Monday, September 22, 2014

A couple more homographic function tricks

A generalized continued fraction is an expression of the form:
You can partly apply a homographic function to a generalized continued fraction if you have a stream of the ai and bi
(define (partly-apply-general hf nums dens)
  (let ((a (first  hf))
        (b (second hf))
        (c (third  hf))
        (d (fourth hf)))
    (if (empty-stream? nums)
        (values (list a a
                      c c)
        (let ((num (head nums))
              (den (head dens)))
          (values (list (+ (* a den) (* b num)) a
                        (+ (* c den) (* d num)) c)
                  (tail nums)
                  (tail dens))))))

(define (print-hf-general hf nums dens)
  (call-with-values (lambda () (partly-evaluate hf))
    (lambda (term hf*)
      (if (not term)
          (call-with-values (lambda () (partly-apply-general hf nums dens))
            (display term) 
            ;; take reciprocal and multiply by 10
            (let ((a (first hf*))
                  (b (second hf*))
                  (c (third hf*))
                  (d (fourth hf*)))
              (print-hf-general (list (* c 10) (* d 10)
                                      a        b)
                           nums dens)))))))
For example, we can compute pi from this generalized continued fraction:
(print-hf-general (list 0 4 1 0)
              ;; [1 1 4 9 16 25 ...]
       (cons-stream 1
      (let squares ((i 1))
    (cons-stream (* i i) (squares (1+ i)))))
              ;; [1 3 5 7 9 11 ...]
       (let odds ((j 1)) 
     (cons-stream j (odds (+ j 2)))))

; Quit!
A bi-homographic function is a function of the form:
(define (bi-homographic a b c d e f g h)
  (lambda (x y)
    (/ (+ (* a x y) (* b x) (* c y) d)
       (+ (* e x y) (* f x) (* g y) h))))
Like a homographic function, you can partly evaluate a bi-homographic function and generate a continued fraction. You can also partly apply a bi-homographic function to a pair of continued fractions. When you do this, you have a choice of which continued fraction to be the object of the partial application. There's about twice as much nasty math involved, but the gist is that a bi-homographic function takes two continued fractions as arguments and produces one continued fraction as a result.

It turns out that addition, subtraction, multiplication and division are bi-homographic functions, so one can incrementally compute sums and products of continued fractions.