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45 строки
1.4 KiB
45 строки
1.4 KiB
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#ifndef STORAGE_LEVELDB_UTIL_CRC32C_H_
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#define STORAGE_LEVELDB_UTIL_CRC32C_H_
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#include <stddef.h>
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#include <stdint.h>
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namespace leveldb {
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namespace crc32c {
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// Return the crc32c of concat(A, data[0,n-1]) where init_crc is the
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// crc32c of some string A. Extend() is often used to maintain the
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// crc32c of a stream of data.
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extern uint32_t Extend(uint32_t init_crc, const char* data, size_t n);
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// Return the crc32c of data[0,n-1]
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inline uint32_t Value(const char* data, size_t n) {
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return Extend(0, data, n);
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}
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static const uint32_t kMaskDelta = 0xa282ead8ul;
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// Return a masked representation of crc.
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//
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// Motivation: it is problematic to compute the CRC of a string that
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// contains embedded CRCs. Therefore we recommend that CRCs stored
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// somewhere (e.g., in files) should be masked before being stored.
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inline uint32_t Mask(uint32_t crc) {
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// Rotate right by 15 bits and add a constant.
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return ((crc >> 15) | (crc << 17)) + kMaskDelta;
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}
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// Return the crc whose masked representation is masked_crc.
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inline uint32_t Unmask(uint32_t masked_crc) {
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uint32_t rot = masked_crc - kMaskDelta;
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return ((rot >> 17) | (rot << 15));
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}
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}
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}
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#endif // STORAGE_LEVELDB_UTIL_CRC32C_H_
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