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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.
 //
 // -----------------------------------------------------------------------------
 // File: crc32c.h
 // -----------------------------------------------------------------------------
 //
 // This header file defines the API for computing CRC32C values as checksums
 // for arbitrary sequences of bytes provided as a string buffer.
 //
 // The API includes the basic functions for computing such CRC32C values and
 // some utility functions for performing more efficient mathematical
 // computations using an existing checksum.
 #ifndef ABSL_CRC_CRC32C_H_
 #define ABSL_CRC_CRC32C_H_
 
 #include <cstdint>
 #include <ostream>
 
 #include "absl/crc/internal/crc32c_inline.h"
 #include "absl/strings/str_format.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
 //-----------------------------------------------------------------------------
 // crc32c_t
 //-----------------------------------------------------------------------------
 
 // `crc32c_t` defines a strongly-typed integer for holding a CRC32C value.
 //
 // Some operators are intentionally omitted. Only equality operators are defined
 // so that `crc32c_t` can be directly compared. Methods for putting `crc32c_t`
 // directly into a set are omitted because this is bug-prone due to checksum
 // collisions. Use an explicit conversion to the `uint32_t` space for operations
 // that treat `crc32c_t` as an integer.
 class crc32c_t final {
  public:
   crc32c_t() = default;
   constexpr explicit crc32c_t(uint32_t crc) : crc_(crc) {}
 
   crc32c_t(const crc32c_t&) = default;
   crc32c_t& operator=(const crc32c_t&) = default;
 
   explicit operator uint32_t() const { return crc_; }
 
   friend bool operator==(crc32c_t lhs, crc32c_t rhs) {
     return static_cast<uint32_t>(lhs) == static_cast<uint32_t>(rhs);
   }
 
   friend bool operator!=(crc32c_t lhs, crc32c_t rhs) { return !(lhs == rhs); }
 
   template <typename Sink>
   friend void AbslStringify(Sink& sink, crc32c_t crc) {
     absl::Format(&sink, "%08x", static_cast<uint32_t>(crc));
   }
 
  private:
   uint32_t crc_;
 };
 
 
 namespace crc_internal {
 // Non-inline code path for `absl::ExtendCrc32c()`. Do not call directly.
 // Call `absl::ExtendCrc32c()` (defined below) instead.
 crc32c_t ExtendCrc32cInternal(crc32c_t initial_crc,
                               absl::string_view buf_to_add);
 }  // namespace crc_internal
 
 // -----------------------------------------------------------------------------
 // CRC32C Computation Functions
 // -----------------------------------------------------------------------------
 
 // ComputeCrc32c()
 //
 // Returns the CRC32C value of the provided string.
 crc32c_t ComputeCrc32c(absl::string_view buf);
 
 // ExtendCrc32c()
 //
 // Computes a CRC32C value from an `initial_crc` CRC32C value including the
 // `buf_to_add` bytes of an additional buffer. Using this function is more
 // efficient than computing a CRC32C value for the combined buffer from
 // scratch.
 //
 // Note: `ExtendCrc32c` with an initial_crc of 0 is equivalent to
 // `ComputeCrc32c`.
 //
 // This operation has a runtime cost of O(`buf_to_add.size()`)
 inline crc32c_t ExtendCrc32c(crc32c_t initial_crc,
                              absl::string_view buf_to_add) {
   // Approximately 75% of calls have size <= 64.
   if (buf_to_add.size() <= 64) {
     uint32_t crc = static_cast<uint32_t>(initial_crc);
     if (crc_internal::ExtendCrc32cInline(&crc, buf_to_add.data(),
                                          buf_to_add.size())) {
       return crc32c_t{crc};
     }
   }
   return crc_internal::ExtendCrc32cInternal(initial_crc, buf_to_add);
 }
 
 // ExtendCrc32cByZeroes()
 //
 // Computes a CRC32C value for a buffer with an `initial_crc` CRC32C value,
 // where `length` bytes with a value of 0 are appended to the buffer. Using this
 // function is more efficient than computing a CRC32C value for the combined
 // buffer from scratch.
 //
 // This operation has a runtime cost of O(log(`length`))
 crc32c_t ExtendCrc32cByZeroes(crc32c_t initial_crc, size_t length);
 
 // MemcpyCrc32c()
 //
 // Copies `src` to `dest` using `memcpy()` semantics, returning the CRC32C
 // value of the copied buffer.
 //
 // Using `MemcpyCrc32c()` is potentially faster than performing the `memcpy()`
 // and `ComputeCrc32c()` operations separately.
 crc32c_t MemcpyCrc32c(void* dest, const void* src, size_t count,
                       crc32c_t initial_crc = crc32c_t{0});
 
 // -----------------------------------------------------------------------------
 // CRC32C Arithmetic Functions
 // -----------------------------------------------------------------------------
 
 // The following functions perform arithmetic on CRC32C values, which are
 // generally more efficient than recalculating any given result's CRC32C value.
 
 // ConcatCrc32c()
 //
 // Calculates the CRC32C value of two buffers with known CRC32C values
 // concatenated together.
 //
 // Given a buffer with CRC32C value `crc1` and a buffer with
 // CRC32C value `crc2` and length, `crc2_length`, returns the CRC32C value of
 // the concatenation of these two buffers.
 //
 // This operation has a runtime cost of O(log(`crc2_length`)).
 crc32c_t ConcatCrc32c(crc32c_t crc1, crc32c_t crc2, size_t crc2_length);
 
 // RemoveCrc32cPrefix()
 //
 // Calculates the CRC32C value of an existing buffer with a series of bytes
 // (the prefix) removed from the beginning of that buffer.
 //
 // Given the CRC32C value of an existing buffer, `full_string_crc`; The CRC32C
 // value of a prefix of that buffer, `prefix_crc`; and the length of the buffer
 // with the prefix removed, `remaining_string_length` , return the CRC32C
 // value of the buffer with the prefix removed.
 //
 // This operation has a runtime cost of O(log(`remaining_string_length`)).
 crc32c_t RemoveCrc32cPrefix(crc32c_t prefix_crc, crc32c_t full_string_crc,
                             size_t remaining_string_length);
 // RemoveCrc32cSuffix()
 //
 // Calculates the CRC32C value of an existing buffer with a series of bytes
 // (the suffix) removed from the end of that buffer.
 //
 // Given a CRC32C value of an existing buffer `full_string_crc`, the CRC32C
 // value of the suffix to remove `suffix_crc`, and the length of that suffix
 // `suffix_len`, returns the CRC32C value of the buffer with suffix removed.
 //
 // This operation has a runtime cost of O(log(`suffix_len`))
 crc32c_t RemoveCrc32cSuffix(crc32c_t full_string_crc, crc32c_t suffix_crc,
                             size_t suffix_length);
 
 // operator<<
 //
 // Streams the CRC32C value `crc` to the stream `os`.
 inline std::ostream& operator<<(std::ostream& os, crc32c_t crc) {
   return os << absl::StreamFormat("%08x", static_cast<uint32_t>(crc));
 }
 
 ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CRC_CRC32C_H_

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