From 557bde6206e5243140307bff964a0b73e204d139 Mon Sep 17 00:00:00 2001
From: Aaron Schaefer <aaron@elasticdog.com>
Date: Sat, 2 Feb 2008 18:53:32 -0500
Subject: [PATCH] Solution to Project Euler problem 40

---
 extra/project-euler/040/040.factor       | 51 ++++++++++++++++++++++++
 extra/project-euler/075/075.factor       |  4 +-
 extra/project-euler/project-euler.factor |  6 +--
 3 files changed, 56 insertions(+), 5 deletions(-)
 create mode 100644 extra/project-euler/040/040.factor

diff --git a/extra/project-euler/040/040.factor b/extra/project-euler/040/040.factor
new file mode 100644
index 0000000000..8984559265
--- /dev/null
+++ b/extra/project-euler/040/040.factor
@@ -0,0 +1,51 @@
+! Copyright (c) 2008 Aaron Schaefer.
+! See http://factorcode.org/license.txt for BSD license.
+USING: kernel math math.parser sequences strings ;
+IN: project-euler.040
+
+! http://projecteuler.net/index.php?section=problems&id=40
+
+! DESCRIPTION
+! -----------
+
+! An irrational decimal fraction is created by concatenating the positive
+! integers:
+
+!     0.123456789101112131415161718192021...
+
+! It can be seen that the 12th digit of the fractional part is 1.
+
+! If dn represents the nth digit of the fractional part, find the value of the
+! following expression.
+
+!     d1 × d10 × d100 × d1000 × d10000 × d100000 × d1000000
+
+
+! SOLUTION
+! --------
+
+<PRIVATE
+
+: (concat-upto) ( n limit str -- str )
+    2dup length > [
+        pick number>string over push-all rot 1+ -rot (concat-upto)
+    ] [
+        2nip
+    ] if ;
+
+: concat-upto ( n -- str )
+    SBUF" " clone 1 -rot (concat-upto) ;
+
+: nth-integer ( n str -- m )
+    [ 1- ] dip nth 1string 10 string>integer ;
+
+PRIVATE>
+
+: euler040 ( -- answer )
+    1000000 concat-upto { 1 10 100 1000 10000 100000 1000000 }
+    [ swap nth-integer ] with map product ;
+
+! [ euler040 ] 100 ave-time
+! 1002 ms run / 43 ms GC ave time - 100 trials
+
+MAIN: euler040
diff --git a/extra/project-euler/075/075.factor b/extra/project-euler/075/075.factor
index f8ee9d50db..8399235c0d 100644
--- a/extra/project-euler/075/075.factor
+++ b/extra/project-euler/075/075.factor
@@ -39,8 +39,8 @@ IN: project-euler.075
 ! Basically, this makes an array of 1000000 zeros, recursively creates
 ! primitive triples using the three transforms and then increments the array at
 ! index [a+b+c] by one for each triple's sum AND its multiples under 1000000
-! (to account for non-primitive triples). The answer is just the number of
-! indexes that equal one.
+! (to account for non-primitive triples). The answer is just the total number
+! of indexes that are equal to one.
 
 SYMBOL: p-count
 
diff --git a/extra/project-euler/project-euler.factor b/extra/project-euler/project-euler.factor
index f5766536ef..eb9d7d1300 100644
--- a/extra/project-euler/project-euler.factor
+++ b/extra/project-euler/project-euler.factor
@@ -11,9 +11,9 @@ USING: definitions io io.files kernel math math.parser project-euler.ave-time
     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.038 project-euler.039 project-euler.067
-    project-euler.075 project-euler.134 project-euler.169 project-euler.173
-    project-euler.175 ;
+    project-euler.037 project-euler.038 project-euler.039 project-euler.040
+    project-euler.067 project-euler.075 project-euler.134 project-euler.169
+    project-euler.173 project-euler.175 ;
 IN: project-euler
 
 <PRIVATE