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96 строки
2.5 KiB
C
96 строки
2.5 KiB
C
/*
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* Copyright (C) 2005 Junio C Hamano
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* The delta-parsing part is almost straight copy of patch-delta.c
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* which is (C) 2005 Nicolas Pitre <nico@cam.org>.
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*/
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#include <stdlib.h>
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#include <string.h>
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#include <limits.h>
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#include "count-delta.h"
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static unsigned long get_hdr_size(const unsigned char **datap)
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{
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const unsigned char *data = *datap;
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unsigned long size;
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unsigned char cmd;
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int i;
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size = i = 0;
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cmd = *data++;
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while (cmd) {
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if (cmd & 1)
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size |= *data++ << i;
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i += 8;
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cmd >>= 1;
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}
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*datap = data;
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return size;
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}
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/*
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* NOTE. We do not _interpret_ delta fully. As an approximation, we
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* just count the number of bytes that are copied from the source, and
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* the number of literal data bytes that are inserted. Number of
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* bytes that are _not_ copied from the source is deletion, and number
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* of inserted literal bytes are addition, so sum of them is what we
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* return. xdelta can express an edit that copies data inside of the
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* destination which originally came from the source. We do not count
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* that in the following routine, so we are undercounting the source
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* material that remains in the final output that way.
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*/
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unsigned long count_delta(void *delta_buf, unsigned long delta_size)
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{
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unsigned long copied_from_source, added_literal;
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const unsigned char *data, *top;
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unsigned char cmd;
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unsigned long src_size, dst_size, out;
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/* the smallest delta size possible is 6 bytes */
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if (delta_size < 6)
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return UINT_MAX;
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data = delta_buf;
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top = delta_buf + delta_size;
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src_size = get_hdr_size(&data);
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dst_size = get_hdr_size(&data);
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added_literal = copied_from_source = out = 0;
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while (data < top) {
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cmd = *data++;
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if (cmd & 0x80) {
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unsigned long cp_off = 0, cp_size = 0;
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if (cmd & 0x01) cp_off = *data++;
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if (cmd & 0x02) cp_off |= (*data++ << 8);
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if (cmd & 0x04) cp_off |= (*data++ << 16);
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if (cmd & 0x08) cp_off |= (*data++ << 24);
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if (cmd & 0x10) cp_size = *data++;
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if (cmd & 0x20) cp_size |= (*data++ << 8);
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if (cp_size == 0) cp_size = 0x10000;
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if (cmd & 0x40)
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/* copy from dst */
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;
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else
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copied_from_source += cp_size;
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out += cp_size;
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} else {
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/* write literal into dst */
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added_literal += cmd;
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out += cmd;
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data += cmd;
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}
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}
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/* sanity check */
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if (data != top || out != dst_size)
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return UINT_MAX;
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/* delete size is what was _not_ copied from source.
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* edit size is that and literal additions.
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*/
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if (src_size + added_literal < copied_from_source)
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/* we ended up overcounting and underflowed */
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return 0;
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return (src_size - copied_from_source) + added_literal;
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}
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