guile/module/language/objcode.scm

;;; Guile Virtual Machine Object Code

;; Copyright (C) 2001 Free Software Foundation, Inc.

;;;; This library is free software; you can redistribute it and/or
;;;; modify it under the terms of the GNU Lesser General Public
;;;; License as published by the Free Software Foundation; either
;;;; version 3 of the License, or (at your option) any later version.
;;;; 
;;;; This library is distributed in the hope that it will be useful,
;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
;;;; Lesser General Public License for more details.
;;;; 
;;;; You should have received a copy of the GNU Lesser General Public
;;;; License along with this library; if not, write to the Free Software
;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

;;; Code:

(define-module (language objcode)
  #:export (encode-length decode-length))


;;;
;;; Variable-length interface
;;;

;; NOTE: decoded in vm_fetch_length in vm.c as well.

(define (encode-length len)
  (cond ((< len 254) (u8vector len))
	((< len (* 256 256))
	 (u8vector 254 (quotient len 256) (modulo len 256)))
	((< len most-positive-fixnum)
	 (u8vector 255
		   (quotient len (* 256 256 256))
		   (modulo (quotient len (* 256 256)) 256)
		   (modulo (quotient len 256) 256)
		   (modulo len 256)))
	(else (error "Too long code length:" len))))

(define (decode-length pop)
  (let ((x (pop)))
    (cond ((< x 254) x)
	  ((= x 254) (+ (ash x 8) (pop)))
	  (else
           (let* ((b2 (pop))
                  (b3 (pop))
                  (b4 (pop)))
             (+ (ash x 24) (ash b2 16) (ash b3 8) b4))))))