Solution to Project Euler problem 38

extra/project-euler/032/032.factor

@@ -1,7 +1,7 @@
 ! Copyright (c) 2008 Aaron Schaefer.
 ! See http://factorcode.org/license.txt for BSD license.
 USING: combinators.lib hashtables kernel math math.combinatorics math.parser
-    math.ranges project-euler.common sequences sorting ;
+    math.ranges project-euler.common sequences ;
 IN: project-euler.032
 
 ! http://projecteuler.net/index.php?section=problems&id=32
@@ -63,9 +63,6 @@ PRIVATE>
 : source-032a ( -- seq )
     50 [1,b] 2000 [1,b] cartesian-product ;
 
-: pandigital? ( n -- ? )
-    number>string natural-sort "123456789" = ;
-
 ! multiplicand/multiplier/product
 : mmp ( pair -- n )
     first2 2dup * [ number>string ] 3apply 3append 10 string>integer ;

extra/project-euler/038/038.factor

@@ -0,0 +1,55 @@
+! Copyright (c) 2008 Aaron Schaefer.
+! See http://factorcode.org/license.txt for BSD license.
+USING: kernel math math.parser math.ranges project-euler.common sequences ;
+IN: project-euler.038
+
+! http://projecteuler.net/index.php?section=problems&id=38
+
+! DESCRIPTION
+! -----------
+
+! Take the number 192 and multiply it by each of 1, 2, and 3:
+
+!     192 × 1 = 192
+!     192 × 2 = 384
+!     192 × 3 = 576
+
+! By concatenating each product we get the 1 to 9 pandigital, 192384576. We
+! will call 192384576 the concatenated product of 192 and (1,2,3)
+
+! The same can be achieved by starting with 9 and multiplying by 1, 2, 3, 4,
+! and 5, giving the pandigital, 918273645, which is the concatenated product of
+! 9 and (1,2,3,4,5).
+
+! What is the largest 1 to 9 pandigital 9-digit number that can be formed as
+! the concatenated product of an integer with (1,2, ... , n) where n > 1?
+
+
+! SOLUTION
+! --------
+
+! Only need to search 4-digit numbers starting with 9 since a 2-digit number
+! starting with 9 would produce 8 or 11 digits, and a 3-digit number starting
+! with 9 would produce 7 or 11 digits.
+
+<PRIVATE
+
+: (concat-product) ( accum n multiplier -- m )
+    pick length 8 > [
+        2drop 10 swap digits>integer
+    ] [
+        [ * number>digits over push-all ] 2keep 1+ (concat-product)
+    ] if ;
+
+: concat-product ( n -- m )
+    V{ } clone swap 1 (concat-product) ;
+
+PRIVATE>
+
+: euler038 ( -- answer )
+    9123 9876 [a,b] [ concat-product ] map [ pandigital? ] subset supremum ;
+
+! [ euler038 ] 100 ave-time
+! 37 ms run / 1 ms GC ave time - 100 trials
+
+MAIN: euler038

extra/project-euler/common/common.factor

@@ -1,5 +1,5 @@
 USING: arrays combinators.lib kernel math math.functions math.miller-rabin
-    math.parser math.primes.factors math.ranges namespaces sequences ;
+    math.parser math.primes.factors math.ranges namespaces sequences sorting ;
 IN: project-euler.common
 
 ! A collection of words used by more than one Project Euler solution
@@ -12,6 +12,7 @@ IN: project-euler.common
 ! log10 - #25, #134
 ! max-path - #18, #67
 ! number>digits - #16, #20, #30, #34
+! pandigital? - #32, #38
 ! propagate-all - #18, #67
 ! sum-proper-divisors - #21
 ! tau* - #12
@@ -67,6 +68,9 @@ PRIVATE>
 : number>digits ( n -- seq )
     number>string string>digits ;
 
+: pandigital? ( n -- ? )
+    number>string natural-sort "123456789" = ;
+
 ! Not strictly needed, but it is nice to be able to dump the triangle after the
 ! propagation
 : propagate-all ( triangle -- newtriangle )

extra/project-euler/project-euler.factor

@@ -11,8 +11,8 @@ USING: definitions io io.files kernel math.parser sequences vocabs
     project-euler.025 project-euler.026 project-euler.027 project-euler.028
     project-euler.029 project-euler.030 project-euler.031 project-euler.032
     project-euler.033 project-euler.034 project-euler.035 project-euler.036
-    project-euler.037 project-euler.067 project-euler.134 project-euler.169
-    project-euler.173 project-euler.175 ;
+    project-euler.037 project-euler.038 project-euler.067 project-euler.134
+    project-euler.169 project-euler.173 project-euler.175 ;
 IN: project-euler
 
 <PRIVATE