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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 "delta.h"
  10#include "count-delta.h"
  11
  12static 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 */
  41unsigned 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}