count-delta.c

   1 /*
   2  * Copyright (C) 2005 Junio C Hamano
   3  * The delta-parsing part is almost straight copy of patch-delta.c
   4  * which is (C) 2005 Nicolas Pitre <nico@cam.org>.
   5  */
   6 #include <stdlib.h>
   7 #include <string.h>
   8 #include <limits.h>
   9 #include "delta.h"
  10 #include "count-delta.h"
  11
  12 /*
  13  * NOTE.  We do not _interpret_ delta fully.  As an approximation, we
  14  * just count the number of bytes that are copied from the source, and
  15  * the number of literal data bytes that are inserted.
  16  *
  17  * Number of bytes that are _not_ copied from the source is deletion,
  18  * and number of inserted literal bytes are addition, so sum of them
  19  * is the extent of damage.  xdelta can express an edit that copies
  20  * data inside of the destination which originally came from the
  21  * source.  We do not count that in the following routine, so we are
  22  * undercounting the source material that remains in the final output
  23  * that way.
  24  */
  25 int count_delta(void *delta_buf, unsigned long delta_size,
  26                 unsigned long *src_copied, unsigned long *literal_added)
  27 {
  28         unsigned long copied_from_source, added_literal;
  29         const unsigned char *data, *top;
  30         unsigned char cmd;
  31         unsigned long src_size, dst_size, out;
  32
  33         if (delta_size < DELTA_SIZE_MIN)
  34                 return -1;
  35
  36         data = delta_buf;
  37         top = delta_buf + delta_size;
  38
  39         src_size = get_delta_hdr_size(&data);
  40         dst_size = get_delta_hdr_size(&data);
  41
  42         added_literal = copied_from_source = out = 0;
  43         while (data < top) {
  44                 cmd = *data++;
  45                 if (cmd & 0x80) {
  46                         unsigned long cp_off = 0, cp_size = 0;
  47                         if (cmd & 0x01) cp_off = *data++;
  48                         if (cmd & 0x02) cp_off |= (*data++ << 8);
  49                         if (cmd & 0x04) cp_off |= (*data++ << 16);
  50                         if (cmd & 0x08) cp_off |= (*data++ << 24);
  51                         if (cmd & 0x10) cp_size = *data++;
  52                         if (cmd & 0x20) cp_size |= (*data++ << 8);
  53                         if (cp_size == 0) cp_size = 0x10000;
  54
  55                         if (cmd & 0x40)
  56                                 /* copy from dst */
  57                                 ;
  58                         else
  59                                 copied_from_source += cp_size;
  60                         out += cp_size;
  61                 } else {
  62                         /* write literal into dst */
  63                         added_literal += cmd;
  64                         out += cmd;
  65                         data += cmd;
  66                 }
  67         }
  68
  69         /* sanity check */
  70         if (data != top || out != dst_size)
  71                 return -1;
  72
  73         /* delete size is what was _not_ copied from source.
  74          * edit size is that and literal additions.
  75          */
  76         *src_copied = copied_from_source;
  77         *literal_added = added_literal;
  78         return 0;
  79 }