82 lines
2.2 KiB
C
82 lines
2.2 KiB
C
///////////////////////////////////////////////////////////////////////////////
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//
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/// \file crc32.c
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/// \brief CRC32 calculation
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///
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/// Calculate the CRC32 using the slice-by-eight algorithm.
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/// It is explained in this document:
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/// http://www.intel.com/technology/comms/perfnet/download/CRC_generators.pdf
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/// The code in this file is not the same as in Intel's paper, but
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/// the basic principle is identical.
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//
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// Author: Lasse Collin
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//
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// This file has been put into the public domain.
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// You can do whatever you want with this file.
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//
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///////////////////////////////////////////////////////////////////////////////
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#include <crc_macros.h>
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extern const uint32_t lzma_crc32_table[8][256];
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// If you make any changes, do some bench marking! Seemingly unrelated
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// changes can very easily ruin the performance (and very probably is
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// very compiler dependent).
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extern uint32_t
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lzma_crc32(const uint8_t *buf, size_t size, uint32_t crc)
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{
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crc = ~crc;
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#ifdef WORDS_BIGENDIAN
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crc = bswap32(crc);
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#endif
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if (size > 8) {
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// Fix the alignment, if needed. The if statement above
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// ensures that this won't read past the end of buf[].
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while ((uintptr_t)(buf) & 7) {
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crc = lzma_crc32_table[0][*buf++ ^ A(crc)] ^ S8(crc);
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--size;
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}
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// Calculate the position where to stop.
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const uint8_t *const limit = buf + (size & ~(size_t)(7));
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// Calculate how many bytes must be calculated separately
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// before returning the result.
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size &= (size_t)(7);
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// Calculate the CRC32 using the slice-by-eight algorithm.
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while (buf < limit) {
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crc ^= *(const uint32_t *)(buf);
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buf += 4;
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crc = lzma_crc32_table[7][A(crc)]
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^ lzma_crc32_table[6][B(crc)]
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^ lzma_crc32_table[5][C(crc)]
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^ lzma_crc32_table[4][D(crc)];
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const uint32_t tmp = *(const uint32_t *)(buf);
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buf += 4;
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// At least with some compilers, it is critical for
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// performance, that the crc variable is XORed
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// between the two table-lookup pairs.
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crc = lzma_crc32_table[3][A(tmp)]
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^ lzma_crc32_table[2][B(tmp)]
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^ crc
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^ lzma_crc32_table[1][C(tmp)]
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^ lzma_crc32_table[0][D(tmp)];
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}
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}
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while (size-- != 0)
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crc = lzma_crc32_table[0][*buf++ ^ A(crc)] ^ S8(crc);
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#ifdef WORDS_BIGENDIAN
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crc = bswap32(crc);
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#endif
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return ~crc;
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}
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