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.  Number of
  33  * bytes that are _not_ copied from the source is deletion, and number
  34  * of inserted literal bytes are addition, so sum of them is what we
  35  * return.  xdelta can express an edit that copies data inside of the
  36  * destination which originally came from the source.  We do not count
  37  * that in the following routine, so we are undercounting the source
  38  * material that remains in the final output that way.
  39  */
  40 unsigned long count_delta(void *delta_buf, unsigned long delta_size)
  41 {
  42         unsigned long copied_from_source, added_literal;
  43         const unsigned char *data, *top;
  44         unsigned char cmd;
  45         unsigned long src_size, dst_size, out;
  46
  47         /* the smallest delta size possible is 6 bytes */
  48         if (delta_size < 6)
  49                 return UINT_MAX;
  50
  51         data = delta_buf;
  52         top = delta_buf + delta_size;
  53
  54         src_size = get_hdr_size(&data);
  55         dst_size = get_hdr_size(&data);
  56
  57         added_literal = copied_from_source = out = 0;
  58         while (data < top) {
  59                 cmd = *data++;
  60                 if (cmd & 0x80) {
  61                         unsigned long cp_off = 0, cp_size = 0;
  62                         if (cmd & 0x01) cp_off = *data++;
  63                         if (cmd & 0x02) cp_off |= (*data++ << 8);
  64                         if (cmd & 0x04) cp_off |= (*data++ << 16);
  65                         if (cmd & 0x08) cp_off |= (*data++ << 24);
  66                         if (cmd & 0x10) cp_size = *data++;
  67                         if (cmd & 0x20) cp_size |= (*data++ << 8);
  68                         if (cp_size == 0) cp_size = 0x10000;
  69
  70                         if (cmd & 0x40)
  71                                 /* copy from dst */
  72                                 ;
  73                         else
  74                                 copied_from_source += cp_size;
  75                         out += cp_size;
  76                 } else {
  77                         /* write literal into dst */
  78                         added_literal += cmd;
  79                         out += cmd;
  80                         data += cmd;
  81                 }
  82         }
  83
  84         /* sanity check */
  85         if (data != top || out != dst_size)
  86                 return UINT_MAX;
  87
  88         /* delete size is what was _not_ copied from source.
  89          * edit size is that and literal additions.
  90          */
  91         if (src_size + added_literal < copied_from_source)
  92                 /* we ended up overcounting and underflowed */
  93                 return 0;
  94         return (src_size - copied_from_source) + added_literal;
  95 }