// Copyright (c) Google LLC 2019

//

// Use of this source code is governed by an MIT-style

// license that can be found in the LICENSE file or at

// https://opensource.org/licenses/MIT.

#ifndef BRUNSLI_DEC_ARITH_DECODE_H_

#define BRUNSLI_DEC_ARITH_DECODE_H_

#include "../common/distributions.h"

#include <brunsli/types.h>

#include "./brunsli_input.h"

namespace brunsli {

// A class used for entropy decoding a sequence of binary values.

// skal@ wrote the original version, szabadka@ ported it for brunsli.

class BinaryArithmeticDecoder {

 public:

  BinaryArithmeticDecoder() {}

  void Init(WordSource* in) {

    low_ = 0;

    high_ = ~0u;

    value_ = in->GetNextWord();

    value_ = (value_ << 16u) | in->GetNextWord();

  }

  // Returns the next bit decoded from the bit stream, based on the given 8-bit

  // precision probability, i.e. P(bit = 0) = prob / 256. This probability must

  // be the same as the one used by the encoder.

  int ReadBit(int prob, WordSource* in) {

    const uint32_t diff = high_ - low_;

    const uint32_t split = low_ + (((uint64_t)diff * prob) >> 8u);

    int bit;

    if (value_ > split) {

      low_ = split + 1;

      bit = 1;

    } else {

      high_ = split;

      bit = 0;

    }

    if (((low_ ^ high_) >> 16u) == 0) {

      value_ = (value_ << 16u) | in->GetNextWord();

      low_ <<= 16u;

      high_ <<= 16u;

      high_ |= 0xFFFFu;

    }

    return bit;

  }

 private:

  uint32_t low_;

  uint32_t high_;

  uint32_t value_;

};

}  // namespace brunsli

#endif  // BRUNSLI_DEC_ARITH_DECODE_H_