#!/usr/bin/sbcl --noinform

; Copy in to out, ommitting any aligned 2048-byte chunks
; that consist entirely of zero-valued bytes.

(defun vcd-filter (in out &key (bufsize 2048))
  (let ((buf (make-sequence '(vector unsigned-byte) bufsize))
        (all-zero (make-sequence '(vector unsigned-byte) bufsize :initial-element 0)))
    (do ((successfully-read (read-sequence buf in) (read-sequence buf in)))
      ((< successfully-read bufsize)
       (write-sequence (subseq buf 0 successfully-read) out))
      (unless (equalp buf all-zero)
      	(write-sequence buf out)))))

(with-open-file (in (third *posix-argv*) :element-type 'unsigned-byte)
  (with-open-file (out (fourth *posix-argv*) :direction :output
  		                             :element-type 'unsigned-byte)
    (vcd-filter in out)))