! --------
: last2 ( seq -- elt last )
- reverse first2 swap ;
+ 2 tail* first2 ;
-: fib-up-to ( n -- seq )
+: fib-upto ( n -- seq )
{ 0 } 1 [ pick dupd < ] [ add dup last2 + ] [ ] while drop nip ;
: euler002 ( -- answer )
- 1000000 fib-up-to [ even? ] subset sum ;
+ 1000000 fib-upto [ even? ] subset sum ;
! [ euler002 ] 100 ave-time
! 0 ms run / 0 ms GC ave time - 100 trials
--- /dev/null
+! Copyright (c) 2007 Aaron Schaefer.
+! See http://factorcode.org/license.txt for BSD license.
+USING: kernel math math.parser memoize sequences ;
+IN: project-euler.025
+
+! http://projecteuler.net/index.php?section=problems&id=25
+
+! DESCRIPTION
+! -----------
+
+! The Fibonacci sequence is defined by the recurrence relation:
+
+! Fn = Fn-1 + Fn-2, where F1 = 1 and F2 = 1.
+
+! Hence the first 12 terms will be:
+
+! F1 = 1
+! F2 = 1
+! F3 = 2
+! F4 = 3
+! F5 = 5
+! F6 = 8
+! F7 = 13
+! F8 = 21
+! F9 = 34
+! F10 = 55
+! F11 = 89
+! F12 = 144
+
+! The 12th term, F12, is the first term to contain three digits.
+
+! What is the first term in the Fibonacci sequence to contain 1000 digits?
+
+
+! SOLUTION
+! --------
+
+MEMO: fib ( m -- n )
+ dup 1 > [ 1 - dup fib swap 1 - fib + ] when ;
+
+<PRIVATE
+
+: (digit-fib) ( n term -- term )
+ 2dup fib number>string length > [ 1+ (digit-fib) ] [ nip ] if ;
+
+: digit-fib ( n -- term )
+ 1 (digit-fib) ;
+
+PRIVATE>
+
+: euler025 ( -- answer )
+ 1000 digit-fib ;
+
+! [ euler025 ] 10 ave-time
+! 5237 ms run / 72 ms GC ave time - 10 trials
+
+MAIN: euler025
project-euler.013 project-euler.014 project-euler.015 project-euler.016
project-euler.017 project-euler.018 project-euler.019 project-euler.020
project-euler.021 project-euler.022 project-euler.023 project-euler.024
+ project-euler.025
project-euler.067 project-euler.134 ;
IN: project-euler