; grade school multiplication
(define (filter pred? xs)
(let loop ((xs xs) (ys '()))
(cond ((null? xs) (reverse ys))
((pred? (car xs))
(loop (cdr xs) (cons (car xs) ys)))
(else (loop (cdr xs) ys)))))
(define (digits n . args)
(let ((b (if (null? args) 10 (car args))))
(let loop ((n n) (d '()))
(if (zero? n) d
(loop (quotient n b)
(cons (modulo n b) d))))))
(define (repeat n c)
(make-string n c))
(define (display-right n len)
(let ((s (number->string n)))
(display (repeat (- len (string-length s)) #\space))
(display s) (newline)))
(define (ilog10 n)
(inexact->exact (floor (/ (log n) (log 10)))))
(define (mult x y)
(let* ((ys (digits y))
(len (length (digits (* x y))))
(lines (length (filter positive? ys))))
(display-right x len)
(display-right y len)
(display (repeat len #\-)) (newline)
(when (< 1 lines)
(let loop ((ys (reverse ys)) (i 0) (z 1))
(when (pair? ys)
(if (zero? (car ys))
(loop (cdr ys) i (* z 10))
(begin (display-right (* x (car ys) z) (- len i))
(loop (cdr ys) (+ i (ilog10 z) 1) 1)))))
(display (repeat len #\-)) (newline))
(display-right (* x y) len)))
(define (problem-a filename)
(with-input-from-file filename
(lambda ()
(let loop ((x (read)) (y (read)) (i 1))
(when (and (positive? x) (positive? y))
(display "Problem ") (display i) (newline)
(mult x y)
(loop (read) (read) (+ i 1)))))))
(mult 432 5678)
(newline)
(mult 200001 90040)
(newline)
(mult 246 70)