## Big Numbers: Division

### June 7, 2011

`(define (big-divide ns ds) ; (values quotient remainder)`

(define (sign x) (if (negative? x) -1 (if (positive? x) 1 0)))

(define (lt? xs ys)

(let ((xlen (length xs)) (ylen (length ys)))

(if (< xlen ylen) #t (if (< ylen xlen) #f

(let loop ((xs (reverse xs)) (ys (reverse ys)))

(cond ((null? xs) #f) ; equal

((< (car xs) (car ys)) #t)

((< (car ys) (car xs)) #f)

(else (loop (cdr xs) (cdr ys)))))))))

(define (reduce xs)

(if (null? (cdr xs)) xs

(if (positive? (car xs)) xs

(reduce (cdr xs)))))

(define (add b1 b2)

(let loop ((b1 b1) (b2 b2) (c 0) (bs '()))

(cond ((null? b1)

(if (zero? c) (reverse bs) (reverse (cons 1 bs))))

((null? b2)

(let* ((sum (+ (car b1) c))

(new-c (if (<= big-base sum) 1 0)))

(loop (cdr b1) b2 new-c

(cons (modulo sum big-base) bs))))

(else (let* ((sum (+ (car b1) (car b2) c))

(new-c (if (<= big-base sum) 1 0)))

(loop (cdr b1) (cdr b2) new-c

(cons (modulo sum big-base) bs)))))))

(define (sub b1 b2)

(let loop ((b1 b1) (b2 b2) (c 0) (bs '()))

(cond ((null? b1) (reverse (reduce bs)))

((null? b2)

(let* ((diff (- (car b1) c))

(new-c (if (< diff 0) 1 0)))

(loop (cdr b1) b2 new-c

(cons (modulo diff big-base) bs))))

(else (let* ((diff (- (car b1) (car b2) c))

(new-c (if (< diff 0) 1 0)))

(loop (cdr b1) (cdr b2) new-c

(cons (modulo diff big-base) bs)))))))

(define (times big1 big2)

(let loop ((b1 big1) (b2 big2) (zs '())

(c 0) (ps '()) (bs '()))

(cond ((null? b1) bs)

((null? b2) (let ((zs (cons 0 zs)))

(loop (cdr b1) big2 zs 0 zs

(add (reverse (if (zero? c) ps (cons c ps))) bs))))

(else (let* ((x (+ (* (car b1) (car b2)) c))

(c (quotient x big-base))

(p (modulo x big-base)))

(loop b1 (cdr b2) zs c (cons p ps) bs))))))

(define (mul1 ns d)

(let loop ((ns ns) (c 0) (ps '()))

(if (null? ns) (reverse (if (zero? c) ps (cons c ps)))

(let* ((x (+ (* (car ns) d) c))

(c (quotient x big-base))

(p (modulo x big-base)))

(loop (cdr ns) c (cons p ps))))))

(define (div1 ns d)

(let loop ((rev-ns (reverse ns)) (qs '()) (r 0))

(if (null? rev-ns) (values (reverse (reduce (reverse qs))) (list r))

(let* ((x (+ (* r big-base) (car rev-ns)))

(q (quotient x d)) (r (modulo x d)))

(loop (cdr rev-ns) (cons q qs) r)))))

(define (nextq x d0 d1 ns)

(let loop ((q (quotient x d0)))

(if (< (* q d1) (+ (* (- x (* q d0)) big-base) (caddr ns))) q

(loop (- q 1)))))

(define (nextn j n rev-ns ds*q)

(let ((zs (append (reverse (sub (reverse (take (+ n 1) rev-ns)) ds*q))

(drop (+ n 1) rev-ns))))

(if (< (length zs) (+ n j)) (cons 0 zs) zs)))

(define (div ns ds)

(if (lt? ns ds) (values '(0) ns)

(let* ((n (length ds)) (m (- (length ns) n)))

(if (= n 1) (div1 ns (car ds))

(let* ((d (quotient big-base (+ (car (reverse ds)) 1)))

(rev-ns (reverse (mul1 ns d)))

(rev-ns (if (= (length ns) (length rev-ns))

(cons 0 rev-ns) rev-ns))

(ds (mul1 ds d))

(d0 (car (reverse ds)))

(d1 (cadr (reverse ds))))

(let loop ((j m) (rev-ns rev-ns) (qs '()))

(if (negative? j)

(call-with-values

(lambda () (div1 (reverse rev-ns) d))

(lambda (q r) (values (reverse (reduce (reverse qs))) q)))

(let* ((x (+ (* (car rev-ns) big-base) (cadr rev-ns)))

(q (nextq x d0 d1 rev-ns))

(ds*q (mul1 ds q)))

(if (lt? (reverse (take (+ n 1) rev-ns)) ds*q)

(let* ((q (- q 1)) (ds*q (sub ds*q ds)))

(loop (- j 1) (nextn j n rev-ns ds*q) (cons q qs)))

(loop (- j 1) (nextn j n rev-ns ds*q) (cons q qs)))))))))))

(if (big-zero? ds) (error 'big-divide "divide by zero")

(let ((sn (sign (car ns))) (sd (sign (car ds))))

(call-with-values

(lambda () (div (cdr (big-abs ns)) (cdr (big-abs ds))))

(lambda (qs rs)

(values (cons (* (if (= sn sd) 1 -1) (length qs)) qs)

(cons (* sn (length rs)) rs)))))))

[...] today’s Programming Praxis exercise, our goal is to add division to our Big Number library. Let’s get [...]

My Haskell solution (see http://bonsaicode.wordpress.com/2011/06/07/programming-praxis-big-numbers-division/ for a version with comments):