fs/jffs2/compr_rtime.c

   1 /*
   2  * JFFS2 -- Journalling Flash File System, Version 2.
   3  *
   4  * Copyright (C) 2001-2003 Red Hat, Inc.
   5  *
   6  * Created by Arjan van de Ven <arjanv@redhat.com>
   7  *
   8  * For licensing information, see the file 'LICENCE' in this directory.
   9  *
  10  * $Id: compr_rtime.c,v 1.11 2003/10/04 08:33:06 dwmw2 Exp $
  11  *
  12  *
  13  * Very simple lz77-ish encoder.
  14  *
  15  * Theory of operation: Both encoder and decoder have a list of "last
  16  * occurrences" for every possible source-value; after sending the
  17  * first source-byte, the second byte indicated the "run" length of
  18  * matches
  19  *
  20  * The algorithm is intended to only send "whole bytes", no bit-messing.
  21  *
  22  */
  23
  24 #include <linux/kernel.h>
  25 #include <linux/types.h>
  26 #include <linux/errno.h>
  27 #include <linux/string.h>
  28
  29 /* _compress returns the compressed size, -1 if bigger */
  30 int jffs2_rtime_compress(unsigned char *data_in, unsigned char *cpage_out,
  31                    uint32_t *sourcelen, uint32_t *dstlen)
  32 {
  33         short positions[256];
  34         int outpos = 0;
  35         int pos=0;
  36
  37         memset(positions,0,sizeof(positions));
  38
  39         while (pos < (*sourcelen) && outpos <= (*dstlen)-2) {
  40                 int backpos, runlen=0;
  41                 unsigned char value;
  42
  43                 value = data_in[pos];
  44
  45                 cpage_out[outpos++] = data_in[pos++];
  46
  47                 backpos = positions[value];
  48                 positions[value]=pos;
  49
  50                 while ((backpos < pos) && (pos < (*sourcelen)) &&
  51                        (data_in[pos]==data_in[backpos++]) && (runlen<255)) {
  52                         pos++;
  53                         runlen++;
  54                 }
  55                 cpage_out[outpos++] = runlen;
  56         }
  57
  58         if (outpos >= pos) {
  59                 /* We failed */
  60                 return -1;
  61         }
  62
  63         /* Tell the caller how much we managed to compress, and how much space it took */
  64         *sourcelen = pos;
  65         *dstlen = outpos;
  66         return 0;
  67 }
  68
  69
  70 void jffs2_rtime_decompress(unsigned char *data_in, unsigned char *cpage_out,
  71                       uint32_t srclen, uint32_t destlen)
  72 {
  73         short positions[256];
  74         int outpos = 0;
  75         int pos=0;
  76
  77         memset(positions,0,sizeof(positions));
  78
  79         while (outpos<destlen) {
  80                 unsigned char value;
  81                 int backoffs;
  82                 int repeat;
  83
  84                 value = data_in[pos++];
  85                 cpage_out[outpos++] = value; /* first the verbatim copied byte */
  86                 repeat = data_in[pos++];
  87                 backoffs = positions[value];
  88
  89                 positions[value]=outpos;
  90                 if (repeat) {
  91                         if (backoffs + repeat >= outpos) {
  92                                 while(repeat) {
  93                                         cpage_out[outpos++] = cpage_out[backoffs++];
  94                                         repeat--;
  95                                 }
  96                         } else {
  97                                 memcpy(&cpage_out[outpos],&cpage_out[backoffs],repeat);
  98                                 outpos+=repeat;
  99                         }
 100                 }
 101         }
 102 }
 103
 104