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 {
  36         unsigned short positions[256];
  37         int outpos = 0;
  38         int pos=0;
  39
  40         if (*dstlen <= 3)
  41                 return -1;
  42
  43         memset(positions,0,sizeof(positions));
  44
  45         while (pos < (*sourcelen) && outpos <= (*dstlen)-2) {
  46                 int backpos, runlen=0;
  47                 unsigned char value;
  48
  49                 value = data_in[pos];
  50
  51                 cpage_out[outpos++] = data_in[pos++];
  52
  53                 backpos = positions[value];
  54                 positions[value]=pos;
  55
  56                 while ((backpos < pos) && (pos < (*sourcelen)) &&
  57                        (data_in[pos]==data_in[backpos++]) && (runlen<255)) {
  58                         pos++;
  59                         runlen++;
  60                 }
  61                 cpage_out[outpos++] = runlen;
  62         }
  63
  64         if (outpos >= pos) {
  65                 /* We failed */
  66                 return -1;
  67         }
  68
  69         /* Tell the caller how much we managed to compress, and how much space it took */
  70         *sourcelen = pos;
  71         *dstlen = outpos;
  72         return 0;
  73 }
  74
  75
  76 static int jffs2_rtime_decompress(unsigned char *data_in,
  77                                   unsigned char *cpage_out,
  78                                   uint32_t srclen, uint32_t destlen)
  79 {
  80         unsigned short positions[256];
  81         int outpos = 0;
  82         int pos=0;
  83
  84         memset(positions,0,sizeof(positions));
  85
  86         while (outpos<destlen) {
  87                 unsigned char value;
  88                 int backoffs;
  89                 int repeat;
  90
  91                 value = data_in[pos++];
  92                 cpage_out[outpos++] = value; /* first the verbatim copied byte */
  93                 repeat = data_in[pos++];
  94                 backoffs = positions[value];
  95
  96                 positions[value]=outpos;
  97                 if (repeat) {
  98                         if (backoffs + repeat >= outpos) {
  99                                 while(repeat) {
 100                                         cpage_out[outpos++] = cpage_out[backoffs++];
 101                                         repeat--;
 102                                 }
 103                         } else {
 104                                 memcpy(&cpage_out[outpos],&cpage_out[backoffs],repeat);
 105                                 outpos+=repeat;
 106                         }
 107                 }
 108         }
 109         return 0;
 110 }
 111
 112 static struct jffs2_compressor jffs2_rtime_comp = {
 113     .priority = JFFS2_RTIME_PRIORITY,
 114     .name = "rtime",
 115     .compr = JFFS2_COMPR_RTIME,
 116     .compress = &jffs2_rtime_compress,
 117     .decompress = &jffs2_rtime_decompress,
 118 #ifdef JFFS2_RTIME_DISABLED
 119     .disabled = 1,
 120 #else
 121     .disabled = 0,
 122 #endif
 123 };
 124
 125 int jffs2_rtime_init(void)
 126 {
 127     return jffs2_register_compressor(&jffs2_rtime_comp);
 128 }
 129
 130 void jffs2_rtime_exit(void)
 131 {
 132     jffs2_unregister_compressor(&jffs2_rtime_comp);
 133 }