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/** \file     BitStream.cpp

    \brief    class for handling bitstream

*/

#include "BitStream.h"

#include <stdint.h>

#include <vector>

#include <string.h>

#include <memory.h>

namespace vvdec

{

void InputBitstream::resetToStart()

{

  m_fifo_idx      = 0;

  m_num_held_bits = 0;

  m_held_bits     = 0;

#if ENABLE_TRACING

  m_numBitsRead   = 0;

#endif

}

/**

 * read uiNumberOfBits from bitstream without updating the bitstream

 * state, storing the result in ruiBits.

 *

 * If reading uiNumberOfBits would overrun the bitstream buffer,

 * the bitstream is effectively padded with sufficient zero-bits to

 * avoid the overrun.

 */

uint32_t InputBitstream::peekBits( uint32_t uiNumberOfBits )

{

  auto saved_fifo_idx      = m_fifo_idx;

  auto saved_num_held_bits = m_num_held_bits;

  auto saved_held_bits     = m_held_bits;

#if ENABLE_TRACING

  auto saved_numBitsRead   = m_numBitsRead;

#endif

  uint32_t num_bits_to_read = std::min(uiNumberOfBits, getNumBitsLeft());

  uint32_t uiBits           = read( num_bits_to_read );

  uiBits <<= (uiNumberOfBits - num_bits_to_read);

  m_fifo_idx      = saved_fifo_idx;

  m_num_held_bits = saved_num_held_bits;

  m_held_bits     = saved_held_bits;

#if ENABLE_TRACING

  m_numBitsRead   = saved_numBitsRead;

#endif

  return uiBits;

}

uint32_t InputBitstream::read (uint32_t uiNumberOfBits)

{

#if ENABLE_TRACING

  m_numBitsRead += uiNumberOfBits;

#endif

  constexpr static uint64_t ONES = ~static_cast<uint64_t>( 0 );   // need to ensure 64 bits for the mask, because shift by 32 is UB for uin32_t

  /* NB, bits are extracted from the MSB of each byte. */

  uint32_t retval = 0;

  if (uiNumberOfBits <= m_num_held_bits)

  {

    /* n=1, len(H)=7:   -VHH HHHH, shift_down=6, mask=0xfe

     * n=3, len(H)=7:   -VVV HHHH, shift_down=4, mask=0xf8

     */

    retval = static_cast<uint32_t>( m_held_bits >> ( m_num_held_bits - uiNumberOfBits ) );

    retval &= ~( ONES << uiNumberOfBits );

    m_num_held_bits -= uiNumberOfBits;

    return retval;

  }

  CHECK( uiNumberOfBits > 32, "Too many bits read" );

  if( m_num_held_bits )

  {

    /* all num_held_bits will go into retval

     *   => need to mask leftover bits from previous extractions

     *   => align retval with top of extracted word */

    /* n=5, len(H)=3: ---- -VVV, mask=0x07, shift_up=5-3=2,

     * n=9, len(H)=3: ---- -VVV, mask=0x07, shift_up=9-3=6 */

    uiNumberOfBits -= m_num_held_bits;

    retval = static_cast<uint32_t>( m_held_bits ) & ~( ONES << m_num_held_bits );   // we can cast to 32 bits, because the held bits are the rightmost bits

    retval <<= uiNumberOfBits;

  }

  /* number of whole bytes that need to be loaded to form retval */

  /* n=32, len(H)=0, load 4bytes, shift_down=0

   * n=32, len(H)=1, load 4bytes, shift_down=1

   * n=31, len(H)=1, load 4bytes, shift_down=1+1

   * n=8,  len(H)=0, load 1byte,  shift_down=0

   * n=8,  len(H)=3, load 1byte,  shift_down=3

   * n=5,  len(H)=1, load 1byte,  shift_down=1+3

   */

  load_next_bits( uiNumberOfBits );

  /* resolve remainder bits */

  m_num_held_bits -= uiNumberOfBits;

  /* copy required part of m_held_bits into retval */

  retval |= static_cast<uint32_t>( m_held_bits >> m_num_held_bits );

  return retval;

}

uint32_t InputBitstream::readOutTrailingBits ()

{

  uint32_t count = 0;

  while( ( getNumBitsLeft() > 0 ) && ( getNumBitsUntilByteAligned() != 0 ) )

  {

    count++;

    read( 1 );

  }

  return count;

}

/**

 Extract substream from the current bitstream.

 \param  uiNumBits    number of bits to transfer

 */

std::unique_ptr<InputBitstream> InputBitstream::extractSubstream( uint32_t uiNumBits )

{

  std::unique_ptr<InputBitstream> substream( new InputBitstream );

  AlignedByteVec& buf = substream->getFifo();

  buf.reserve( ( uiNumBits + 7 ) / 8 + 1 );    // +1 because a zero byte might be added later

  const uint32_t uiNumBytes = uiNumBits / 8;   // don't round up here, because the remaing bits will be copied later

  if( m_num_held_bits == 0 )

  {

    CHECK( m_fifo_idx + uiNumBytes > m_fifo.size(), "Exceeded FIFO size" );

    std::size_t currentOutputBufferSize = buf.size();

    buf.resize( currentOutputBufferSize + uiNumBytes );

    const uint32_t uiNumBytesToReadFromFifo = std::min<uint32_t>( uiNumBytes, (uint32_t) m_fifo.size() - m_fifo_idx );

    memcpy( &( buf[currentOutputBufferSize] ), &( m_fifo[m_fifo_idx] ), uiNumBytesToReadFromFifo );

    m_fifo_idx += uiNumBytesToReadFromFifo;

    if( uiNumBytesToReadFromFifo != uiNumBytes )

    {

      memset( &( buf[currentOutputBufferSize + uiNumBytesToReadFromFifo] ), 0, uiNumBytes - uiNumBytesToReadFromFifo );

    }

  }

  else

  {

    for( uint32_t ui = 0; ui < uiNumBytes; ui++ )

    {

      uint32_t uiByte = read( 8 );

      buf.push_back( uiByte );

    }

  }

  if( uiNumBits & 0x7 )

  {

    uint32_t uiByte = read( uiNumBits & 0x7 );

    uiByte <<= 8 - ( uiNumBits & 0x7 );

    buf.push_back( uiByte );

  }

  return substream;

}

uint32_t InputBitstream::readByteAlignment()

{

  uint32_t code = read( 1 );

  CHECK( code != 1, "Code is not '1'" );

  uint32_t numBits = getNumBitsUntilByteAligned();

  if(numBits)

  {

    CHECK( numBits > getNumBitsLeft(), "More bits available than left" );

    code = read( numBits );

    CHECK( code != 0, "Code not '0'" );

  }

  return numBits+1;

}

}