fs/jffs2/compr_rtime.c

   1 /*
   2  * JFFS2 -- Journalling Flash File System, Version 2.
   3  *
   4  * Copyright © 2001-2007 Red Hat, Inc.
   5  * Copyright © 2004-2010 David Woodhouse <dwmw2@infradead.org>
   6  *
   7  * Created by Arjan van de Ven <arjanv@redhat.com>
   8  *
   9  * For licensing information, see the file 'LICENCE' in this directory.
  10  *
  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 #include <linux/jffs2.h>
  29 #include "compr.h"
  30
  31 /* _compress returns the compressed size, -1 if bigger */
  32 static int jffs2_rtime_compress(unsigned char *data_in,
  33                                 unsigned char *cpage_out,
  34                                 uint32_t *sourcelen, uint32_t *dstlen,
  35                                 void *model)
  36 {
  37         short positions[256];
  38         int outpos = 0;
  39         int pos=0;
  40
  41         memset(positions,0,sizeof(positions));
  42
  43         while (pos < (*sourcelen) && outpos <= (*dstlen)-2) {
  44                 int backpos, runlen=0;
  45                 unsigned char value;
  46
  47                 value = data_in[pos];
  48
  49                 cpage_out[outpos++] = data_in[pos++];
  50
  51                 backpos = positions[value];
  52                 positions[value]=pos;
  53
  54                 while ((backpos < pos) && (pos < (*sourcelen)) &&
  55                        (data_in[pos]==data_in[backpos++]) && (runlen<255)) {
  56                         pos++;
  57                         runlen++;
  58                 }
  59                 cpage_out[outpos++] = runlen;
  60         }
  61
  62         if (outpos >= pos) {
  63                 /* We failed */
  64                 return -1;
  65         }
  66
  67         /* Tell the caller how much we managed to compress, and how much space it took */
  68         *sourcelen = pos;
  69         *dstlen = outpos;
  70         return 0;
  71 }
  72
  73
  74 static int jffs2_rtime_decompress(unsigned char *data_in,
  75                                   unsigned char *cpage_out,
  76                                   uint32_t srclen, uint32_t destlen,
  77                                   void *model)
  78 {
  79         short positions[256];
  80         int outpos = 0;
  81         int pos=0;
  82
  83         memset(positions,0,sizeof(positions));
  84
  85         while (outpos<destlen) {
  86                 unsigned char value;
  87                 int backoffs;
  88                 int repeat;
  89
  90                 value = data_in[pos++];
  91                 cpage_out[outpos++] = value; /* first the verbatim copied byte */
  92                 repeat = data_in[pos++];
  93                 backoffs = positions[value];
  94
  95                 positions[value]=outpos;
  96                 if (repeat) {
  97                         if (backoffs + repeat >= outpos) {
  98                                 while(repeat) {
  99                                         cpage_out[outpos++] = cpage_out[backoffs++];
 100                                         repeat--;
 101                                 }
 102                         } else {
 103                                 memcpy(&cpage_out[outpos],&cpage_out[backoffs],repeat);
 104                                 outpos+=repeat;
 105                         }
 106                 }
 107         }
 108         return 0;
 109 }
 110
 111 static struct jffs2_compressor jffs2_rtime_comp = {
 112     .priority = JFFS2_RTIME_PRIORITY,
 113     .name = "rtime",
 114     .compr = JFFS2_COMPR_RTIME,
 115     .compress = &jffs2_rtime_compress,
 116     .decompress = &jffs2_rtime_decompress,
 117 #ifdef JFFS2_RTIME_DISABLED
 118     .disabled = 1,
 119 #else
 120     .disabled = 0,
 121 #endif
 122 };
 123
 124 int jffs2_rtime_init(void)
 125 {
 126     return jffs2_register_compressor(&jffs2_rtime_comp);
 127 }
 128
 129 void jffs2_rtime_exit(void)
 130 {
 131     jffs2_unregister_compressor(&jffs2_rtime_comp);
 132 }