sicp-all-tasks/sicp/1_002e19

Exercise 1.19: There is a clever algorithm for
computing the Fibonacci numbers in a logarithmic number of steps.  Recall the
transformation of the state variables 
  a
 and 
  b
 in the fib-iter
process of 1.2.2: 
  a
  ←
  a
  +
  b
 and 
  b
  ←
  a
.
Call this transformation 
  T
, and observe that applying 
  T
 over and over
again 
  n
 times, starting with 1 and 0, produces the pair 
  
    Fib
    (
    n
    +
    1
    )
  
 and 

  
    Fib
    (
    n
    )
  
.  In other words, the Fibonacci numbers are produced
by applying 
  
    T
    n
  
, the 
  
    n
    
      th
    
  
 power of the transformation 
  T
,
starting with the pair (1, 0).  Now consider 
  T
 to be the special case of

  
    p
    =
    0
  
 and 
  
    q
    =
    1
  
 in a family of transformations 
  
    T
    
      p
      q
    
  
,
where 
  
    T
    
      p
      q
    
  
 transforms the pair 
  
    (
    a
    ,
    b
    )
  
 according to 

  a
  ←
  
    b
    q
  
  +
  
    a
    q
  
  +
  
    a
    p
  
 and 
  b
  ←
  
    b
    p
  
  +
  
    a
    q
  
.
Show that if we apply such a transformation 
  
    T
    
      p
      q
    
  
 twice, the
effect is the same as using a single transformation 
  
    T
    
      
        p
        ′
      
      
        q
        ′
      
    
  
 of the
same form, and compute 
  
    p
    ′
  
  
 and 
  
    q
    ′
  
  
 in terms of 
  p
 and 
  q
.  This
gives us an explicit way to square these transformations, and thus we can
compute 
  
    T
    n
  
 using successive squaring, as in the fast-expt
procedure.  Put this all together to complete the following procedure, which
runs in a logarithmic number of steps:41


(define (fib n)
  (fib-iter 1 0 0 1 n))

(define (fib-iter a b p q count)
  (cond ((= count 0) 
         b)
        ((even? count)
         (fib-iter a
                   b
                   ⟨??⟩  ;compute p'
                   ⟨??⟩  ;compute q'
                   (/ count 2)))
        (else 
         (fib-iter (+ (* b q) 
                      (* a q) 
                      (* a p))
                   (+ (* b p) 
                      (* a q))
                   p
                   q
                   (- count 1)))))