src/impala/bit-stream-utils.h

// Copyright 2012 Cloudera Inc.
//
// 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
//
// 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 <boost/cstdint.hpp>
#include <string.h>
#include "impala/compiler-util.h"
#include "impala/bit-util.h"
#include "impala/logging.h"

namespace impala {

// 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).
// This class does not allocate memory.
class BitWriter {
 public:
  // buffer: buffer to write bits to.  Buffer should be preallocated with
  // 'buffer_len' bytes.
  BitWriter(uint8_t* buffer, int buffer_len) :
      buffer_(buffer),
      max_bytes_(buffer_len) {
    Clear();
  }

  void Clear() {
    buffered_values_ = 0;
    byte_offset_ = 0;
    bit_offset_ = 0;
  }

  // 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); }
  uint8_t* buffer() const { return buffer_; }
  int buffer_len() const { return max_bytes_; }

  // Writes a value to buffered_values_, flushing to buffer_ if necessary.  This is bit
  // packed.  Returns false if there was not enough space. num_bits must be <= 32.
  bool PutValue(uint64_t v, int num_bits);

  // Writes v to the next aligned byte using num_bytes. If T is larger than num_bytes, the
  // extra high-order bytes will be ignored. Returns false if there was not enough space.
  template<typename T>
  bool PutAligned(T v, int num_bytes);

  // Write a Vlq encoded int to the buffer.  Returns false if there was not enough
  // room.  The value is written byte aligned.
  // For more details on vlq:
  // en.wikipedia.org/wiki/Variable-length_quantity
  bool PutVlqInt(uint32_t v);
  bool PutZigZagVlqInt(int32_t v);

  // Get a pointer to the next aligned byte and advance the underlying buffer
  // by num_bytes.
  // Returns NULL if there was not enough space.
  uint8_t* GetNextBytePtr(int num_bytes = 1);

  // 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:
  uint8_t* buffer_;
  int max_bytes_;

  // 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_;

  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) :
      buffer_(buffer),
      max_bytes_(buffer_len),
      byte_offset_(0),
      bit_offset_(0) {
    int num_bytes = std::min(8, max_bytes_ - byte_offset_);
    memcpy(&buffered_values_, buffer_ + byte_offset_, num_bytes);
  }

  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(uint64_t* v);
  bool GetZigZagVlqInt(int64_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)); }

  const uint8_t* current_ptr() { return buffer_ + byte_offset_; }
  void SkipBytes(int num_bytes) {
    byte_offset_ += num_bytes;
    num_bytes = std::min(8, max_bytes_ - byte_offset_);
    memcpy(&buffered_values_, buffer_ + byte_offset_, num_bytes);
  }

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

 private:
  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_
};

}

#endif