text: // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file. See the AUTHORS file for names of contributors.
  
  #ifndef STORAGE_HYPERLEVELDB_DB_LOG_READER_H_
  #define STORAGE_HYPERLEVELDB_DB_LOG_READER_H_
  
  #include <stdint.h>
  
  #include "log_format.h"
  #include "../hyperleveldb/slice.h"
  #include "../hyperleveldb/status.h"
  
  namespace hyperleveldb {
  
  class SequentialFile;
  
  namespace log {
  
  class Reader {
   public:
    // Interface for reporting errors.
    class Reporter {
     public:
      virtual ~Reporter();
  
      // Some corruption was detected.  "size" is the approximate number
      // of bytes dropped due to the corruption.
      virtual void Corruption(size_t bytes, const Status& status) = 0;
    };
  
    // Create a reader that will return log records from "*file".
    // "*file" must remain live while this Reader is in use.
    //
    // If "reporter" is non-NULL, it is notified whenever some data is
    // dropped due to a detected corruption.  "*reporter" must remain
    // live while this Reader is in use.
    //
    // If "checksum" is true, verify checksums if available.
    //
    // The Reader will start reading at the first record located at physical
    // position >= initial_offset within the file.
    Reader(SequentialFile* file, Reporter* reporter, bool checksum,
           uint64_t initial_offset);
  
    ~Reader();
  
    // Read the next record into *record.  Returns true if read
    // successfully, false if we hit end of the input.  May use
    // "*scratch" as temporary storage.  The contents filled in *record
    // will only be valid until the next mutating operation on this
    // reader or the next mutation to *scratch.
    bool ReadRecord(Slice* record, std::string* scratch);
  
    // Returns the physical offset of the last record returned by ReadRecord.
    //
    // Undefined before the first call to ReadRecord.
    uint64_t LastRecordOffset();
  
   private:
    SequentialFile* const file_;
    Reporter* const reporter_;
    bool const checksum_;
    char* const backing_store_;
    Slice buffer_;
    bool eof_;   // Last Read() indicated EOF by returning < kBlockSize
  
    // Offset of the last record returned by ReadRecord.
    uint64_t last_record_offset_;
    // Offset of the first location past the end of buffer_.
    uint64_t end_of_buffer_offset_;
  
    // Offset at which to start looking for the first record to return
    uint64_t const initial_offset_;
  
    // Extend record types with the following special values
    enum {
      kEof = kMaxRecordType + 1,
      // Returned whenever we find an invalid physical record.
      // Currently there are three situations in which this happens:
      // * The record has an invalid CRC (ReadPhysicalRecord reports a drop)
      // * The record is a 0-length record (No drop is reported)
      // * The record is below constructor's initial_offset (No drop is reported)
      kBadRecord = kMaxRecordType + 2
    };
  
    // Skips all blocks that are completely before "initial_offset_".
    //
    // Returns true on success. Handles reporting.
    bool SkipToInitialBlock();
  
    // Return type, or one of the preceding special values
    unsigned int ReadPhysicalRecord(Slice* result);
  
    // Reports dropped bytes to the reporter.
    // buffer_ must be updated to remove the dropped bytes prior to invocation.
    void ReportCorruption(size_t bytes, const char* reason);
    void ReportDrop(size_t bytes, const Status& reason);
  
    // No copying allowed
    Reader(const Reader&);
    void operator=(const Reader&);
  };
  
  }  // namespace log
  }  // namespace hyperleveldb
  
  #endif  // STORAGE_HYPERLEVELDB_DB_LOG_READER_H_
  