be/src/kudu/util/bit-stream-utils.h

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you 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
//
//   http://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 IMPALA_UTIL_BIT_STREAM_UTILS_H
#define IMPALA_UTIL_BIT_STREAM_UTILS_H

#include "kudu/gutil/port.h"
#include "kudu/util/bit-util.h"
#include "kudu/util/faststring.h"

namespace kudu {

// Utility class to write bit/byte streams.  This class can write data to either be
// bit packed or byte aligned (and a single stream that has a mix of both).
class BitWriter {
 public:
  // buffer: buffer to write bits to.
  explicit BitWriter(faststring *buffer)
    : buffer_(buffer) {
    Clear();
  }

  void Clear() {
    buffered_values_ = 0;
    byte_offset_ = 0;
    bit_offset_ = 0;
    buffer_->clear();
  }

  // Returns a pointer to the underlying buffer
  faststring *buffer() const { return buffer_; }

  // The number of current bytes written, including the current byte (i.e. may include a
  // fraction of a byte). Includes buffered values.
  int bytes_written() const { return byte_offset_ + BitUtil::Ceil(bit_offset_, 8); }

  // Writes a value to buffered_values_, flushing to buffer_ if necessary.  This is bit
  // packed. num_bits must be <= 32. If 'v' is larger than 'num_bits' bits, the higher
  // bits are ignored.
  void PutValue(uint64_t v, int num_bits);

  // Writes v to the next aligned byte using num_bits. If T is larger than num_bits, the
  // extra high-order bits will be ignored.
  template<typename T>
  void PutAligned(T v, int num_bits);

  // Write a Vlq encoded int to the buffer. The value is written byte aligned.
  // For more details on vlq: en.wikipedia.org/wiki/Variable-length_quantity
  void PutVlqInt(int32_t v);

  // Get the index to the next aligned byte and advance the underlying buffer by num_bytes.
  size_t GetByteIndexAndAdvance(int num_bytes) {
    uint8_t* ptr = GetNextBytePtr(num_bytes);
    return ptr - buffer_->data();
  }

  // Get a pointer to the next aligned byte and advance the underlying buffer by num_bytes.
  uint8_t* GetNextBytePtr(int num_bytes);

  // Flushes all buffered values to the buffer. Call this when done writing to the buffer.
  // If 'align' is true, buffered_values_ is reset and any future writes will be written
  // to the next byte boundary.
  void Flush(bool align = false);

 private:
  // Bit-packed values are initially written to this variable before being memcpy'd to
  // buffer_. This is faster than writing values byte by byte directly to buffer_.
  uint64_t buffered_values_;

  faststring *buffer_;
  int byte_offset_;       // Offset in buffer_
  int bit_offset_;        // Offset in buffered_values_
};

// Utility class to read bit/byte stream.  This class can read bits or bytes
// that are either byte aligned or not.  It also has utilities to read multiple
// bytes in one read (e.g. encoded int).
class BitReader {
 public:
  // 'buffer' is the buffer to read from.  The buffer's length is 'buffer_len'.
  BitReader(const uint8_t* buffer, int buffer_len);

  BitReader() : buffer_(NULL), max_bytes_(0) {}

  // Gets the next value from the buffer.  Returns true if 'v' could be read or false if
  // there are not enough bytes left. num_bits must be <= 32.
  template<typename T>
  bool GetValue(int num_bits, T* v);

  // Reads a 'num_bytes'-sized value from the buffer and stores it in 'v'. T needs to be a
  // little-endian native type and big enough to store 'num_bytes'. The value is assumed
  // to be byte-aligned so the stream will be advanced to the start of the next byte
  // before 'v' is read. Returns false if there are not enough bytes left.
  template<typename T>
  bool GetAligned(int num_bytes, T* v);

  // Reads a vlq encoded int from the stream.  The encoded int must start at the
  // beginning of a byte. Return false if there were not enough bytes in the buffer.
  bool GetVlqInt(int32_t* v);

  // Returns the number of bytes left in the stream, not including the current byte (i.e.,
  // there may be an additional fraction of a byte).
  int bytes_left() { return max_bytes_ - (byte_offset_ + BitUtil::Ceil(bit_offset_, 8)); }

  // Current position in the stream, by bit.
  int position() const { return byte_offset_ * 8 + bit_offset_; }

  // Rewind the stream by 'num_bits' bits
  void Rewind(int num_bits);

  // Seek to a specific bit in the buffer
  void SeekToBit(uint stream_position);

  // Maximum byte length of a vlq encoded int
  static const int MAX_VLQ_BYTE_LEN = 5;

  bool is_initialized() const { return buffer_ != NULL; }

 private:
  // Used by SeekToBit() and GetValue() to fetch the
  // the next word into buffer_.
  void BufferValues();

  const uint8_t* buffer_;
  int max_bytes_;

  // Bytes are memcpy'd from buffer_ and values are read from this variable. This is
  // faster than reading values byte by byte directly from buffer_.
  uint64_t buffered_values_;

  int byte_offset_;       // Offset in buffer_
  int bit_offset_;        // Offset in buffered_values_
};

} // namespace kudu

#endif