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 int *hash_shift)
  34 {
  35         unsigned long hsize;
  36         unsigned int hshift, i;
  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 < 16; i++);
  44         hsize = 1 << i;
  45         hshift = i - 8;
  46         *hash_shift = hshift;
  47
  48         /* allocate lookup index */
  49         mem = malloc(hsize * sizeof(*hash) + bufsize * sizeof(*entry));
  50         if (!mem)
  51                 return NULL;
  52         hash = mem;
  53         entry = mem + hsize * sizeof(*hash);
  54         memset(hash, 0, hsize * sizeof(*hash));
  55
  56         /* then populate it */
  57         data = buf + bufsize - 2;
  58         while (data > buf) {
  59                 entry->ptr = --data;
  60                 i = data[0] ^ data[1] ^ (data[2] << hshift);
  61                 entry->next = hash[i];
  62                 hash[i] = entry++;
  63         }
  64
  65         return hash;
  66 }
  67
  68 /* provide the size of the copy opcode given the block offset and size */
  69 #define COPYOP_SIZE(o, s) \
  70     (!!(o & 0xff) + !!(o & 0xff00) + !!(o & 0xff0000) + !!(o & 0xff000000) + \
  71      !!(s & 0xff) + !!(s & 0xff00) + 1)
  72
  73 /* the maximum size for any opcode */
  74 #define MAX_OP_SIZE COPYOP_SIZE(0xffffffff, 0xffffffff)
  75
  76 void *diff_delta(void *from_buf, unsigned long from_size,
  77                  void *to_buf, unsigned long to_size,
  78                  unsigned long *delta_size,
  79                  unsigned long max_size)
  80 {
  81         unsigned int i, outpos, outsize, inscnt, hash_shift;
  82         const unsigned char *ref_data, *ref_top, *data, *top;
  83         unsigned char *out;
  84         struct index *entry, **hash;
  85
  86         if (!from_size || !to_size)
  87                 return NULL;
  88         hash = delta_index(from_buf, from_size, &hash_shift);
  89         if (!hash)
  90                 return NULL;
  91
  92         outpos = 0;
  93         outsize = 8192;
  94         if (max_size && outsize >= max_size)
  95                 outsize = max_size + MAX_OP_SIZE + 1;
  96         out = malloc(outsize);
  97         if (!out) {
  98                 free(hash);
  99                 return NULL;
 100         }
 101
 102         ref_data = from_buf;
 103         ref_top = from_buf + from_size;
 104         data = to_buf;
 105         top = to_buf + to_size;
 106
 107         /* store reference buffer size */
 108         out[outpos++] = from_size;
 109         from_size >>= 7;
 110         while (from_size) {
 111                 out[outpos - 1] |= 0x80;
 112                 out[outpos++] = from_size;
 113                 from_size >>= 7;
 114         }
 115
 116         /* store target buffer size */
 117         out[outpos++] = to_size;
 118         to_size >>= 7;
 119         while (to_size) {
 120                 out[outpos - 1] |= 0x80;
 121                 out[outpos++] = to_size;
 122                 to_size >>= 7;
 123         }
 124
 125         inscnt = 0;
 126
 127         while (data < top) {
 128                 unsigned int moff = 0, msize = 0;
 129                 if (data + 2 < top) {
 130                         i = data[0] ^ data[1] ^ (data[2] << hash_shift);
 131                         for (entry = hash[i]; entry; entry = entry->next) {
 132                                 const unsigned char *ref = entry->ptr;
 133                                 const unsigned char *src = data;
 134                                 unsigned int ref_size = ref_top - ref;
 135                                 if (ref_size > top - src)
 136                                         ref_size = top - src;
 137                                 if (ref_size > 0x10000)
 138                                         ref_size = 0x10000;
 139                                 if (ref_size <= msize)
 140                                         break;
 141                                 while (ref_size && *src++ == *ref) {
 142                                         ref++;
 143                                         ref_size--;
 144                                 }
 145                                 ref_size = ref - entry->ptr;
 146                                 if (msize < ref - entry->ptr) {
 147                                         /* this is our best match so far */
 148                                         msize = ref - entry->ptr;
 149                                         moff = entry->ptr - ref_data;
 150                                 }
 151                         }
 152                 }
 153
 154                 if (!msize || msize < COPYOP_SIZE(moff, msize)) {
 155                         if (!inscnt)
 156                                 outpos++;
 157                         out[outpos++] = *data++;
 158                         inscnt++;
 159                         if (inscnt == 0x7f) {
 160                                 out[outpos - inscnt - 1] = inscnt;
 161                                 inscnt = 0;
 162                         }
 163                 } else {
 164                         unsigned char *op;
 165
 166                         if (inscnt) {
 167                                 out[outpos - inscnt - 1] = inscnt;
 168                                 inscnt = 0;
 169                         }
 170
 171                         data += msize;
 172                         op = out + outpos++;
 173                         i = 0x80;
 174
 175                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x01; }
 176                         moff >>= 8;
 177                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x02; }
 178                         moff >>= 8;
 179                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x04; }
 180                         moff >>= 8;
 181                         if (moff & 0xff) { out[outpos++] = moff; i |= 0x08; }
 182
 183                         if (msize & 0xff) { out[outpos++] = msize; i |= 0x10; }
 184                         msize >>= 8;
 185                         if (msize & 0xff) { out[outpos++] = msize; i |= 0x20; }
 186
 187                         *op = i;
 188                 }
 189
 190                 if (outpos >= outsize - MAX_OP_SIZE) {
 191                         void *tmp = out;
 192                         outsize = outsize * 3 / 2;
 193                         if (max_size && outsize >= max_size)
 194                                 outsize = max_size + MAX_OP_SIZE + 1;
 195                         if (max_size && outpos > max_size)
 196                                 out = NULL;
 197                         else
 198                                 out = realloc(out, outsize);
 199                         if (!out) {
 200                                 free(tmp);
 201                                 free(hash);
 202                                 return NULL;
 203                         }
 204                 }
 205         }
 206
 207         if (inscnt)
 208                 out[outpos - inscnt - 1] = inscnt;
 209
 210         free(hash);
 211         *delta_size = outpos;
 212         return out;
 213 }