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/** \file     BinDecoder.cpp

 *  \brief    Low level binary symbol writer

 */

#include "BinDecoder.h"

#include "CommonLib/Rom.h"

#include "CommonLib/BitStream.h"

#include "CommonLib/dtrace_next.h"

#include <limits>

#define CNT_OFFSET 0

namespace vvdec

{

// BinDecoder uses the `signed >> 31` sign-extract idiom, which is implementation-defined

// for negative operands before C++20. Require two's-complement representation so the idiom

// behaves as intended (mandatory anyway in C++20).

static_assert( std::numeric_limits<int>::min() == -std::numeric_limits<int>::max() - 1,

               "two's complement signed integer representation required" );

static_assert( ( -1 >> 1 ) == -1,

               "right shift of negative number needs to be implemented as arithmetic shift." );

void BinDecoder::init( InputBitstream* bitstream )

{

  m_Bitstream = bitstream;

}

void BinDecoder::uninit()

{

  m_Bitstream = 0;

}

void BinDecoder::start()

{

  CHECK( m_Bitstream->getNumBitsUntilByteAligned(), "Bitstream is not byte aligned." );

  m_Range       = 510;

  m_Value       = ( uint32_t( m_Bitstream->readByte() ) << 8 ) + m_Bitstream->readByte();

  m_bitsNeeded  = -8;

}

void BinDecoder::finish()

{

  unsigned lastByte = m_Bitstream->peekPreviousByte();

  CHECK( ( ( lastByte << ( 8 + m_bitsNeeded ) ) & 0xff ) != 0x80,

        "No proper stop/alignment pattern at end of CABAC stream." );

}

void BinDecoder::reset( int qp, int initId )

{

  m_Ctx.init( qp, initId );

  start();

}

// decodeBinEP() is defined ALWAYS_INLINE in BinDecoder.h (moved for inlining into the CABAC hot loops).

unsigned BinDecoder::decodeBinsEP( unsigned numBins )

{

#if ENABLE_TRACING

  int numBinsOrig = numBins;

#endif

  if( m_Range == 256 )

  {

    return decodeAlignedBinsEP( numBins );

  }

  unsigned remBins = numBins;

  unsigned bins    = 0;

  unsigned value   = m_Value;

  unsigned range = m_Range;

  int bitsNeeded = m_bitsNeeded;

  while( remBins > 8 )

  {

    value       = ( value << 8 ) + ( m_Bitstream->readByte() << ( 8 + bitsNeeded ) );

    unsigned SR =   range << 15;

    for( int i = 0; i < 8; i++ )

    {

      bins += bins;

      SR  >>= 1;

      if( value >= SR )

      {

        bins    ++;

        value -= SR;

      }

    }

    remBins -= 8;

  }

  bitsNeeded   += (int)remBins;

  value       <<= remBins;

  if( bitsNeeded >= 0 )

  {

    value      += m_Bitstream->readByte() << bitsNeeded;

    bitsNeeded -= 8;

  }

  unsigned SR = range << ( remBins + 7 );

  for ( unsigned i = 0; i < remBins; i++ )

  {

    bins += bins;

    SR  >>= 1;

    if( value >= SR )

    {

      bins    ++;

      value -= SR;

    }

  }

  m_Value = value;

  m_Range = range;

  m_bitsNeeded = bitsNeeded;

#if ENABLE_TRACING

  for( int i = 0; i < numBinsOrig; i++ )

  {

    DTRACE( g_trace_ctx, D_CABAC, "%d" "  " "%d" "  EP=%d \n", DTRACE_GET_COUNTER( g_trace_ctx, D_CABAC ), m_Range, ( bins >> ( numBinsOrig - 1 - i ) ) & 1 );

  }

#endif

  return bins;

}

unsigned BinDecoder::decodeRemAbsEP(unsigned goRicePar, unsigned cutoff, int maxLog2TrDynamicRange)

{

  unsigned prefix = 0;

  {

    const unsigned  maxPrefix = 32 - maxLog2TrDynamicRange;

    unsigned        codeWord = 0;

    do

    {

      prefix++;

      codeWord = decodeBinEP();

    } while (codeWord && prefix < maxPrefix);

    prefix -= 1 - (int)codeWord;

  }

  unsigned length = goRicePar, offset;

  if (prefix < cutoff)

  {

    offset = prefix << goRicePar;

  }

  else

  {

    offset = (((1u << (prefix - cutoff)) + cutoff - 1) << goRicePar);

    {

      length += (prefix == (32u - maxLog2TrDynamicRange) ? maxLog2TrDynamicRange - goRicePar : prefix - cutoff);

    }

  }

  return offset + decodeBinsEP(length);

}

// decodeBinTrm() is defined ALWAYS_INLINE in BinDecoder.h (moved for inlining into the CABAC hot loops).

void BinDecoder::align()

{

  m_Range = 256;

}

#if ENABLE_TRACING

unsigned int BinDecoder::getNumBitsRead() const

{

  return m_Bitstream->getNumBitsRead() + m_bitsNeeded;

}

#endif   // ENABLE_TRACING

unsigned BinDecoder::decodeAlignedBinsEP( unsigned numBins )

{

#if ENABLE_TRACING

  int numBinsOrig = numBins;

#endif

  unsigned remBins = numBins;

  unsigned bins    = 0;

  while( remBins > 0 )

  {

    // The MSB of m_Value is known to be 0 because range is 256. Therefore:

    //   > The comparison against the symbol range of 128 is simply a test on the next-most-significant bit

    //   > "Subtracting" the symbol range if the decoded bin is 1 simply involves clearing that bit.

    //  As a result, the required bins are simply the <binsToRead> next-most-significant bits of m_Value

    //  (m_Value is stored MSB-aligned in a 16-bit buffer - hence the shift of 15)

    //

    //    m_Value = |0|V|V|V|V|V|V|V|V|B|B|B|B|B|B|B|        (V = usable bit, B = potential buffered bit (buffer refills when m_bitsNeeded >= 0))

    //

    unsigned binsToRead = std::min<unsigned>( remBins, 8 ); //read bytes if able to take advantage of the system's byte-read function

    unsigned binMask    = ( 1 << binsToRead ) - 1;

    unsigned newBins    = ( m_Value >> (15 - binsToRead) ) & binMask;

    bins                = ( bins    << binsToRead) | newBins;

    m_Value             = ( m_Value << binsToRead) & 0x7FFF;

    remBins            -= binsToRead;

    m_bitsNeeded       += (int)binsToRead;

    if( m_bitsNeeded >= 0 )

    {

      m_Value          |= m_Bitstream->readByte() << m_bitsNeeded;

      m_bitsNeeded     -= 8;

    }

  }

#if ENABLE_TRACING

  for( int i = 0; i < numBinsOrig; i++ )

  {

    DTRACE( g_trace_ctx, D_CABAC, "%d" "  " "%d" "  " "EP=%d \n", DTRACE_GET_COUNTER( g_trace_ctx, D_CABAC ), m_Range, ( bins >> ( numBinsOrig - 1 - i ) ) & 1 );

  }

#endif

  return bins;

}

// decodeBin() is defined ALWAYS_INLINE in BinDecoder.h (moved for inlining into the CABAC hot loops).

}