SST Footer 格式 

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class Footer {
 public:
  // Constructs a footer without specifying its table magic number.
  // In such case, the table magic number of such footer should be
  // initialized via @ReadFooterFromFile().
  // Use this when you plan to load Footer with DecodeFrom(). Never use this
  // when you plan to EncodeTo.
  Footer() : Footer(kInvalidTableMagicNumber, 0) {}

  // Use this constructor when you plan to write out the footer using
  // EncodeTo(). Never use this constructor with DecodeFrom().
  Footer(uint64_t table_magic_number, uint32_t version);

  // The version of the footer in this file
  uint32_t version() const { return version_; }

  // The checksum type used in this file
  ChecksumType checksum() const { return checksum_; }
  void set_checksum(const ChecksumType c) { checksum_ = c; }

  // The block handle for the metaindex block of the table
  const BlockHandle& metaindex_handle() const { return metaindex_handle_; }
  void set_metaindex_handle(const BlockHandle& h) { metaindex_handle_ = h; }

  // The block handle for the index block of the table
  const BlockHandle& index_handle() const { return index_handle_; }

  void set_index_handle(const BlockHandle& h) { index_handle_ = h; }

  uint64_t table_magic_number() const { return table_magic_number_; }

  void EncodeTo(std::string* dst) const;

  // Set the current footer based on the input slice.
  //
  // REQUIRES: table_magic_number_ is not set (i.e.,
  // HasInitializedTableMagicNumber() is true). The function will initialize the
  // magic number
  Status DecodeFrom(Slice* input);

  // Encoded length of a Footer.  Note that the serialization of a Footer will
  // always occupy at least kMinEncodedLength bytes.  If fields are changed
  // the version number should be incremented and kMaxEncodedLength should be
  // increased accordingly.
  enum {
    // Footer version 0 (legacy) will always occupy exactly this many bytes.
    // It consists of two block handles, padding, and a magic number.
    kVersion0EncodedLength = 2 * BlockHandle::kMaxEncodedLength + 8,
    // Footer of versions 1 and higher will always occupy exactly this many
    // bytes. It consists of the checksum type, two block handles, padding,
    // a version number (bigger than 1), and a magic number
    kNewVersionsEncodedLength = 1 + 2 * BlockHandle::kMaxEncodedLength + 4 + 8,
    kMinEncodedLength = kVersion0EncodedLength,
    kMaxEncodedLength = kNewVersionsEncodedLength,
  };

  static const uint64_t kInvalidTableMagicNumber = 0;

  // convert this object to a human readable form
  std::string ToString() const;

 private:
  // REQUIRES: magic number wasn't initialized.
  void set_table_magic_number(uint64_t magic_number) {
    assert(!HasInitializedTableMagicNumber());
    table_magic_number_ = magic_number;
  }

  // return true if @table_magic_number_ is set to a value different
  // from @kInvalidTableMagicNumber.
  bool HasInitializedTableMagicNumber() const {
    return (table_magic_number_ != kInvalidTableMagicNumber);
  }

  uint32_t version_;
  ChecksumType checksum_;
  BlockHandle metaindex_handle_;
  BlockHandle index_handle_;
  uint64_t table_magic_number_ = 0;
}; // class Footer