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+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CRC_INTERNAL_CRC_INTERNAL_H_
+#define ABSL_CRC_INTERNAL_CRC_INTERNAL_H_
+
+#include <cstdint>
+#include <memory>
+#include <vector>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/crc/internal/crc.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace crc_internal {
+
+// Prefetch constants used in some Extend() implementations
+constexpr int kPrefetchHorizon = ABSL_CACHELINE_SIZE * 4; // Prefetch this far
+static_assert(kPrefetchHorizon >= 64, "CRCPrefetchHorizon less than loop len");
+
+// We require the Scramble() function:
+// - to be reversible (Unscramble() must exist)
+// - to be non-linear in the polynomial's Galois field (so the CRC of a
+// scrambled CRC is not linearly affected by the scrambled CRC, even if
+// using the same polynomial)
+// - not to be its own inverse. Preferably, if X=Scramble^N(X) and N!=0, then
+// N is large.
+// - to be fast.
+// - not to change once defined.
+// We introduce non-linearity in two ways:
+// Addition of a constant.
+// - The carries introduce non-linearity; we use bits of an irrational
+// (phi) to make it unlikely that we introduce no carries.
+// Rotate by a constant number of bits.
+// - We use floor(degree/2)+1, which does not divide the degree, and
+// splits the bits nearly evenly, which makes it less likely the
+// halves will be the same or one will be all zeroes.
+// We do both things to improve the chances of non-linearity in the face of
+// bit patterns with low numbers of bits set, while still being fast.
+// Below is the constant that we add. The bits are the first 128 bits of the
+// fractional part of phi, with a 1 ored into the bottom bit to maximize the
+// cycle length of repeated adds.
+constexpr uint64_t kScrambleHi = (static_cast<uint64_t>(0x4f1bbcdcU) << 32) |
+ static_cast<uint64_t>(0xbfa53e0aU);
+constexpr uint64_t kScrambleLo = (static_cast<uint64_t>(0xf9ce6030U) << 32) |
+ static_cast<uint64_t>(0x2e76e41bU);
+
+class CRCImpl : public CRC { // Implemention of the abstract class CRC
+ public:
+ using Uint32By256 = uint32_t[256];
+
+ CRCImpl() {}
+ ~CRCImpl() override = default;
+
+ // The internal version of CRC::New().
+ static CRCImpl* NewInternal();
+
+ void Empty(uint32_t* crc) const override;
+
+ // Fill in a table for updating a CRC by one word of 'word_size' bytes
+ // [last_lo, last_hi] contains the answer if the last bit in the word
+ // is set.
+ static void FillWordTable(uint32_t poly, uint32_t last, int word_size,
+ Uint32By256* t);
+
+ // Build the table for extending by zeroes, returning the number of entries.
+ // For a in {1, 2, ..., ZEROES_BASE-1}, b in {0, 1, 2, 3, ...},
+ // entry j=a-1+(ZEROES_BASE-1)*b
+ // contains a polynomial Pi such that multiplying
+ // a CRC by Pi mod P, where P is the CRC polynomial, is equivalent to
+ // appending a*2**(ZEROES_BASE_LG*b) zero bytes to the original string.
+ static int FillZeroesTable(uint32_t poly, Uint32By256* t);
+
+ virtual void InitTables() = 0;
+
+ private:
+ CRCImpl(const CRCImpl&) = delete;
+ CRCImpl& operator=(const CRCImpl&) = delete;
+};
+
+// This is the 32-bit implementation. It handles all sizes from 8 to 32.
+class CRC32 : public CRCImpl {
+ public:
+ CRC32() {}
+ ~CRC32() override {}
+
+ void Extend(uint32_t* crc, const void* bytes, size_t length) const override;
+ void ExtendByZeroes(uint32_t* crc, size_t length) const override;
+ void Scramble(uint32_t* crc) const override;
+ void Unscramble(uint32_t* crc) const override;
+ void UnextendByZeroes(uint32_t* crc, size_t length) const override;
+
+ void InitTables() override;
+
+ private:
+ // Common implementation guts for ExtendByZeroes and UnextendByZeroes().
+ //
+ // zeroes_table is a table as returned by FillZeroesTable(), containing
+ // polynomials representing CRCs of strings-of-zeros of various lenghts,
+ // and which can be combined by polynomial multiplication. poly_table is
+ // a table of CRC byte extension values. These tables are determined by
+ // the generator polynomial.
+ //
+ // These will be set to reverse_zeroes_ and reverse_table0_ for Unextend, and
+ // CRC32::zeroes_ and CRC32::table0_ for Extend.
+ void ExtendByZeroesImpl(uint32_t* crc, size_t length,
+ const uint32_t zeroes_table[256],
+ const uint32_t poly_table[256]) const;
+
+ uint32_t table0_[256]; // table of byte extensions
+ uint32_t zeroes_[256]; // table of zero extensions
+
+ // table of 4-byte extensions shifted by 12 bytes of zeroes
+ uint32_t table_[4][256];
+
+ // Reverse lookup tables, using the alternate polynomial used by
+ // UnextendByZeroes().
+ uint32_t reverse_table0_[256]; // table of reverse byte extensions
+ uint32_t reverse_zeroes_[256]; // table of reverse zero extensions
+
+ CRC32(const CRC32&) = delete;
+ CRC32& operator=(const CRC32&) = delete;
+};
+
+// Helpers
+
+// Return a bit mask containing len 1-bits.
+// Requires 0 < len <= sizeof(T)
+template <typename T>
+T MaskOfLength(int len) {
+ // shift 2 by len-1 rather than 1 by len because shifts of wordsize
+ // are undefined.
+ return (T(2) << (len - 1)) - 1;
+}
+
+// Rotate low-order "width" bits of "in" right by "r" bits,
+// setting other bits in word to arbitrary values.
+template <typename T>
+T RotateRight(T in, int width, int r) {
+ return (in << (width - r)) | ((in >> r) & MaskOfLength<T>(width - r));
+}
+
+// RoundUp<N>(p) returns the lowest address >= p aligned to an N-byte
+// boundary. Requires that N is a power of 2.
+template <int alignment>
+const uint8_t* RoundUp(const uint8_t* p) {
+ static_assert((alignment & (alignment - 1)) == 0, "alignment is not 2^n");
+ constexpr uintptr_t mask = alignment - 1;
+ const uintptr_t as_uintptr = reinterpret_cast<uintptr_t>(p);
+ return reinterpret_cast<const uint8_t*>((as_uintptr + mask) & ~mask);
+}
+
+// Return a newly created CRC32AcceleratedX86ARMCombined if we can use Intel's
+// or ARM's CRC acceleration for a given polynomial. Return nullptr otherwise.
+CRCImpl* TryNewCRC32AcceleratedX86ARMCombined();
+
+// Return all possible hardware accelerated implementations. For testing only.
+std::vector<std::unique_ptr<CRCImpl>> NewCRC32AcceleratedX86ARMCombinedAll();
+
+} // namespace crc_internal
+ABSL_NAMESPACE_END
+} // namespace absl
+
+#endif // ABSL_CRC_INTERNAL_CRC_INTERNAL_H_