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(const void *from_buf, unsigned long from_size,
 135                  const 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 }