Solution to Project Euler problem 49

extra/project-euler/049/049-tests.factor

@@ -0,0 +1,4 @@
+USING: project-euler.049 tools.test ;
+IN: project-euler.049.tests
+
+[ 296962999629 ] [ euler049 ] unit-test

extra/project-euler/049/049.factor

@@ -0,0 +1,74 @@
+! Copyright (c) 2009 Aaron Schaefer.
+! See http://factorcode.org/license.txt for BSD license.
+USING: arrays byte-arrays fry hints kernel math math.combinatorics
+    math.functions math.parser math.primes project-euler.common sequences sets ;
+IN: project-euler.049
+
+! http://projecteuler.net/index.php?section=problems&id=49
+
+! DESCRIPTION
+! -----------
+
+! The arithmetic sequence, 1487, 4817, 8147, in which each of the terms
+! increases by 3330, is unusual in two ways: (i) each of the three terms are
+! prime, and, (ii) each of the 4-digit numbers are permutations of one another.
+
+! There are no arithmetic sequences made up of three 1-, 2-, or 3-digit primes,
+! exhibiting this property, but there is one other 4-digit increasing sequence.
+
+! What 12-digit number do you form by concatenating the three terms in this
+! sequence?
+
+
+! SOLUTION
+! --------
+
+<PRIVATE
+
+: count-digits ( n -- byte-array )
+    10 <byte-array> [
+        '[ 10 /mod _ [ 1+ ] change-nth dup 0 > ] loop drop
+    ] keep ;
+
+HINTS: count-digits fixnum ;
+
+: permutations? ( n m -- ? )
+    [ count-digits ] bi@ = ;
+
+: collect-permutations ( seq -- seq )
+    [ V{ } clone ] [ dup ] bi* [
+        dupd '[ _ permutations? ] filter
+        [ diff ] keep pick push
+    ] each drop ;
+
+: potential-sequences ( -- seq )
+    1000 9999 primes-between
+    collect-permutations [ length 3 > ] filter ;
+
+: arithmetic-terms ( m n -- seq )
+    2dup [ swap - ] keep + 3array ;
+
+: (find-unusual-terms) ( n seq -- seq/f )
+    [ [ arithmetic-terms ] with map ] keep
+    '[ _ [ peek ] dip member? ] find nip ;
+
+: find-unusual-terms ( seq -- seq/? )
+    unclip-slice over (find-unusual-terms) [
+        nip
+    ] [
+        dup length 3 >= [ find-unusual-terms ] [ drop f ] if
+    ] if* ;
+
+: 4digit-concat ( seq -- str )
+    0 [ [ 10000 * ] dip + ] reduce ;
+
+PRIVATE>
+
+: euler049 ( -- answer )
+    potential-sequences [ find-unusual-terms ] map sift
+    [ { 1487 4817 8147 } = not ] find nip 4digit-concat ;
+
+! [ euler049 ] 100 ave-time
+! 206 ms ave run time - 10.25 SD (100 trials)
+
+SOLUTION: euler049

extra/project-euler/project-euler.factor

@@ -14,14 +14,14 @@ USING: definitions io io.files io.pathnames kernel math math.parser
     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.046 project-euler.047 project-euler.048
-    project-euler.052 project-euler.053 project-euler.055 project-euler.056
-    project-euler.057 project-euler.059 project-euler.067 project-euler.071
-    project-euler.073 project-euler.075 project-euler.076 project-euler.079
-    project-euler.092 project-euler.097 project-euler.099 project-euler.100
-    project-euler.116 project-euler.117 project-euler.134 project-euler.148
-    project-euler.150 project-euler.151 project-euler.164 project-euler.169
-    project-euler.173 project-euler.175 project-euler.186 project-euler.190
-    project-euler.203 project-euler.215 ;
+    project-euler.049 project-euler.052 project-euler.053 project-euler.055
+    project-euler.056 project-euler.057 project-euler.059 project-euler.067
+    project-euler.071 project-euler.073 project-euler.075 project-euler.076
+    project-euler.079 project-euler.092 project-euler.097 project-euler.099
+    project-euler.100 project-euler.116 project-euler.117 project-euler.134
+    project-euler.148 project-euler.150 project-euler.151 project-euler.164
+    project-euler.169 project-euler.173 project-euler.175 project-euler.186
+    project-euler.190 project-euler.203 project-euler.215 ;
 IN: project-euler
 
 <PRIVATE