FreeBSD/Linux Kernel Cross Reference
sys/fs/jffs2/compr_rtime.c

     /*
      * JFFS2 -- Journalling Flash File System, Version 2.
      *
      * Copyright (C) 2001 Red Hat, Inc.
      *
      * Created by Arjan van de Ven <arjanv@redhat.com>
      *
      * The original JFFS, from which the design for JFFS2 was derived,
      * was designed and implemented by Axis Communications AB.
     *
     * The contents of this file are subject to the Red Hat eCos Public
     * License Version 1.1 (the "Licence"); you may not use this file
     * except in compliance with the Licence.  You may obtain a copy of
     * the Licence at http://www.redhat.com/
     *
     * Software distributed under the Licence is distributed on an "AS IS"
     * basis, WITHOUT WARRANTY OF ANY KIND, either express or implied.
     * See the Licence for the specific language governing rights and
     * limitations under the Licence.
     *
     * The Original Code is JFFS2 - Journalling Flash File System, version 2
     *
     * Alternatively, the contents of this file may be used under the
     * terms of the GNU General Public License version 2 (the "GPL"), in
     * which case the provisions of the GPL are applicable instead of the
     * above.  If you wish to allow the use of your version of this file
     * only under the terms of the GPL and not to allow others to use your
     * version of this file under the RHEPL, indicate your decision by
     * deleting the provisions above and replace them with the notice and
     * other provisions required by the GPL.  If you do not delete the
     * provisions above, a recipient may use your version of this file
     * under either the RHEPL or the GPL.
     *
     * $Id: compr_rtime.c,v 1.5 2001/03/15 15:38:23 dwmw2 Exp $
     *
     *
     * Very simple lz77-ish encoder.
     *
     * Theory of operation: Both encoder and decoder have a list of "last
     * occurances" for every possible source-value; after sending the
     * first source-byte, the second byte indicated the "run" length of
     * matches
     *
     * The algorithm is intended to only send "whole bytes", no bit-messing.
     *
     */
    
    #include <linux/kernel.h>
    #include <linux/types.h>
    #include <linux/errno.h>
    #include <linux/string.h> 
    
    /* _compress returns the compressed size, -1 if bigger */
    int rtime_compress(unsigned char *data_in, unsigned char *cpage_out, 
                       __u32 *sourcelen, __u32 *dstlen)
    {
            int positions[256];
            int outpos = 0;
            int pos=0;
    
            memset(positions,0,sizeof(positions)); 
            
            while (pos < (*sourcelen) && outpos <= (*dstlen)-2) {
                    int backpos, runlen=0;
                    unsigned char value;
                    
                    value = data_in[pos];
    
                    cpage_out[outpos++] = data_in[pos++];
                    
                    backpos = positions[value];
                    positions[value]=pos;
                    
                    while ((backpos < pos) && (pos < (*sourcelen)) &&
                           (data_in[pos]==data_in[backpos++]) && (runlen<255)) {
                            pos++;
                            runlen++;
                    }
                    cpage_out[outpos++] = runlen;
            }
    
            if (outpos >= pos) {
                    /* We failed */
                    return -1;
            }
            
            /* Tell the caller how much we managed to compress, and how much space it took */
            *sourcelen = pos;
            *dstlen = outpos;
            return 0;
    }                  
    
    
    void rtime_decompress(unsigned char *data_in, unsigned char *cpage_out,
                          __u32 srclen, __u32 destlen)
    {
            int positions[256];
            int outpos = 0;
            int pos=0;
           
           memset(positions,0,sizeof(positions)); 
           
           while (outpos<destlen) {
                   unsigned char value;
                   int backoffs;
                   int repeat;
                   
                   value = data_in[pos++];
                   cpage_out[outpos++] = value; /* first the verbatim copied byte */
                   repeat = data_in[pos++];
                   backoffs = positions[value];
                   
                   positions[value]=outpos;
                   if (repeat) {
                           if (backoffs + repeat >= outpos) {
                                   while(repeat) {
                                           cpage_out[outpos++] = cpage_out[backoffs++];
                                           repeat--;
                                   }
                           } else {
                                   memcpy(&cpage_out[outpos],&cpage_out[backoffs],repeat);
                                   outpos+=repeat;         
                           }
                   }
           }               
   }                  
   
   

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