LXR linux/fs/jffs2/compr

   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 */
  31static 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
  73static 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
 110static 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
 123int jffs2_rtime_init(void)
 124{
 125    return jffs2_register_compressor(&jffs2_rtime_comp);
 126}
 127
 128void jffs2_rtime_exit(void)
 129{
 130    jffs2_unregister_compressor(&jffs2_rtime_comp);
 131}
 132