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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
+// Shorter prefetch distance for smaller buffers
+constexpr int kPrefetchHorizonMedium = ABSL_CACHELINE_SIZE * 1;
+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 {  // Implementation of the abstract class CRC
+ public:
+  using Uint32By256 = uint32_t[256];
+
+  CRCImpl() = default;
+  ~CRCImpl() override = default;
+
+  // The internal version of CRC::New().
+  static CRCImpl* NewInternal();
+
+  // 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() = default;
+  ~CRC32() override = default;
+
+  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 lengths,
+  // 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.
+  static void ExtendByZeroesImpl(uint32_t* crc, size_t length,
+                                 const uint32_t zeroes_table[256],
+                                 const uint32_t poly_table[256]);
+
+  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_