folded_integer_bit_encoder.h

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// Copyright 2016 The Draco Authors.
//
// 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.
//
// File provides direct encoding of bits with arithmetic encoder interface.
#ifndef DRACO_COMPRESSION_BIT_CODERS_FOLDED_INTEGER_BIT_ENCODER_H_
#define DRACO_COMPRESSION_BIT_CODERS_FOLDED_INTEGER_BIT_ENCODER_H_
 
#include <vector>
 
#include "draco/core/encoder_buffer.h"
 
namespace draco {
 
// This coding scheme considers every bit of an (up to) 32bit integer as a
// separate context. This can be a significant advantage when encoding numbers
// where it is more likely that the front bits are zero.
// The behavior is essentially the same as other arithmetic encoding schemes,
// the only difference is that encoding and decoding of bits must be absolutely
// symmetric, bits handed in by EncodeBit32 must be also decoded in this way.
// This is the FoldedBit32Encoder, see also FoldedBit32Decoder.
template <class BitEncoderT>
class FoldedBit32Encoder {
 public:
  FoldedBit32Encoder() {}
  ~FoldedBit32Encoder() {}
 
  // Must be called before any Encode* function is called.
  void StartEncoding() {
    for (int i = 0; i < 32; i++) {
      folded_number_encoders_[i].StartEncoding();
    }
    bit_encoder_.StartEncoding();
  }
 
  // Encode one bit. If |bit| is true encode a 1, otherwise encode a 0.
  void EncodeBit(bool bit) { bit_encoder_.EncodeBit(bit); }
 
  // Encode |nbits| of |value|, starting from the least significant bit.
  // |nbits| must be > 0 and <= 32.
  void EncodeLeastSignificantBits32(int nbits, uint32_t value) {
    uint32_t selector = 1 << (nbits - 1);
    for (int i = 0; i < nbits; i++) {
      const bool bit = (value & selector);
      folded_number_encoders_[i].EncodeBit(bit);
      selector = selector >> 1;
    }
  }
 
  // Ends the bit encoding and stores the result into the target_buffer.
  void EndEncoding(EncoderBuffer *target_buffer) {
    for (int i = 0; i < 32; i++) {
      folded_number_encoders_[i].EndEncoding(target_buffer);
    }
    bit_encoder_.EndEncoding(target_buffer);
  }
 
 private:
  void Clear() {
    for (int i = 0; i < 32; i++) {
      folded_number_encoders_[i].Clear();
    }
    bit_encoder_.Clear();
  }
 
  std::array<BitEncoderT, 32> folded_number_encoders_;
  BitEncoderT bit_encoder_;
};
 
}  // namespace draco
 
#endif  // DRACO_COMPRESSION_BIT_CODERS_FOLDED_INTEGER_BIT_ENCODER_H_