SDRPlusPlus/decoder_modules/m17_decoder/src/golay24.h

// Copyright 2020 Rob Riggs <rob@mobilinkd.com>
// All rights reserved.

#pragma once

#include <array>
#include <cstdint>
#include <algorithm>
#include <utility>

namespace mobilinkd {

    // Parts are adapted from:
    // http://aqdi.com/articles/using-the-golay-error-detection-and-correction-code-3/

    namespace Golay24 {

        int popcount(uint32_t n) {
            int count = 0;
            for (int i = 0; i < 32; i++) {
                count += ((n >> i) & 1);
            }
            return count;
        }

        namespace detail {

            // Need a constexpr sort.
            // https://stackoverflow.com/a/40030044/854133
            template <class T>
            void swap(T& l, T& r) {
                T tmp = std::move(l);
                l = std::move(r);
                r = std::move(tmp);
            }

            template <typename T, size_t N>
            struct array {
                constexpr T& operator[](size_t i) {
                    return arr[i];
                }

                constexpr const T& operator[](size_t i) const {
                    return arr[i];
                }

                constexpr const T* begin() const {
                    return arr;
                }
                constexpr const T* end() const {
                    return arr + N;
                }

                T arr[N];
            };

            template <typename T, size_t N>
            void sort_impl(array<T, N>& array, size_t left, size_t right) {
                if (left < right) {
                    size_t m = left;

                    for (size_t i = left + 1; i < right; i++)
                        if (array[i] < array[left])
                            swap(array[++m], array[i]);

                    swap(array[left], array[m]);

                    sort_impl(array, left, m);
                    sort_impl(array, m + 1, right);
                }
            }

            template <typename T, size_t N>
            array<T, N> sort(array<T, N> array) {
                auto sorted = array;
                sort_impl(sorted, 0, N);
                return sorted;
            }

        } // detail

        // static constexpr uint16_t POLY = 0xAE3;
        constexpr uint16_t POLY = 0xC75;

#pragma pack(push, 1)
        struct SyndromeMapEntry {
            uint32_t a{ 0 };
            uint16_t b{ 0 };
        };
#pragma pack(pop)

        /**
 * Calculate the syndrome of a [23,12] Golay codeword.
 *
 * @return the 11-bit syndrome of the codeword in bits [22:12].
 */
        uint32_t syndrome(uint32_t codeword) {
            codeword &= 0xffffffl;
            for (size_t i = 0; i != 12; ++i) {
                if (codeword & 1)
                    codeword ^= POLY;
                codeword >>= 1;
            }
            return (codeword << 12);
        }

        bool parity(uint32_t codeword) {
            return popcount(codeword) & 1;
        }

        SyndromeMapEntry makeSyndromeMapEntry(uint64_t val) {
            return SyndromeMapEntry{ uint32_t(val >> 16), uint16_t(val & 0xFFFF) };
        }

        uint64_t makeSME(uint64_t syndrome, uint32_t bits) {
            return (syndrome << 24) | (bits & 0xFFFFFF);
        }

        constexpr size_t LUT_SIZE = 2048;

        std::array<SyndromeMapEntry, LUT_SIZE> make_lut() {
            constexpr size_t VECLEN = 23;
            detail::array<uint64_t, LUT_SIZE> result{};

            size_t index = 0;
            result[index++] = makeSME(syndrome(0), 0);

            for (size_t i = 0; i != VECLEN; ++i) {
                auto v = (1 << i);
                result[index++] = makeSME(syndrome(v), v);
            }

            for (size_t i = 0; i != VECLEN - 1; ++i) {
                for (size_t j = i + 1; j != VECLEN; ++j) {
                    auto v = (1 << i) | (1 << j);
                    result[index++] = makeSME(syndrome(v), v);
                }
            }

            for (size_t i = 0; i != VECLEN - 2; ++i) {
                for (size_t j = i + 1; j != VECLEN - 1; ++j) {
                    for (size_t k = j + 1; k != VECLEN; ++k) {
                        auto v = (1 << i) | (1 << j) | (1 << k);
                        result[index++] = makeSME(syndrome(v), v);
                    }
                }
            }

            result = detail::sort(result);

            std::array<SyndromeMapEntry, LUT_SIZE> tmp;
            for (size_t i = 0; i != LUT_SIZE; ++i) {
                tmp[i] = makeSyndromeMapEntry(result[i]);
            }

            return tmp;
        }

        inline auto LUT = make_lut();

        /**
 * Calculate [23,12] Golay codeword.
 *
 * @return checkbits(11)|data(12).
 */
        uint32_t encode23(uint16_t data) {
            // data &= 0xfff;
            uint32_t codeword = data;
            for (size_t i = 0; i != 12; ++i) {
                if (codeword & 1)
                    codeword ^= POLY;
                codeword >>= 1;
            }
            return codeword | (data << 11);
        }

        uint32_t encode24(uint16_t data) {
            auto codeword = encode23(data);
            return ((codeword << 1) | parity(codeword));
        }

        bool decode(uint32_t input, uint32_t& output) {
            auto syndrm = syndrome(input >> 1);
            auto it = std::lower_bound(LUT.begin(), LUT.end(), syndrm,
                                       [](const SyndromeMapEntry& sme, uint32_t val) {
                                           return (sme.a >> 8) < val;
                                       });

            if ((it->a >> 8) == syndrm) {
                // Build the correction from the compressed entry.
                auto correction = ((((it->a & 0xFF) << 16) | it->b) << 1);
                // Apply the correction to the input.
                output = input ^ correction;
                // Only test parity for 3-bit errors.
                return popcount(syndrm) < 3 || !parity(output);
            }

            return false;
        }

    } // Golay24

} // mobilinkd