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   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
  11static 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 */
  40unsigned 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}