1 ! Copyright (c) 2007 Samuel Tardieu.
2 ! See http://factorcode.org/license.txt for BSD license.
3 USING: combinators kernel math math.parser math.ranges sequences vectors ;
6 ! http://projecteuler.net/index.php?section=problems&id=175
11 ! Define f(0)=1 and f(n) to be the number of ways to write n as a sum of
12 ! powers of 2 where no power occurs more than twice.
14 ! For example, f(10)=5 since there are five different ways to express
15 ! 10: 10 = 8+2 = 8+1+1 = 4+4+2 = 4+2+2+1+1 = 4+4+1+1
17 ! It can be shown that for every fraction p/q (p0, q0) there exists at
18 ! least one integer n such that f(n)/f(n-1)=p/q.
20 ! For instance, the smallest n for which f(n)/f(n-1)=13/17 is 241. The
21 ! binary expansion of 241 is 11110001. Reading this binary number from
22 ! the most significant bit to the least significant bit there are 4
23 ! one's, 3 zeroes and 1 one. We shall call the string 4,3,1 the
24 ! Shortened Binary Expansion of 241.
26 ! Find the Shortened Binary Expansion of the smallest n for which
27 ! f(n)/f(n-1)=123456789/987654321.
29 ! Give your answer as comma separated integers, without any whitespaces.
34 : add-bits ( vec n b -- )
38 pick length 1 bitand = [ over pop + ] when swap push
41 : compute ( vec ratio -- )
43 { [ dup integer? ] [ 1- 0 add-bits ] }
44 { [ dup 1 < ] [ 1 over - / dupd compute 1 1 add-bits ] }
45 { [ t ] [ [ 1 mod compute ] 2keep >integer 0 add-bits ] }
48 : euler175 ( -- result )
49 V{ 1 } clone dup 123456789/987654321 compute [ number>string ] map "," join ;
51 ! [ euler175 ] 100 ave-time
52 ! 0 ms run / 0 ms GC ave time - 100 trials