17 lines
475 B
Scheme
17 lines
475 B
Scheme
#lang sicp
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(define (fib n)
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(fib-iter 1 0 0 1 n))
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(define (fib-iter a b p q count)
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(cond ((= count 0) b)
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((even? count)
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(fib-iter a
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b
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(+ (* p p) (* q q))
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(+ (* 2 p q) (* q q))
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(/ count 2)))
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(else (fib-iter (+ (* b q) (* a q) (* a p))
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(+ (* b p) (* a q))
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p
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q
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(- count 1)))))
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