module/language/objcode.scm

   1 ;;; Guile Virtual Machine Object Code
   2
   3 ;; Copyright (C) 2001 Free Software Foundation, Inc.
   4
   5 ;;;; This library is free software; you can redistribute it and/or
   6 ;;;; modify it under the terms of the GNU Lesser General Public
   7 ;;;; License as published by the Free Software Foundation; either
   8 ;;;; version 3 of the License, or (at your option) any later version.
   9 ;;;;
  10 ;;;; This library is distributed in the hope that it will be useful,
  11 ;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
  12 ;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13 ;;;; Lesser General Public License for more details.
  14 ;;;;
  15 ;;;; You should have received a copy of the GNU Lesser General Public
  16 ;;;; License along with this library; if not, write to the Free Software
  17 ;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  18
  19 ;;; Code:
  20
  21 (define-module (language objcode)
  22   #:export (encode-length decode-length))
  23
  24 \f
  25 ;;;
  26 ;;; Variable-length interface
  27 ;;;
  28
  29 ;; NOTE: decoded in vm_fetch_length in vm.c as well.
  30
  31 (define (encode-length len)
  32   (cond ((< len 254) (u8vector len))
  33         ((< len (* 256 256))
  34          (u8vector 254 (quotient len 256) (modulo len 256)))
  35         ((< len most-positive-fixnum)
  36          (u8vector 255
  37                    (quotient len (* 256 256 256))
  38                    (modulo (quotient len (* 256 256)) 256)
  39                    (modulo (quotient len 256) 256)
  40                    (modulo len 256)))
  41         (else (error "Too long code length:" len))))
  42
  43 (define (decode-length pop)
  44   (let ((x (pop)))
  45     (cond ((< x 254) x)
  46           ((= x 254) (+ (ash x 8) (pop)))
  47           (else
  48            (let* ((b2 (pop))
  49                   (b3 (pop))
  50                   (b4 (pop)))
  51              (+ (ash x 24) (ash b2 16) (ash b3 8) b4))))))