials
+
+MAIN: euler046

extra/project-euler/092/092.factor

@@ -1,6 +1,6 @@
-! Copyright (c) 2008 Aaron Schaefer.
+! Copyright (c) 2008 Aaron Schaefer, Slava Pestov.
 ! See http://factorcode.org/license.txt for BSD license.
-USING: kernel math math.ranges project-euler.common sequences ;
+USING: kernel math math.ranges sequences ;
 IN: project-euler.092
 
 ! http://projecteuler.net/index.php?section=problems&id=92

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, #35, #38, #43, #52, #55, #56, #92
+! number>digits - #16, #20, #30, #34, #35, #38, #43, #52, #55, #56
 ! palindrome? - #4, #36, #55
 ! pandigital? - #32, #38
 ! pentagonal? - #44, #45

extra/project-euler/project-euler.factor

@@ -13,10 +13,10 @@ USING: definitions io io.files kernel math math.parser project-euler.ave-time
     project-euler.033 project-euler.034 project-euler.035 project-euler.036
     project-euler.037 project-euler.038 project-euler.039 project-euler.040
     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.045 project-euler.046 project-euler.048 project-euler.052
-    project-euler.056 project-euler.067 project-euler.075 project-euler.079
+    project-euler.053 project-euler.056 project-euler.067 project-euler.075
-    project-euler.092 project-euler.097 project-euler.134 project-euler.169
+    project-euler.079 project-euler.092 project-euler.097 project-euler.134
-    project-euler.173 project-euler.175 ;
+    project-euler.169 project-euler.173 project-euler.175 ;
 IN: project-euler
 
 <PRIVATE