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