Elegant code for binary addition?

Question

I just wrote the function add-registers for binary addition of two n-bit registers in Racket (using bit-add function as a helper):

(define (bit-add x y c)
  (values (bitwise-xor x y c) (bitwise-ior (bitwise-and x y)
                                           (bitwise-and x c)
                                           (bitwise-and y c))))

(define (add-registers xs ys)
  (let ([carry 0])
    (values (reverse (for/list ([b1 (reverse xs)] [b2 (reverse ys)])
                       (let-values ([(nb nc) (bit-add b1 b2 carry)])
                         (set! carry nc)
                         nb)))
            carry)))

But I found my code pretty ugly. So I wonder if this could be written more concisely and elegant?

Sam Tobin-Hochstadt · Accepted Answer · 2012-04-10 14:16:15Z

6

Here's a new version of add-registers that looks somewhat nicer:

(define (add-registers xs ys)
  (for/fold ([carry 0] [bs empty])
     ([b1 (reverse xs)] [b2 (reverse ys)])
     (define-values (nb nc) (bit-add b1 b2 carry))
     (values nc (cons nb bs))))

answered Apr 10, 2012 at 14:16

Sam Tobin-Hochstadt

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