! Copyright (c) 2008 Aaron Schaefer.
! See http://factorcode.org/license.txt for BSD license.
USING: kernel locals make math project-euler.common sequences ;
IN: project-euler.073

! http://projecteuler.net/index.php?section=problems&id=73

! DESCRIPTION
! -----------

! Consider the fraction, n/d, where n and d are positive integers. If n<d and
! HCF(n,d) = 1, it is called a reduced proper fraction.

! If we list the set of reduced proper fractions for d <= 8 in ascending order of
! size, we get:

!     1/8, 1/7, 1/6, 1/5, 1/4, 2/7, 1/3, 3/8, 2/5, 3/7, 1/2, 4/7, 3/5, 5/8,
!     2/3, 5/7, 3/4, 4/5, 5/6, 6/7, 7/8

! It can be seen that there are 3 fractions between 1/3 and 1/2.

! How many fractions lie between 1/3 and 1/2 in the sorted set of reduced
! proper fractions for d <= 10,000?


! SOLUTION
! --------

! Use the properties of a Farey sequence by setting an upper bound of 3/7 and
! then taking the mediant of that fraction and the one to its immediate left
! repeatedly until the denominator is as close to 1000000 as possible without
! going over.

<PRIVATE

! MESSY STACK SHUFFLING, NEED RECURSION HELP?

: (euler073) ( limit lo hi -- )
    pick [ 2dup mediant denominator ] dip <= [
        2dup mediant dup , tuck swap [ pick ] 2dip (euler073) (euler073)
    ] [ 3drop ] if ;

! SLIGHTLY CLEANER USING LOCALS, STILL NOT OPTIMAL

:: (euler073a) ( limit lo hi -- )
    lo hi mediant denominator limit <= [
        lo hi mediant dup ,
        [ [ limit lo ] dip (euler073a) ]
        [ [ limit ] dip hi (euler073a) ] bi
    ] when ;

PRIVATE>

: euler073 ( -- answer )
    [ 10000 1/3 1/2 (euler073a) ] { } make length ;

! Takes about 30 seconds right now, either way...any suggestions?