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