1 ! Copyright (c) 2007, 2008 Aaron Schaefer, Alexander Solovyov, Vishal Talwar.
2 ! See https://factorcode.org/license.txt for BSD license.
3 USING: kernel math sequences project-euler.common ;
6 ! https://projecteuler.net/problem=2
11 ! Each new term in the Fibonacci sequence is generated by adding
12 ! the previous two terms. By starting with 1 and 2, the first 10
15 ! 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
17 ! Find the sum of all the even-valued terms in the sequence
18 ! which do not exceed four million.
26 : (fib-upto) ( seq n limit -- seq )
27 2dup <= [ [ suffix! dup 2 tail* sum ] dip (fib-upto) ] [ 2drop ] if ;
31 : fib-upto ( n -- seq )
32 V{ 0 } clone 1 rot (fib-upto) ;
34 : euler002 ( -- answer )
35 4,000,000 fib-upto [ even? ] filter sum ;
37 ! [ euler002 ] 100 ave-time
38 ! 0 ms ave run time - 0.22 SD (100 trials)
44 : fib-upto* ( n -- seq )
45 [ 1 0 ] dip '[ [ + ] 2keep dup _ < ] [ ] produce 3nip ;
47 : euler002a ( -- answer )
48 4,000,000 fib-upto* [ even? ] filter sum ;
50 ! [ euler002a ] 100 ave-time
51 ! 0 ms ave run time - 0.2 SD (100 trials)
56 : next-fibs ( x y -- y x+y )
59 : ?retotal ( total fib- fib+ -- retotal fib- fib+ )
60 dup even? [ [ nip + ] 2keep ] when ;
62 : (sum-even-fibs-below) ( partial fib- fib+ max -- total )
66 [ ?retotal next-fibs ] dip (sum-even-fibs-below)
71 : sum-even-fibs-below ( max -- sum )
72 [ 0 0 1 ] dip (sum-even-fibs-below) ;
74 : euler002b ( -- answer )
75 4000000 sum-even-fibs-below ;
77 ! [ euler002b ] 100 ave-time
78 ! 0 ms ave run time - 0.0 SD (100 trials)