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 */
jffs2_rtime_compress(unsigned char * data_in,unsigned char * cpage_out,uint32_t * sourcelen,uint32_t * dstlen)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  
jffs2_rtime_decompress(unsigned char * data_in,unsigned char * cpage_out,uint32_t srclen,uint32_t destlen)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  
jffs2_rtime_init(void)125  int jffs2_rtime_init(void)
126  {
127      return jffs2_register_compressor(&jffs2_rtime_comp);
128  }
129  
jffs2_rtime_exit(void)130  void jffs2_rtime_exit(void)
131  {
132      jffs2_unregister_compressor(&jffs2_rtime_comp);
133  }
134