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 "count-delta.h"
  10
  11 static unsigned long get_hdr_size(const unsigned char **datap)
  12 {
  13         const unsigned char *data = *datap;
  14         unsigned long size;
  15         unsigned char cmd;
  16         int i;
  17         size = i = 0;
  18         cmd = *data++;
  19         while (cmd) {
  20                 if (cmd & 1)
  21                         size |= *data++ << i;
  22                 i += 8;
  23                 cmd >>= 1;
  24         }
  25         *datap = data;
  26         return size;
  27 }
  28
  29 /*
  30  * NOTE.  We do not _interpret_ delta fully.  As an approximation, we
  31  * just count the number of bytes that are copied from the source, and
  32  * the number of literal data bytes that are inserted.
  33  *
  34  * Number of bytes that are _not_ copied from the source is deletion,
  35  * and number of inserted literal bytes are addition, so sum of them
  36  * is the extent of damage.  xdelta can express an edit that copies
  37  * data inside of the destination which originally came from the
  38  * source.  We do not count that in the following routine, so we are
  39  * undercounting the source material that remains in the final output
  40  * that way.
  41  */
  42 int count_delta(void *delta_buf, unsigned long delta_size,
  43                 unsigned long *src_copied, unsigned long *literal_added)
  44 {
  45         unsigned long copied_from_source, added_literal;
  46         const unsigned char *data, *top;
  47         unsigned char cmd;
  48         unsigned long src_size, dst_size, out;
  49
  50         /* the smallest delta size possible is 6 bytes */
  51         if (delta_size < 6)
  52                 return -1;
  53
  54         data = delta_buf;
  55         top = delta_buf + delta_size;
  56
  57         src_size = get_hdr_size(&data);
  58         dst_size = get_hdr_size(&data);
  59
  60         added_literal = copied_from_source = out = 0;
  61         while (data < top) {
  62                 cmd = *data++;
  63                 if (cmd & 0x80) {
  64                         unsigned long cp_off = 0, cp_size = 0;
  65                         if (cmd & 0x01) cp_off = *data++;
  66                         if (cmd & 0x02) cp_off |= (*data++ << 8);
  67                         if (cmd & 0x04) cp_off |= (*data++ << 16);
  68                         if (cmd & 0x08) cp_off |= (*data++ << 24);
  69                         if (cmd & 0x10) cp_size = *data++;
  70                         if (cmd & 0x20) cp_size |= (*data++ << 8);
  71                         if (cp_size == 0) cp_size = 0x10000;
  72
  73                         if (cmd & 0x40)
  74                                 /* copy from dst */
  75                                 ;
  76                         else
  77                                 copied_from_source += cp_size;
  78                         out += cp_size;
  79                 } else {
  80                         /* write literal into dst */
  81                         added_literal += cmd;
  82                         out += cmd;
  83                         data += cmd;
  84                 }
  85         }
  86
  87         /* sanity check */
  88         if (data != top || out != dst_size)
  89                 return -1;
  90
  91         /* delete size is what was _not_ copied from source.
  92          * edit size is that and literal additions.
  93          */
  94         *src_copied = copied_from_source;
  95         *literal_added = added_literal;
  96         return 0;
  97 }