LXR linux/fs/jffs2/compr

   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 */
  32static int jffs2_rtime_compress(unsigned char *data_in,
  33                                unsigned char *cpage_out,
  34                                uint32_t *sourcelen, uint32_t *dstlen)
  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
  73static int jffs2_rtime_decompress(unsigned char *data_in,
  74                                  unsigned char *cpage_out,
  75                                  uint32_t srclen, uint32_t destlen)
  76{
  77        short positions[256];
  78        int outpos = 0;
  79        int pos=0;
  80
  81        memset(positions,0,sizeof(positions));
  82
  83        while (outpos<destlen) {
  84                unsigned char value;
  85                int backoffs;
  86                int repeat;
  87
  88                value = data_in[pos++];
  89                cpage_out[outpos++] = value; /* first the verbatim copied byte */
  90                repeat = data_in[pos++];
  91                backoffs = positions[value];
  92
  93                positions[value]=outpos;
  94                if (repeat) {
  95                        if (backoffs + repeat >= outpos) {
  96                                while(repeat) {
  97                                        cpage_out[outpos++] = cpage_out[backoffs++];
  98                                        repeat--;
  99                                }
 100                        } else {
 101                                memcpy(&cpage_out[outpos],&cpage_out[backoffs],repeat);
 102                                outpos+=repeat;
 103                        }
 104                }
 105        }
 106        return 0;
 107}
 108
 109static struct jffs2_compressor jffs2_rtime_comp = {
 110    .priority = JFFS2_RTIME_PRIORITY,
 111    .name = "rtime",
 112    .compr = JFFS2_COMPR_RTIME,
 113    .compress = &jffs2_rtime_compress,
 114    .decompress = &jffs2_rtime_decompress,
 115#ifdef JFFS2_RTIME_DISABLED
 116    .disabled = 1,
 117#else
 118    .disabled = 0,
 119#endif
 120};
 121
 122int jffs2_rtime_init(void)
 123{
 124    return jffs2_register_compressor(&jffs2_rtime_comp);
 125}
 126
 127void jffs2_rtime_exit(void)
 128{
 129    jffs2_unregister_compressor(&jffs2_rtime_comp);
 130}
 131