rle-encoding.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_RLE_ENCODING_H
#define IMPALA_RLE_ENCODING_H

#include "common/compiler-util.h"
#include "util/bit-stream-utils.inline.h"
#include "util/bit-util.h"

namespace impala {

// Utility classes to do run length encoding (RLE) for fixed bit width values.  If runs
// are sufficiently long, RLE is used, otherwise, the values are just bit-packed
// (literal encoding).
// For both types of runs, there is a byte-aligned indicator which encodes the length
// of the run and the type of the run.
// This encoding has the benefit that when there aren't any long enough runs, values
// are always decoded at fixed (can be precomputed) bit offsets OR both the value and
// the run length are byte aligned. This allows for very efficient decoding
// implementations.
// The encoding is:
//    encoded-block := run*
//    run := literal-run | repeated-run
//    literal-run := literal-indicator < literal bytes >
//    repeated-run := repeated-indicator < repeated value. padded to byte boundary >
//    literal-indicator := varint_encode( number_of_groups << 1 | 1)
//    repeated-indicator := varint_encode( number_of_repetitions << 1 )
//
// Each run is preceded by a varint. The varint's least significant bit is
// used to indicate whether the run is a literal run or a repeated run. The rest
// of the varint is used to determine the length of the run (eg how many times the
// value repeats).
//
// In the case of literal runs, the run length is always a multiple of 8 (i.e. encode
// in groups of 8), so that no matter the bit-width of the value, the sequence will end
// on a byte boundary without padding.
// Given that we know it is a multiple of 8, we store the number of 8-groups rather than
// the actual number of encoded ints. (This means that the total number of encoded values
// can not be determined from the encoded data, since the number of values in the last
// group may not be a multiple of 8).
// There is a break-even point when it is more storage efficient to do run length
// encoding.  For 1 bit-width values, that point is 8 values.  They require 2 bytes
// for both the repeated encoding or the literal encoding.  This value can always
// be computed based on the bit-width.
// TODO: think about how to use this for strings.  The bit packing isn't quite the same.
//
// Examples with bit-width 1 (eg encoding booleans):
// ----------------------------------------
// 100 1s followed by 100 0s:
// <varint(100 << 1)> <1, padded to 1 byte>  <varint(100 << 1)> <0, padded to 1 byte>  
//  - (total 4 bytes)
//
// alternating 1s and 0s (200 total):
// 200 ints = 25 groups of 8
// <varint((25 << 1) | 1)> <25 bytes of values, bitpacked>  
// (total 26 bytes, 1 byte overhead)
//
// TODO: this implementation is tailored to bit-width 1 and will need more work to
// make it general.

// Decoder class for RLE encoded data.
class RleDecoder {
 public:
  // Create a decoder object. buffer/buffer_len is the decoded data.
  // bit_width is the width of each value (before encoding).
  // TODO: support bit_width.  Only 1 is implemented.
  RleDecoder(uint8_t* buffer, int buffer_len, int bit_width)
    : bit_reader_(buffer, buffer_len),
      bit_width_(bit_width),
      current_value_(0),
      repeat_count_(0),
      literal_count_(0) {
    DCHECK_GE(bit_width_, 1);
    DCHECK_LE(bit_width_, 64);
  }

  RleDecoder() {}

  // Gets the next value.  Returns false if there are no more.
  template<typename T>
  bool Get(T* val);

 private:
  BitReader bit_reader_;
  int bit_width_;
  uint64_t current_value_;
  uint32_t repeat_count_;
  uint32_t literal_count_;
};

// Class to incrementally build the rle data.   This class does not allocate any memory.
// The encoding has two modes: encoding repeated runs and literal runs.
// If the run is sufficiently short, it is more efficient to encode as a literal run.
// This class does so by buffering 8 values at a time.  If they are not all the same
// they are added to the literal run.  If they are the same, they are added to the 
// repeated run.  When we switch modes, the previous run is flushed out.
class RleEncoder {
 public:
  // buffer/buffer_len: preallocated output buffer.  
  // bit_width: max number of bits for value.
  // TODO: consider adding a min_repeated_run_length so the caller can control
  // when values should be encoded as repeated runs.  Currently this is derived
  // based on the bit_width, which can determine a storage optimal choice.
  RleEncoder(uint8_t* buffer, int buffer_len, int bit_width)
    : bit_width_(bit_width),
      bit_writer_(buffer, buffer_len) {
    DCHECK_GE(bit_width_, 1);
    DCHECK_LE(bit_width_, 64);
    Clear();
  }

  // Encode value.  Returns true if the value fits in buffer, false otherwise.
  // This value must be representable with bit_width_ bits.
  bool Put(uint64_t value);

  // Flushes any pending vales to the underlying buffer.  Also updates buffer_full_
  // if we can't encode any more values.
  // Returns the total number of bytes written
  int Flush();

  // Resets all the state in the encoder.
  void Clear();

  // Returns pointer to underlying buffer
  uint8_t* buffer() { return bit_writer_.buffer(); }
  int32_t len() { return bit_writer_.bytes_written(); }
  
 private:
  // Flushes any buffered values.  If this is part of a repeated run, this is largely 
  // a no-op.
  // If it is part of a literal run, this will call FlushLiteralRun, which writes
  // out the buffered literal values.
  // If 'done' is true, the current run would be written even if it would normally
  // have been buffered more.  This should only be called at the end, when the
  // encoder has received all values even if it would normally continue to be
  // buffered.
  void FlushBufferedValues(bool done);

  // Flushes literal values to the underlying buffer.  If update_indicator_byte,
  // then the current literal run is complete and the indicator byte is updated.
  void FlushLiteralRun(bool update_indicator_byte);

  // Flushes a repeated run to the underlying buffer.
  void FlushRepeatedRun();

  // Number of bits needed to encode the value.
  const int bit_width_;

  // Underlying buffer.
  BitWriter bit_writer_;

  // If true, the buffer is full and subsequent put's will fail.
  bool buffer_full_;

  // We need to buffer at most 8 values for literals.  This happens when the
  // bit_width is 1 (so 8 values fit in one byte).
  // TODO: generalize this to other bit widths
  int64_t buffered_values_[8];

  // Number of values in buffered_values_
  int num_buffered_values_;

  // The current (also last) value that was written and the count of how
  // many times in a row that value has been seen.  This is maintained even
  // if we are in a literal run.  If the repeat_count_ get high enough, we switch 
  // to encoding repeated runs.
  int64_t current_value_;
  int repeat_count_;

  // Number of literals in the current run.  This does not include the literals
  // that might be in buffered_values_.  Only after we've got a group big enough
  // can we decide if they should part of the literal_count_ or repeat_count_
  int literal_count_;

  // Pointer to a byte in the underlying buffer that stores the indicator byte.
  // This is reserved as soon as we need a literal run but the value is written
  // when the literal run is complete.
  uint8_t* literal_indicator_byte_;
};
   
template<typename T>
inline bool RleDecoder::Get(T* val) {
  if (UNLIKELY(literal_count_ == 0 && repeat_count_ == 0)) {
    // Read the next run's indicator int, it could be a literal or repeated run
    // The int is encoded as a vlq-encoded value.
    int32_t indicator_value = 0;
    bool result = bit_reader_.GetVlqInt(&indicator_value);
    DCHECK(result);

    // lsb indicates if it is a literal run or repeated run
    bool is_literal = indicator_value & 1;
    if (is_literal) {
      literal_count_ = (indicator_value >> 1) * 8;
    } else {
      repeat_count_ = indicator_value >> 1;
      bool result = bit_reader_.GetAligned<T>(
          bit_width_, reinterpret_cast<T*>(&current_value_));
      DCHECK(result);
    }
  }

  if (LIKELY(repeat_count_ > 0)) {
    *val = current_value_;
    --repeat_count_;
  } else {
    DCHECK(literal_count_ > 0);
    bool result = bit_reader_.GetValue(bit_width_, val);
    DCHECK(result);
    --literal_count_;
  }

  return true;
}

// This function buffers input values 8 at a time.  After seeing all 8 values,
// it decides whether they should be encoded as a literal or repeated run.
inline bool RleEncoder::Put(uint64_t value) {
  DCHECK(bit_width_ == 64 || value < (1LL << bit_width_));
  if (UNLIKELY(buffer_full_)) return false;

  if (LIKELY(current_value_ == value)) {
    ++repeat_count_;
    if (repeat_count_ > 8) {
      // This is just a continuation of the current run, no need to buffer the
      // values.
      // Note that this is the fast path for long repeated runs.
      return true;
    }
  } else {
    if (repeat_count_ >= 8) {
      // We had a run that was long enough but it has ended.  Flush the
      // current repeated run.
      DCHECK_EQ(literal_count_, 0);
      FlushRepeatedRun();
    }
    repeat_count_ = 1;
    current_value_ = value;
  } 

  buffered_values_[num_buffered_values_] = value;
  if (++num_buffered_values_ == 8) {
    DCHECK_EQ(literal_count_ % 8, 0);
    FlushBufferedValues(false);
  }
  return true;
}

inline void RleEncoder::FlushLiteralRun(bool update_indicator_byte) {
  if (literal_indicator_byte_ == NULL) {
    // The literal indicator byte has not been reserved yet, get one now.
    literal_indicator_byte_ = bit_writer_.GetNextBytePtr();
    DCHECK(literal_indicator_byte_ != NULL);
  }
  
  // Write all the buffered values as bit packed literals
  bool result = true;
  for (int i = 0; i < num_buffered_values_; ++i) {
    result &= bit_writer_.PutValue(buffered_values_[i], bit_width_);
  }
  DCHECK(result);
  num_buffered_values_ = 0;

  if (update_indicator_byte) {
    // At this point we need to write the indicator byte for the literal run.
    // We only reserve one byte, to allow for streaming writes of literal values.
    // The logic makes sure we flush literal runs often enough to not overrun
    // the 1 byte.
    int num_groups = BitUtil::Ceil(literal_count_, 8);
    int32_t indicator_value = (num_groups << 1) | 1;
    DCHECK_EQ(indicator_value & 0xFFFFFF00, 0);
    *literal_indicator_byte_ = indicator_value;
    literal_indicator_byte_ = NULL;
    literal_count_ = 0;
  }
}

inline void RleEncoder::FlushRepeatedRun() {
  DCHECK_GT(repeat_count_, 0);
  bool result = true;
  // The lsb of 0 indicates this is a repeated run
  int32_t indicator_value = repeat_count_ << 1 | 0;
  result &= bit_writer_.PutVlqInt(indicator_value);
  result &= bit_writer_.PutAligned<int64_t>(current_value_, bit_width_);
  DCHECK(result);
  num_buffered_values_ = 0;
  repeat_count_ = 0;
}

// Flush the values that have been buffered.  At this point we decide whether
// we need to switch between the run types or continue the current one.
inline void RleEncoder::FlushBufferedValues(bool done) {
  if (repeat_count_ >= 8) {
    // Clear the buffered values.  They are part of the repeated run now and we
    // don't want to flush them out as literals.
    num_buffered_values_ = 0;
    if (literal_count_ != 0) {
      // There was a current literal run.  All the values in it have been flushed
      // but we still need to update the indicator byte.
      DCHECK_EQ(literal_count_ % 8, 0);
      DCHECK_EQ(repeat_count_, 8);
      FlushLiteralRun(true);
    } 
    DCHECK_EQ(literal_count_, 0);
    return;
  }

  literal_count_ += num_buffered_values_;
  int num_groups = BitUtil::Ceil(literal_count_, 8);
  if (num_groups + 1 >= (1 << 6)) {
    // We need to start a new literal run because the indicator byte we've reserved
    // cannot store more values.
    DCHECK(literal_indicator_byte_ != NULL);
    FlushLiteralRun(true);
  } else {
    FlushLiteralRun(done);
  } 
  repeat_count_ = 0;
}

inline int RleEncoder::Flush() {
  if (literal_count_ > 0 || repeat_count_ > 0 || num_buffered_values_ > 0) {
    bool all_repeat = literal_count_ == 0 &&
        (repeat_count_ == num_buffered_values_ || num_buffered_values_ == 0);
    // There is something pending, figure out if it's a repeated or literal run
    if (repeat_count_ > 0 && all_repeat) {
      FlushRepeatedRun();
    } else  {
      literal_count_ += num_buffered_values_;
      FlushLiteralRun(true);
      repeat_count_ = 0;
    }
  }
  DCHECK_EQ(num_buffered_values_, 0);
  DCHECK_EQ(literal_count_, 0);
  DCHECK_EQ(repeat_count_, 0);

  int bytes_written = bit_writer_.bytes_written();
  if (bytes_written + BitReader::MAX_VLQ_BYTE_LEN >= bit_writer_.buffer_len()) {
    buffer_full_ = true;
  }
  return bytes_written;
}

inline void RleEncoder::Clear() {
  buffer_full_ = false;
  current_value_ = 0;
  repeat_count_ = 0;
  num_buffered_values_ = 0;
  literal_count_ = 0;
  literal_indicator_byte_ = NULL;
  bit_writer_.Clear();
}

}
#endif