fs/jffs2/compr_rtime.c

   1 /*
   2  * JFFS2 -- Journalling Flash File System, Version 2.
   3  *
   4  * Copyright © 2001-2007 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  *
  11  *
  12  * Very simple lz77-ish encoder.
  13  *
  14  * Theory of operation: Both encoder and decoder have a list of "last
  15  * occurrences" for every possible source-value; after sending the
  16  * first source-byte, the second byte indicated the "run" length of
  17  * matches
  18  *
  19  * The algorithm is intended to only send "whole bytes", no bit-messing.
  20  *
  21  */
  22
  23 #include <linux/kernel.h>
  24 #include <linux/types.h>
  25 #include <linux/errno.h>
  26 #include <linux/string.h>
  27 #include <linux/jffs2.h>
  28 #include "compr.h"
  29
  30 /* _compress returns the compressed size, -1 if bigger */
  31 static int jffs2_rtime_compress(unsigned char *data_in,
  32                                 unsigned char *cpage_out,
  33                                 uint32_t *sourcelen, uint32_t *dstlen,
  34                                 void *model)
  35 {
  36         short positions[256];
  37         int outpos = 0;
  38         int pos=0;
  39
  40         memset(positions,0,sizeof(positions));
  41
  42         while (pos < (*sourcelen) && outpos <= (*dstlen)-2) {
  43                 int backpos, runlen=0;
  44                 unsigned char value;
  45
  46                 value = data_in[pos];
  47
  48                 cpage_out[outpos++] = data_in[pos++];
  49
  50                 backpos = positions[value];
  51                 positions[value]=pos;
  52
  53                 while ((backpos < pos) && (pos < (*sourcelen)) &&
  54                        (data_in[pos]==data_in[backpos++]) && (runlen<255)) {
  55                         pos++;
  56                         runlen++;
  57                 }
  58                 cpage_out[outpos++] = runlen;
  59         }
  60
  61         if (outpos >= pos) {
  62                 /* We failed */
  63                 return -1;
  64         }
  65
  66         /* Tell the caller how much we managed to compress, and how much space it took */
  67         *sourcelen = pos;
  68         *dstlen = outpos;
  69         return 0;
  70 }
  71
  72
  73 static int jffs2_rtime_decompress(unsigned char *data_in,
  74                                   unsigned char *cpage_out,
  75                                   uint32_t srclen, uint32_t destlen,
  76                                   void *model)
  77 {
  78         short positions[256];
  79         int outpos = 0;
  80         int pos=0;
  81
  82         memset(positions,0,sizeof(positions));
  83
  84         while (outpos<destlen) {
  85                 unsigned char value;
  86                 int backoffs;
  87                 int repeat;
  88
  89                 value = data_in[pos++];
  90                 cpage_out[outpos++] = value; /* first the verbatim copied byte */
  91                 repeat = data_in[pos++];
  92                 backoffs = positions[value];
  93
  94                 positions[value]=outpos;
  95                 if (repeat) {
  96                         if (backoffs + repeat >= outpos) {
  97                                 while(repeat) {
  98                                         cpage_out[outpos++] = cpage_out[backoffs++];
  99                                         repeat--;
 100                                 }
 101                         } else {
 102                                 memcpy(&cpage_out[outpos],&cpage_out[backoffs],repeat);
 103                                 outpos+=repeat;
 104                         }
 105                 }
 106         }
 107         return 0;
 108 }
 109
 110 static struct jffs2_compressor jffs2_rtime_comp = {
 111     .priority = JFFS2_RTIME_PRIORITY,
 112     .name = "rtime",
 113     .compr = JFFS2_COMPR_RTIME,
 114     .compress = &jffs2_rtime_compress,
 115     .decompress = &jffs2_rtime_decompress,
 116 #ifdef JFFS2_RTIME_DISABLED
 117     .disabled = 1,
 118 #else
 119     .disabled = 0,
 120 #endif
 121 };
 122
 123 int jffs2_rtime_init(void)
 124 {
 125     return jffs2_register_compressor(&jffs2_rtime_comp);
 126 }
 127
 128 void jffs2_rtime_exit(void)
 129 {
 130     jffs2_unregister_compressor(&jffs2_rtime_comp);
 131 }