bitreader.h

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/*
 * Copyright (c) 2022 Scott Theisen
 *
 * This file is part of MythTV.
 *
 * MythTV is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * MythTV is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with MythTV; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */
 
#ifndef BIT_READER_H
#define BIT_READER_H
 
#include <array>
#include <cstdint>
#include <climits> // for CHAR_BIT
 
#if __has_include(<bit>) // C++20
#include <bit>
#endif
 
class BitReader
{
  public:
    BitReader(const uint8_t* buffer, uint64_t size) :
        m_buffer(buffer),
        m_bufferEnd(buffer + size)
    {
    }
    ~BitReader() = default;
 
    void skip_bits(unsigned n)
    {
        if (m_cacheSize > n)
        {
            m_cache <<= n;
            m_cacheSize -= n;
        }
        else
        {
            n -= m_cacheSize;
            m_cacheSize = 0;
            m_cache      = 0;
            m_bitIndex += n;
            int quotient = m_bitIndex / CHAR_BIT;
            m_bitIndex %= CHAR_BIT;
            m_buffer    += quotient;
        }
    }
    uint32_t show_bits(unsigned n)
    {
        //assert(n <= 32);
        if (m_cacheSize < n)
        {
            refill_cache(32);
        }
        return uint32_t(get_upper_bits(m_cache, n));
    }
    uint64_t show_bits64(unsigned n)
    {
        //assert(n <= 64);
        if (m_cacheSize < n)
        {
            refill_cache(64);
        }
        return get_upper_bits(m_cache, n);
    }
    bool next_bit() { return get_bits(1) == 1; }
    uint32_t get_bits(unsigned n)
    {
        uint32_t ret = show_bits(n);
        skip_bits(n);
        return ret;
    }
    uint64_t get_bits64(unsigned n)
    {
        uint64_t ret = show_bits64(n);
        skip_bits(n);
        return ret;
    }
 
    int get_ue_golomb()
    {
        return get_ue_golomb(13);
    }
 
    uint32_t get_ue_golomb_long()
    {
        uint32_t buf    = show_bits(32);
        unsigned lz     = clz(buf);
        unsigned length = lz + 1;
        skip_bits(lz);
 
        if (length > 32)
        {
            skip_bits(length);
            return UINT32_MAX;
        }
 
        return get_bits(length) - 1;
    }
 
    int get_ue_golomb_31()
    {
        return get_ue_golomb(5);
    }
 
    int get_se_golomb()
    {
        uint32_t buf = get_ue_golomb_long() + 1;
        // revert the -1 offset to get the raw value
 
        if (buf & 1)
        {
            return -(buf >> 1);
        }
        return buf >> 1;
    }
 
    int64_t get_bits_left()
    {
        return m_cacheSize + CHAR_BIT * int64_t(m_bufferEnd - m_buffer) - m_bitIndex;
    }
 
  private:
    int get_ue_golomb(unsigned max_length);
    static unsigned clz(uint32_t v)
    {
#if defined(__cpp_lib_bitops) && __cpp_lib_bitops >= 201907L // C++20 <bit>
        return std::countl_zero(v);
#else
        // based on public domain log2_floor
        // http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn
        if (v == 0)
        {
            return 32;
        }
        static constexpr std::array<int,32> MultiplyDeBruijnBitPosition =
        {
             0,  9,  1, 10, 13, 21,  2, 29, 11, 14, 16, 18, 22, 25,  3, 30,
             8, 12, 20, 28, 15, 17, 24,  7, 19, 27, 23,  6, 26,  5,  4, 31
        };
 
        v |= v >> 1; // first round down to one less than a power of 2 
        v |= v >> 2;
        v |= v >> 4;
        v |= v >> 8;
        v |= v >> 16;
 
        return 31 - MultiplyDeBruijnBitPosition[(uint32_t)(v * 0x07C4ACDDU) >> 27];
#endif
    }
 
    static constexpr uint64_t get_upper_bits(uint64_t val, unsigned bits)
    {
        // protect against undefined behavior
        if (bits == 0)
        {
            return UINT64_C(0);
        }
        if (bits > 64)
        {
            return val;
        }
        return val >> (64 - bits);
    }
 
    void refill_cache(unsigned min_bits);
 
    const uint8_t *m_buffer; 
    const uint8_t * const m_bufferEnd; 
    unsigned m_bitIndex {0}; 
 
    uint64_t m_cache {0};
    unsigned m_cacheSize {0};
 
};
 
inline int BitReader::get_ue_golomb(unsigned max_length)
{
    uint32_t buf    = show_bits(32);
    unsigned lz     = clz(buf);
    unsigned length = lz + 1;
    unsigned size   = lz + length; // total length
 
    skip_bits(size);
 
    if (length > max_length || length > 16)
    {
        return -1;
    }
 
    buf >>= 32 - size; // remove irrelevant trailing bits
 
    return buf - 1; // offset for storing 0
}
 
inline void BitReader::refill_cache(unsigned min_bits)
{
    while (m_cacheSize < min_bits && m_buffer < m_bufferEnd)
    {
        int shift = 64 - m_cacheSize;
        int bits = CHAR_BIT - m_bitIndex;
        if (shift > bits)
        {
            m_cache |= int64_t(*m_buffer & ((1 << bits) - 1)) << (shift - bits);
            m_bitIndex   = 0;
            m_buffer++;
            m_cacheSize += bits;
        }
        else
        {
            m_cache |= int64_t(*m_buffer & ((1 << bits) - 1)) >> (bits - shift);
            m_bitIndex  += shift;
            m_cacheSize += shift;
        }
 
    }
}
 
#endif // BIT_READER_H