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 /* maximum hash entry list for the same hash bucket */
  31 #define HASH_LIMIT 64
  32
  33 #define GR_PRIME 0x9e370001
  34 #define HASH(v, shift) (((unsigned int)(v) * GR_PRIME) >> (shift))
  35
  36 struct index_entry {
  37         const unsigned char *ptr;
  38         unsigned int val;
  39         struct index_entry *next;
  40 };
  41
  42 struct delta_index {
  43         const void *src_buf;
  44         unsigned long src_size;
  45         unsigned int hash_shift;
  46         struct index_entry *hash[0];
  47 };
  48
  49 struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
  50 {
  51         unsigned int i, hsize, hshift, entries, *hash_count;
  52         const unsigned char *data, *buffer = buf;
  53         struct delta_index *index;
  54         struct index_entry *entry, **hash;
  55         void *mem;
  56
  57         if (!buf || !bufsize)
  58                 return NULL;
  59
  60         /* determine index hash size */
  61         entries = bufsize  / BLK_SIZE;
  62         hsize = entries / 4;
  63         for (i = 4; (1 << i) < hsize && i < 31; i++);
  64         hsize = 1 << i;
  65         hshift = 32 - i;
  66
  67         /* allocate lookup index */
  68         mem = malloc(sizeof(*index) +
  69                      sizeof(*hash) * hsize +
  70                      sizeof(*entry) * entries);
  71         if (!mem)
  72                 return NULL;
  73         index = mem;
  74         mem = index + 1;
  75         hash = mem;
  76         mem = hash + hsize;
  77         entry = mem;
  78
  79         index->src_buf = buf;
  80         index->src_size = bufsize;
  81         index->hash_shift = hshift;
  82         memset(hash, 0, hsize * sizeof(*hash));
  83
  84         /* allocate an array to count hash entries */
  85         hash_count = calloc(hsize, sizeof(*hash_count));
  86         if (!hash_count) {
  87                 free(index);
  88                 return NULL;
  89         }
  90
  91         /* then populate the index */
  92         data = buffer + entries * BLK_SIZE - BLK_SIZE;
  93         while (data >= buffer) {
  94                 unsigned int val = adler32(0, data, BLK_SIZE);
  95                 i = HASH(val, hshift);
  96                 entry->ptr = data;
  97                 entry->val = val;
  98                 entry->next = hash[i];
  99                 hash[i] = entry++;
 100                 hash_count[i]++;
 101                 data -= BLK_SIZE;
 102         }
 103
 104         /*
 105          * Determine a limit on the number of entries in the same hash
 106          * bucket.  This guard us against patological data sets causing
 107          * really bad hash distribution with most entries in the same hash
 108          * bucket that would bring us to O(m*n) computing costs (m and n
 109          * corresponding to reference and target buffer sizes).
 110          *
 111          * Make sure none of the hash buckets has more entries than
 112          * we're willing to test.  Otherwise we cull the entry list
 113          * uniformly to still preserve a good repartition across
 114          * the reference buffer.
 115          */
 116         for (i = 0; i < hsize; i++) {
 117                 if (hash_count[i] < HASH_LIMIT)
 118                         continue;
 119                 entry = hash[i];
 120                 do {
 121                         struct index_entry *keep = entry;
 122                         int skip = hash_count[i] / HASH_LIMIT / 2;
 123                         do {
 124                                 entry = entry->next;
 125                         } while(--skip && entry);
 126                         keep->next = entry;
 127                 } while(entry);
 128         }
 129         free(hash_count);
 130
 131         return index;
 132 }
 133
 134 void free_delta_index(struct delta_index *index)
 135 {
 136         free(index);
 137 }
 138
 139 /* provide the size of the copy opcode given the block offset and size */
 140 #define COPYOP_SIZE(o, s) \
 141     (!!(o & 0xff) + !!(o & 0xff00) + !!(o & 0xff0000) + !!(o & 0xff000000) + \
 142      !!(s & 0xff) + !!(s & 0xff00) + 1)
 143
 144 /* the maximum size for any opcode */
 145 #define MAX_OP_SIZE COPYOP_SIZE(0xffffffff, 0xffffffff)
 146
 147 void *
 148 create_delta(const struct delta_index *index,
 149              const void *trg_buf, unsigned long trg_size,
 150              unsigned long *delta_size, unsigned long max_size)
 151 {
 152         unsigned int i, outpos, outsize, hash_shift;
 153         int inscnt;
 154         const unsigned char *ref_data, *ref_top, *data, *top;
 155         unsigned char *out;
 156
 157         if (!trg_buf || !trg_size)
 158                 return NULL;
 159
 160         outpos = 0;
 161         outsize = 8192;
 162         if (max_size && outsize >= max_size)
 163                 outsize = max_size + MAX_OP_SIZE + 1;
 164         out = malloc(outsize);
 165         if (!out)
 166                 return NULL;
 167
 168         /* store reference buffer size */
 169         i = index->src_size;
 170         while (i >= 0x80) {
 171                 out[outpos++] = i | 0x80;
 172                 i >>= 7;
 173         }
 174         out[outpos++] = i;
 175
 176         /* store target buffer size */
 177         i = trg_size;
 178         while (i >= 0x80) {
 179                 out[outpos++] = i | 0x80;
 180                 i >>= 7;
 181         }
 182         out[outpos++] = i;
 183
 184         ref_data = index->src_buf;
 185         ref_top = ref_data + index->src_size;
 186         data = trg_buf;
 187         top = trg_buf + trg_size;
 188         hash_shift = index->hash_shift;
 189         inscnt = 0;
 190
 191         while (data < top) {
 192                 unsigned int moff = 0, msize = 0;
 193                 struct index_entry *entry;
 194                 unsigned int val = adler32(0, data, BLK_SIZE);
 195                 i = HASH(val, hash_shift);
 196                 for (entry = index->hash[i]; entry; entry = entry->next) {
 197                         const unsigned char *ref = entry->ptr;
 198                         const unsigned char *src = data;
 199                         unsigned int ref_size = ref_top - ref;
 200                         if (entry->val != val)
 201                                 continue;
 202                         if (ref_size > top - src)
 203                                 ref_size = top - src;
 204                         if (ref_size > 0x10000)
 205                                 ref_size = 0x10000;
 206                         if (ref_size <= msize)
 207                                 break;
 208                         while (ref_size-- && *src++ == *ref)
 209                                 ref++;
 210                         if (msize < ref - entry->ptr) {
 211                                 /* this is our best match so far */
 212                                 msize = ref - entry->ptr;
 213                                 moff = entry->ptr - ref_data;
 214                         }
 215                 }
 216
 217                 if (!msize || msize < COPYOP_SIZE(moff, msize)) {
 218                         if (!inscnt)
 219                                 outpos++;
 220                         out[outpos++] = *data++;
 221                         inscnt++;
 222                         if (inscnt == 0x7f) {
 223                                 out[outpos - inscnt - 1] = inscnt;
 224                                 inscnt = 0;
 225                         }
 226                 } else {
 227                         unsigned char *op;
 228
 229                         if (inscnt) {
 230                                 while (moff && ref_data[moff-1] == data[-1]) {
 231                                         if (msize == 0x10000)
 232                                                 break;
 233                                         /* we can match one byte back */
 234                                         msize++;
 235                                         moff--;
 236                                         data--;
 237                                         outpos--;
 238                                         if (--inscnt)
 239                                                 continue;
 240                                         outpos--;  /* remove count slot */
 241                                         inscnt--;  /* make it -1 */
 242                                         break;
 243                                 }
 244                                 out[outpos - inscnt - 1] = inscnt;
 245                                 inscnt = 0;
 246                         }
 247
 248                         data += msize;
 249                         op = out + outpos++;
 250                         i = 0x80;
 251
 252                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x01; }
 253                         moff >>= 8;
 254                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x02; }
 255                         moff >>= 8;
 256                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x04; }
 257                         moff >>= 8;
 258                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x08; }
 259
 260                         if (msize & 0xff) { out[outpos++] = msize; i |= 0x10; }
 261                         msize >>= 8;
 262                         if (msize & 0xff) { out[outpos++] = msize; i |= 0x20; }
 263
 264                         *op = i;
 265                 }
 266
 267                 if (outpos >= outsize - MAX_OP_SIZE) {
 268                         void *tmp = out;
 269                         outsize = outsize * 3 / 2;
 270                         if (max_size && outsize >= max_size)
 271                                 outsize = max_size + MAX_OP_SIZE + 1;
 272                         if (max_size && outpos > max_size)
 273                                 out = NULL;
 274                         else
 275                                 out = realloc(out, outsize);
 276                         if (!out) {
 277                                 free(tmp);
 278                                 return NULL;
 279                         }
 280                 }
 281         }
 282
 283         if (inscnt)
 284                 out[outpos - inscnt - 1] = inscnt;
 285
 286         *delta_size = outpos;
 287         return out;
 288 }