25 lines
519 B
Factor
25 lines
519 B
Factor
! Copyright (c) 2008 Eric Mertens
|
|
! See http://factorcode.org/license.txt for BSD license.
|
|
USING: kernel math math.functions sequences sequences.lib ;
|
|
|
|
IN: project-euler.148
|
|
|
|
<PRIVATE
|
|
|
|
: sum-1toN ( n -- sum )
|
|
dup 1+ * 2/ ; inline
|
|
|
|
: >base7 ( x -- y )
|
|
[ dup 0 > ] [ 7 /mod ] [ ] unfold nip ;
|
|
|
|
: (use-digit) ( prev x index -- next )
|
|
[ [ 1+ * ] [ sum-1toN 7 sum-1toN ] bi ] dip ^ * + ;
|
|
|
|
PRIVATE>
|
|
|
|
: (euler148) ( x -- y )
|
|
>base7 0 [ (use-digit) ] reduce-index ;
|
|
|
|
: euler148 ( -- y )
|
|
10 9 ^ (euler148) ;
|