xref: /linux/fs/jffs2/compr_rtime.c (revision 2da68a77) 
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 }
134