/***************************************************************************
 *   Copyright (C) 2021 by Federico Amedeo Izzo IU2NUO,                    *
 *                         Niccolò Izzo IU2KIN                             *
 *                         Wojciech Kaczmarski SP5WWP                      *
 *                         Frederik Saraci IU2NRO                          *
 *                         Silvano Seva IU2KWO                             *
 *                                                                         *
 *                                                                         *
 *   This program is free software; you can redistribute it and/or modify  *
 *   it under the terms of the GNU General Public License as published by  *
 *   the Free Software Foundation; either version 3 of the License, or     *
 *   (at your option) any later version.                                   *
 *                                                                         *
 *   This program 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 General Public License for more details.                          *
 *                                                                         *
 *   You should have received a copy of the GNU General Public License     *
 *   along with this program; if not, see <http://www.gnu.org/licenses/>   *
 ***************************************************************************/

#ifndef M17_GOLAY_H
#define M17_GOLAY_H

#ifndef __cplusplus
#error This header is C++ only!
#endif

#include <cstdint>

namespace Golay24
{

/**
 * Data encoding matrix for Golay(24,12) code with generator polynomial 0xC75;
 */
static const uint16_t encode_matrix[12] =
{
    0x8eb, 0x93e, 0xa97, 0xdc6, 0x367, 0x6cd,
    0xd99, 0x3da, 0x7b4, 0xf68, 0x63b, 0xc75
};


/**
 * Data decoding matrix for Golay(24,12) code with generator polynomial 0xC75;
 */
static const uint16_t decode_matrix[12] =
{
    0xc75, 0x49f, 0x93e, 0x6e3, 0xdc6, 0xf13,
    0xab9, 0x1ed, 0x3da, 0x7b4, 0xf68, 0xa4f
};


/**
 * Function computing the Golay(24,12) checksum of a given 12-bit data block.
 *
 * @param value: input data.
 * @return Golay(24,12) checksum.
 */
static uint16_t calcChecksum(const uint16_t& value)
{
    uint16_t checksum = 0;

    for(uint8_t i = 0; i < 12; i++)
    {
        if(value & (1 << i))
        {
            checksum ^= encode_matrix[i];
        }
    }

    return checksum;
}


/**
 * Detect and correct errors in a Golay(24,12) codeword.
 *
 * @param codeword: input codeword.
 * @return bitmask corresponding to detected bit errors in the codeword, or
 * 0xFFFFFFFF if bit errors are unrecoverable.
 */
static uint32_t detectErrors(const uint32_t& codeword)
{
    uint16_t data   = codeword >> 12;
    uint16_t parity = codeword & 0xFFF;

    uint16_t syndrome = parity ^ calcChecksum(data);

    if(__builtin_popcount(syndrome) <= 3)
    {
        return syndrome;
    }

    for(uint8_t i = 0; i<12; i++)
    {
        uint16_t e = 1 << i;
        uint16_t coded_error = encode_matrix[i];

        if(__builtin_popcount(syndrome^coded_error) <= 2)
        {
            return (e << 12) | (syndrome ^ coded_error);
        }
    }


    uint16_t inv_syndrome = 0;
    for(uint8_t i = 0; i < 12; i++)
    {
        if(syndrome & (1 << i))
        {
            inv_syndrome ^= decode_matrix[i];
        }
    }

    if(__builtin_popcount(inv_syndrome) <= 3)
    {
        return inv_syndrome << 12;
    }

    for(uint8_t i = 0; i < 12; i++ )
    {
        uint16_t e = 1 << i;
        uint16_t coding_error = decode_matrix[i];
        if(__builtin_popcount(inv_syndrome ^ coding_error) <= 2 )
        {
            return ((inv_syndrome ^ coding_error) << 12) | e;
        }
    }

    return 0xFFFFFFFF;
}

}   // namespace Golay24


/**
 * Compute the Golay(24,12) codeword of a given block of data using the
 * generator polynomial 0xC75.
 * Result is composed as follows:
 *
 * +--------+----------+--------+
 * | parity | checksum |  data  |
 * +--------+----------+--------+
 * | 1 bit  |   11 bit | 12 bit |
 * +--------+----------+--------+
 *
 * \param data: input data, upper four bits are discarded.
 * \return resulting 24 bit codeword.
 */
static inline uint32_t golay24_encode(const uint16_t& data)
{
    return (data << 12) | Golay24::calcChecksum(data);
}


/**
 * Decode a Golay(24,12) codeword, correcting eventual bit errors. In case the
 * bit errors are not correctable, the function returns 0xFFFF, a value outside
 * the range allowed for the 12-bit input data required by Golay coding.
 *
 * \param codeword: input Golay(24,12) codeword.
 * \return original data block or 0xFFFF in case of unrecoverable errors.
 */
static inline uint16_t golay24_decode(const uint32_t& codeword)
{
    uint32_t errors = Golay24::detectErrors(codeword);
    if(errors == 0xFFFFFFFF) return 0xFFFF;
    return ((codeword ^ errors) >> 12) & 0x0FFF;
}

#endif /* M17_GOLAY_H */