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 "delta.h"
  24
  25
  26 struct index {
  27         const unsigned char *ptr;
  28         struct index *next;
  29 };
  30
  31 static struct index ** delta_index(const unsigned char *buf,
  32                                    unsigned long bufsize,
  33                                    unsigned long trg_bufsize)
  34 {
  35         unsigned long hsize;
  36         unsigned int i, hshift, hlimit, *hash_count;
  37         const unsigned char *data;
  38         struct index *entry, **hash;
  39         void *mem;
  40
  41         /* determine index hash size */
  42         hsize = bufsize / 4;
  43         for (i = 8; (1 << i) < hsize && i < 24; i += 2);
  44         hsize = 1 << i;
  45         hshift = (i - 8) / 2;
  46
  47         /*
  48          * Allocate lookup index.  Note the first hash pointer
  49          * is used to store the hash shift value.
  50          */
  51         mem = malloc((1 + hsize) * sizeof(*hash) + bufsize * sizeof(*entry));
  52         if (!mem)
  53                 return NULL;
  54         hash = mem;
  55         *hash++ = (void *)hshift;
  56         entry = mem + (1 + hsize) * sizeof(*hash);
  57         memset(hash, 0, hsize * sizeof(*hash));
  58
  59         /* allocate an array to count hash entries */
  60         hash_count = calloc(hsize, sizeof(*hash_count));
  61         if (!hash_count) {
  62                 free(hash);
  63                 return NULL;
  64         }
  65
  66         /* then populate the index */
  67         data = buf + bufsize - 2;
  68         while (data > buf) {
  69                 entry->ptr = --data;
  70                 i = data[0] ^ ((data[1] ^ (data[2] << hshift)) << hshift);
  71                 entry->next = hash[i];
  72                 hash[i] = entry++;
  73                 hash_count[i]++;
  74         }
  75
  76         /*
  77          * Determine a limit on the number of entries in the same hash
  78          * bucket.  This guard us against patological data sets causing
  79          * really bad hash distribution with most entries in the same hash
  80          * bucket that would bring us to O(m*n) computing costs (m and n
  81          * corresponding to reference and target buffer sizes).
  82          *
  83          * The more the target buffer is large, the more it is important to
  84          * have small entry lists for each hash buckets.  With such a limit
  85          * the cost is bounded to something more like O(m+n).
  86          */
  87         hlimit = (1 << 26) / trg_bufsize;
  88         if (hlimit < 16)
  89                 hlimit = 16;
  90
  91         /*
  92          * Now make sure none of the hash buckets has more entries than
  93          * we're willing to test.  Otherwise we short-circuit the entry
  94          * list uniformly to still preserve a good repartition across
  95          * the reference buffer.
  96          */
  97         for (i = 0; i < hsize; i++) {
  98                 if (hash_count[i] < hlimit)
  99                         continue;
 100                 entry = hash[i];
 101                 do {
 102                         struct index *keep = entry;
 103                         int skip = hash_count[i] / hlimit / 2;
 104                         do {
 105                                 entry = entry->next;
 106                         } while(--skip && entry);
 107                         keep->next = entry;
 108                 } while(entry);
 109         }
 110         free(hash_count);
 111
 112         return hash-1;
 113 }
 114
 115 /* provide the size of the copy opcode given the block offset and size */
 116 #define COPYOP_SIZE(o, s) \
 117     (!!(o & 0xff) + !!(o & 0xff00) + !!(o & 0xff0000) + !!(o & 0xff000000) + \
 118      !!(s & 0xff) + !!(s & 0xff00) + 1)
 119
 120 /* the maximum size for any opcode */
 121 #define MAX_OP_SIZE COPYOP_SIZE(0xffffffff, 0xffffffff)
 122
 123 void *diff_delta(void *from_buf, unsigned long from_size,
 124                  void *to_buf, unsigned long to_size,
 125                  unsigned long *delta_size,
 126                  unsigned long max_size,
 127                  void **from_index)
 128 {
 129         unsigned int i, outpos, outsize, inscnt, hash_shift;
 130         const unsigned char *ref_data, *ref_top, *data, *top;
 131         unsigned char *out;
 132         struct index *entry, **hash;
 133
 134         if (!from_size || !to_size)
 135                 return NULL;
 136         if (from_index && *from_index) {
 137                 hash = *from_index;
 138         } else {
 139                 hash = delta_index(from_buf, from_size, to_size);
 140                 if (!hash)
 141                         return NULL;
 142                 if (from_index)
 143                         *from_index = hash;
 144         }
 145         hash_shift = (unsigned int)(*hash++);
 146
 147         outpos = 0;
 148         outsize = 8192;
 149         if (max_size && outsize >= max_size)
 150                 outsize = max_size + MAX_OP_SIZE + 1;
 151         out = malloc(outsize);
 152         if (!out) {
 153                 if (!from_index)
 154                         free(hash-1);
 155                 return NULL;
 156         }
 157
 158         ref_data = from_buf;
 159         ref_top = from_buf + from_size;
 160         data = to_buf;
 161         top = to_buf + to_size;
 162
 163         /* store reference buffer size */
 164         out[outpos++] = from_size;
 165         from_size >>= 7;
 166         while (from_size) {
 167                 out[outpos - 1] |= 0x80;
 168                 out[outpos++] = from_size;
 169                 from_size >>= 7;
 170         }
 171
 172         /* store target buffer size */
 173         out[outpos++] = to_size;
 174         to_size >>= 7;
 175         while (to_size) {
 176                 out[outpos - 1] |= 0x80;
 177                 out[outpos++] = to_size;
 178                 to_size >>= 7;
 179         }
 180
 181         inscnt = 0;
 182
 183         while (data < top) {
 184                 unsigned int moff = 0, msize = 0;
 185                 if (data + 3 <= top) {
 186                         i = data[0] ^ ((data[1] ^ (data[2] << hash_shift)) << hash_shift);
 187                         for (entry = hash[i]; entry; entry = entry->next) {
 188                                 const unsigned char *ref = entry->ptr;
 189                                 const unsigned char *src = data;
 190                                 unsigned int ref_size = ref_top - ref;
 191                                 if (ref_size > top - src)
 192                                         ref_size = top - src;
 193                                 if (ref_size > 0x10000)
 194                                         ref_size = 0x10000;
 195                                 if (ref_size <= msize)
 196                                         break;
 197                                 if (*ref != *src)
 198                                         continue;
 199                                 while (ref_size-- && *++src == *++ref);
 200                                 if (msize < ref - entry->ptr) {
 201                                         /* this is our best match so far */
 202                                         msize = ref - entry->ptr;
 203                                         moff = entry->ptr - ref_data;
 204                                 }
 205                         }
 206                 }
 207
 208                 if (!msize || msize < COPYOP_SIZE(moff, msize)) {
 209                         if (!inscnt)
 210                                 outpos++;
 211                         out[outpos++] = *data++;
 212                         inscnt++;
 213                         if (inscnt == 0x7f) {
 214                                 out[outpos - inscnt - 1] = inscnt;
 215                                 inscnt = 0;
 216                         }
 217                 } else {
 218                         unsigned char *op;
 219
 220                         if (inscnt) {
 221                                 out[outpos - inscnt - 1] = inscnt;
 222                                 inscnt = 0;
 223                         }
 224
 225                         data += msize;
 226                         op = out + outpos++;
 227                         i = 0x80;
 228
 229                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x01; }
 230                         moff >>= 8;
 231                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x02; }
 232                         moff >>= 8;
 233                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x04; }
 234                         moff >>= 8;
 235                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x08; }
 236
 237                         if (msize & 0xff) { out[outpos++] = msize; i |= 0x10; }
 238                         msize >>= 8;
 239                         if (msize & 0xff) { out[outpos++] = msize; i |= 0x20; }
 240
 241                         *op = i;
 242                 }
 243
 244                 if (outpos >= outsize - MAX_OP_SIZE) {
 245                         void *tmp = out;
 246                         outsize = outsize * 3 / 2;
 247                         if (max_size && outsize >= max_size)
 248                                 outsize = max_size + MAX_OP_SIZE + 1;
 249                         if (max_size && outpos > max_size)
 250                                 out = NULL;
 251                         else
 252                                 out = realloc(out, outsize);
 253                         if (!out) {
 254                                 free(tmp);
 255                                 if (!from_index)
 256                                         free(hash-1);
 257                                 return NULL;
 258                         }
 259                 }
 260         }
 261
 262         if (inscnt)
 263                 out[outpos - inscnt - 1] = inscnt;
 264
 265         if (!from_index)
 266                 free(hash-1);
 267         *delta_size = outpos;
 268         return out;
 269 }