read-cache.c: use xcalloc() not calloc()
[git/git.git] / diff-delta.c
1 /*
2 * diff-delta.c: generate a delta between two buffers
3 *
4 * Many parts of this file have been lifted from LibXDiff version 0.10.
5 * http://www.xmailserver.org/xdiff-lib.html
6 *
7 * LibXDiff was written by Davide Libenzi <davidel@xmailserver.org>
8 * Copyright (C) 2003 Davide Libenzi
9 *
10 * Many mods for GIT usage by Nicolas Pitre <nico@cam.org>, (C) 2005.
11 *
12 * This file is free software; you can redistribute it and/or
13 * modify it under the terms of the GNU Lesser General Public
14 * License as published by the Free Software Foundation; either
15 * version 2.1 of the License, or (at your option) any later version.
16 *
17 * Use of this within git automatically means that the LGPL
18 * licensing gets turned into GPLv2 within this project.
19 */
20
21 #include <stdlib.h>
22 #include <string.h>
23 #include <zlib.h>
24 #include "delta.h"
25
26
27 /* block size: min = 16, max = 64k, power of 2 */
28 #define BLK_SIZE 16
29
30 #define MIN(a, b) ((a) < (b) ? (a) : (b))
31
32 #define GR_PRIME 0x9e370001
33 #define HASH(v, shift) (((unsigned int)(v) * GR_PRIME) >> (shift))
34
35 struct index {
36 const unsigned char *ptr;
37 unsigned int val;
38 struct index *next;
39 };
40
41 static struct index ** delta_index(const unsigned char *buf,
42 unsigned long bufsize,
43 unsigned long trg_bufsize,
44 unsigned int *hash_shift)
45 {
46 unsigned int i, hsize, hshift, hlimit, entries, *hash_count;
47 const unsigned char *data;
48 struct index *entry, **hash;
49 void *mem;
50
51 /* determine index hash size */
52 entries = bufsize / BLK_SIZE;
53 hsize = entries / 4;
54 for (i = 4; (1 << i) < hsize && i < 31; i++);
55 hsize = 1 << i;
56 hshift = 32 - i;
57 *hash_shift = hshift;
58
59 /* allocate lookup index */
60 mem = malloc(hsize * sizeof(*hash) + entries * sizeof(*entry));
61 if (!mem)
62 return NULL;
63 hash = mem;
64 entry = mem + hsize * sizeof(*hash);
65 memset(hash, 0, hsize * sizeof(*hash));
66
67 /* allocate an array to count hash entries */
68 hash_count = calloc(hsize, sizeof(*hash_count));
69 if (!hash_count) {
70 free(hash);
71 return NULL;
72 }
73
74 /* then populate the index */
75 data = buf + entries * BLK_SIZE - BLK_SIZE;
76 while (data >= buf) {
77 unsigned int val = adler32(0, data, BLK_SIZE);
78 i = HASH(val, hshift);
79 entry->ptr = data;
80 entry->val = val;
81 entry->next = hash[i];
82 hash[i] = entry++;
83 hash_count[i]++;
84 data -= BLK_SIZE;
85 }
86
87 /*
88 * Determine a limit on the number of entries in the same hash
89 * bucket. This guard us against patological data sets causing
90 * really bad hash distribution with most entries in the same hash
91 * bucket that would bring us to O(m*n) computing costs (m and n
92 * corresponding to reference and target buffer sizes).
93 *
94 * The more the target buffer is large, the more it is important to
95 * have small entry lists for each hash buckets. With such a limit
96 * the cost is bounded to something more like O(m+n).
97 */
98 hlimit = (1 << 26) / trg_bufsize;
99 if (hlimit < 4*BLK_SIZE)
100 hlimit = 4*BLK_SIZE;
101
102 /*
103 * Now make sure none of the hash buckets has more entries than
104 * we're willing to test. Otherwise we cull the entry list
105 * uniformly to still preserve a good repartition across
106 * the reference buffer.
107 */
108 for (i = 0; i < hsize; i++) {
109 if (hash_count[i] < hlimit)
110 continue;
111 entry = hash[i];
112 do {
113 struct index *keep = entry;
114 int skip = hash_count[i] / hlimit / 2;
115 do {
116 entry = entry->next;
117 } while(--skip && entry);
118 keep->next = entry;
119 } while(entry);
120 }
121 free(hash_count);
122
123 return hash;
124 }
125
126 /* provide the size of the copy opcode given the block offset and size */
127 #define COPYOP_SIZE(o, s) \
128 (!!(o & 0xff) + !!(o & 0xff00) + !!(o & 0xff0000) + !!(o & 0xff000000) + \
129 !!(s & 0xff) + !!(s & 0xff00) + 1)
130
131 /* the maximum size for any opcode */
132 #define MAX_OP_SIZE COPYOP_SIZE(0xffffffff, 0xffffffff)
133
134 void *diff_delta(void *from_buf, unsigned long from_size,
135 void *to_buf, unsigned long to_size,
136 unsigned long *delta_size,
137 unsigned long max_size)
138 {
139 unsigned int i, outpos, outsize, hash_shift;
140 int inscnt;
141 const unsigned char *ref_data, *ref_top, *data, *top;
142 unsigned char *out;
143 struct index *entry, **hash;
144
145 if (!from_size || !to_size)
146 return NULL;
147 hash = delta_index(from_buf, from_size, to_size, &hash_shift);
148 if (!hash)
149 return NULL;
150
151 outpos = 0;
152 outsize = 8192;
153 if (max_size && outsize >= max_size)
154 outsize = max_size + MAX_OP_SIZE + 1;
155 out = malloc(outsize);
156 if (!out) {
157 free(hash);
158 return NULL;
159 }
160
161 ref_data = from_buf;
162 ref_top = from_buf + from_size;
163 data = to_buf;
164 top = to_buf + to_size;
165
166 /* store reference buffer size */
167 out[outpos++] = from_size;
168 from_size >>= 7;
169 while (from_size) {
170 out[outpos - 1] |= 0x80;
171 out[outpos++] = from_size;
172 from_size >>= 7;
173 }
174
175 /* store target buffer size */
176 out[outpos++] = to_size;
177 to_size >>= 7;
178 while (to_size) {
179 out[outpos - 1] |= 0x80;
180 out[outpos++] = to_size;
181 to_size >>= 7;
182 }
183
184 inscnt = 0;
185
186 while (data < top) {
187 unsigned int moff = 0, msize = 0;
188 if (data + BLK_SIZE <= top) {
189 unsigned int val = adler32(0, data, BLK_SIZE);
190 i = HASH(val, hash_shift);
191 for (entry = hash[i]; entry; entry = entry->next) {
192 const unsigned char *ref = entry->ptr;
193 const unsigned char *src = data;
194 unsigned int ref_size = ref_top - ref;
195 if (entry->val != val)
196 continue;
197 if (ref_size > top - src)
198 ref_size = top - src;
199 if (ref_size > 0x10000)
200 ref_size = 0x10000;
201 if (ref_size <= msize)
202 break;
203 while (ref_size-- && *src++ == *ref)
204 ref++;
205 if (msize < ref - entry->ptr) {
206 /* this is our best match so far */
207 msize = ref - entry->ptr;
208 moff = entry->ptr - ref_data;
209 }
210 }
211 }
212
213 if (!msize || msize < COPYOP_SIZE(moff, msize)) {
214 if (!inscnt)
215 outpos++;
216 out[outpos++] = *data++;
217 inscnt++;
218 if (inscnt == 0x7f) {
219 out[outpos - inscnt - 1] = inscnt;
220 inscnt = 0;
221 }
222 } else {
223 unsigned char *op;
224
225 if (inscnt) {
226 while (moff && ref_data[moff-1] == data[-1]) {
227 if (msize == 0x10000)
228 break;
229 /* we can match one byte back */
230 msize++;
231 moff--;
232 data--;
233 outpos--;
234 if (--inscnt)
235 continue;
236 outpos--; /* remove count slot */
237 inscnt--; /* make it -1 */
238 break;
239 }
240 out[outpos - inscnt - 1] = inscnt;
241 inscnt = 0;
242 }
243
244 data += msize;
245 op = out + outpos++;
246 i = 0x80;
247
248 if (moff & 0xff) { out[outpos++] = moff; i |= 0x01; }
249 moff >>= 8;
250 if (moff & 0xff) { out[outpos++] = moff; i |= 0x02; }
251 moff >>= 8;
252 if (moff & 0xff) { out[outpos++] = moff; i |= 0x04; }
253 moff >>= 8;
254 if (moff & 0xff) { out[outpos++] = moff; i |= 0x08; }
255
256 if (msize & 0xff) { out[outpos++] = msize; i |= 0x10; }
257 msize >>= 8;
258 if (msize & 0xff) { out[outpos++] = msize; i |= 0x20; }
259
260 *op = i;
261 }
262
263 if (outpos >= outsize - MAX_OP_SIZE) {
264 void *tmp = out;
265 outsize = outsize * 3 / 2;
266 if (max_size && outsize >= max_size)
267 outsize = max_size + MAX_OP_SIZE + 1;
268 if (max_size && outpos > max_size)
269 out = NULL;
270 else
271 out = realloc(out, outsize);
272 if (!out) {
273 free(tmp);
274 free(hash);
275 return NULL;
276 }
277 }
278 }
279
280 if (inscnt)
281 out[outpos - inscnt - 1] = inscnt;
282
283 free(hash);
284 *delta_size = outpos;
285 return out;
286 }