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 int *hash_shift)
  44 {
  45         unsigned int hsize, hshift, entries, blksize, i;
  46         const unsigned char *data;
  47         struct index *entry, **hash;
  48         void *mem;
  49
  50         /* determine index hash size */
  51         entries = (bufsize + BLK_SIZE - 1) / BLK_SIZE;
  52         hsize = entries / 4;
  53         for (i = 4; (1 << i) < hsize && i < 16; i++);
  54         hsize = 1 << i;
  55         hshift = 32 - i;
  56         *hash_shift = hshift;
  57
  58         /* allocate lookup index */
  59         mem = malloc(hsize * sizeof(*hash) + entries * sizeof(*entry));
  60         if (!mem)
  61                 return NULL;
  62         hash = mem;
  63         entry = mem + hsize * sizeof(*hash);
  64         memset(hash, 0, hsize * sizeof(*hash));
  65
  66         /* then populate it */
  67         data = buf + entries * BLK_SIZE - BLK_SIZE;
  68         blksize = bufsize - (data - buf);
  69         while (data >= buf) {
  70                 unsigned int val = adler32(0, data, blksize);
  71                 i = HASH(val, hshift);
  72                 entry->ptr = data;
  73                 entry->val = val;
  74                 entry->next = hash[i];
  75                 hash[i] = entry++;
  76                 blksize = BLK_SIZE;
  77                 data -= BLK_SIZE;
  78         }
  79
  80         return hash;
  81 }
  82
  83 /* provide the size of the copy opcode given the block offset and size */
  84 #define COPYOP_SIZE(o, s) \
  85     (!!(o & 0xff) + !!(o & 0xff00) + !!(o & 0xff0000) + !!(o & 0xff000000) + \
  86      !!(s & 0xff) + !!(s & 0xff00) + 1)
  87
  88 /* the maximum size for any opcode */
  89 #define MAX_OP_SIZE COPYOP_SIZE(0xffffffff, 0xffffffff)
  90
  91 void *diff_delta(void *from_buf, unsigned long from_size,
  92                  void *to_buf, unsigned long to_size,
  93                  unsigned long *delta_size,
  94                  unsigned long max_size)
  95 {
  96         unsigned int i, outpos, outsize, inscnt, hash_shift;
  97         const unsigned char *ref_data, *ref_top, *data, *top;
  98         unsigned char *out;
  99         struct index *entry, **hash;
 100
 101         if (!from_size || !to_size)
 102                 return NULL;
 103         hash = delta_index(from_buf, from_size, &hash_shift);
 104         if (!hash)
 105                 return NULL;
 106
 107         outpos = 0;
 108         outsize = 8192;
 109         if (max_size && outsize >= max_size)
 110                 outsize = max_size + MAX_OP_SIZE + 1;
 111         out = malloc(outsize);
 112         if (!out) {
 113                 free(hash);
 114                 return NULL;
 115         }
 116
 117         ref_data = from_buf;
 118         ref_top = from_buf + from_size;
 119         data = to_buf;
 120         top = to_buf + to_size;
 121
 122         /* store reference buffer size */
 123         out[outpos++] = from_size;
 124         from_size >>= 7;
 125         while (from_size) {
 126                 out[outpos - 1] |= 0x80;
 127                 out[outpos++] = from_size;
 128                 from_size >>= 7;
 129         }
 130
 131         /* store target buffer size */
 132         out[outpos++] = to_size;
 133         to_size >>= 7;
 134         while (to_size) {
 135                 out[outpos - 1] |= 0x80;
 136                 out[outpos++] = to_size;
 137                 to_size >>= 7;
 138         }
 139
 140         inscnt = 0;
 141
 142         while (data < top) {
 143                 unsigned int moff = 0, msize = 0;
 144                 unsigned int blksize = MIN(top - data, BLK_SIZE);
 145                 unsigned int val = adler32(0, data, blksize);
 146                 i = HASH(val, hash_shift);
 147                 for (entry = hash[i]; entry; entry = entry->next) {
 148                         const unsigned char *ref = entry->ptr;
 149                         const unsigned char *src = data;
 150                         unsigned int ref_size = ref_top - ref;
 151                         if (entry->val != val)
 152                                 continue;
 153                         if (ref_size > top - src)
 154                                 ref_size = top - src;
 155                         while (ref_size && *src++ == *ref) {
 156                                 ref++;
 157                                 ref_size--;
 158                         }
 159                         ref_size = ref - entry->ptr;
 160                         if (ref_size > msize) {
 161                                 /* this is our best match so far */
 162                                 moff = entry->ptr - ref_data;
 163                                 msize = ref_size;
 164                                 if (msize >= 0x10000) {
 165                                         msize = 0x10000;
 166                                         break;
 167                                 }
 168                         }
 169                 }
 170
 171                 if (!msize || msize < COPYOP_SIZE(moff, msize)) {
 172                         if (!inscnt)
 173                                 outpos++;
 174                         out[outpos++] = *data++;
 175                         inscnt++;
 176                         if (inscnt == 0x7f) {
 177                                 out[outpos - inscnt - 1] = inscnt;
 178                                 inscnt = 0;
 179                         }
 180                 } else {
 181                         unsigned char *op;
 182
 183                         if (inscnt) {
 184                                 out[outpos - inscnt - 1] = inscnt;
 185                                 inscnt = 0;
 186                         }
 187
 188                         data += msize;
 189                         op = out + outpos++;
 190                         i = 0x80;
 191
 192                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x01; }
 193                         moff >>= 8;
 194                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x02; }
 195                         moff >>= 8;
 196                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x04; }
 197                         moff >>= 8;
 198                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x08; }
 199
 200                         if (msize & 0xff) { out[outpos++] = msize; i |= 0x10; }
 201                         msize >>= 8;
 202                         if (msize & 0xff) { out[outpos++] = msize; i |= 0x20; }
 203
 204                         *op = i;
 205                 }
 206
 207                 if (outpos >= outsize - MAX_OP_SIZE) {
 208                         void *tmp = out;
 209                         outsize = outsize * 3 / 2;
 210                         if (max_size && outsize >= max_size)
 211                                 outsize = max_size + MAX_OP_SIZE + 1;
 212                         if (max_size && outpos > max_size)
 213                                 out = NULL;
 214                         else
 215                                 out = realloc(out, outsize);
 216                         if (!out) {
 217                                 free(tmp);
 218                                 free(hash);
 219                                 return NULL;
 220                         }
 221                 }
 222         }
 223
 224         if (inscnt)
 225                 out[outpos - inscnt - 1] = inscnt;
 226
 227         free(hash);
 228         *delta_size = outpos;
 229         return out;
 230 }