Function encode

src/stream.cpp:649–725

    };

    static fec_t encode(const std::string_view &payload, size_t blocksize, size_t fecpercentage, size_t minparityshards, size_t prefixsize) {
      auto payload_size = payload.size();

      auto pad = payload_size % blocksize != 0;

      auto aligned_data_shards = payload_size / blocksize;
      auto data_shards = aligned_data_shards + (pad ? 1 : 0);
      auto parity_shards = (data_shards * fecpercentage + 99) / 100;

      // increase the FEC percentage for this frame if the parity shard minimum is not met
      if (parity_shards < minparityshards && fecpercentage != 0) {
        parity_shards = minparityshards;
        fecpercentage = (100 * parity_shards) / data_shards;

        BOOST_LOG(verbose) << "Increasing FEC percentage to "sv << fecpercentage << " to meet parity shard minimum"sv << std::endl;
      }

      auto nr_shards = data_shards + parity_shards;

      // If we need to store a zero-padded data shard, allocate that first to
      // to keep the shards in order and reduce buffer fragmentation
      auto parity_shard_offset = pad ? 1 : 0;
      util::buffer_t<char> shards {(parity_shard_offset + parity_shards) * blocksize};
      util::buffer_t<uint8_t *> shards_p {nr_shards};
      std::vector<platf::buffer_descriptor_t> payload_buffers;
      payload_buffers.reserve(2);

      // Point into the payload buffer for all except the final padded data shard
      auto next = std::begin(payload);
      for (auto x = 0; x < aligned_data_shards; ++x) {
        shards_p[x] = (uint8_t *) next;
        next += blocksize;
      }
      payload_buffers.emplace_back(std::begin(payload), aligned_data_shards * blocksize);

      // If the last data shard needs to be zero-padded, we must use the shards buffer
      if (pad) {
        shards_p[aligned_data_shards] = (uint8_t *) &shards[0];

        // GCC doesn't figure out that std::copy_n() can be replaced with memcpy() here
        // and ends up compiling a horribly slow element-by-element copy loop, so we
        // help it by using memcpy()/memset() directly.
        auto copy_len = std::min<size_t>(blocksize, std::end(payload) - next);
        std::memcpy(shards_p[aligned_data_shards], next, copy_len);
        if (copy_len < blocksize) {
          // Zero any additional space after the end of the payload
          std::memset(shards_p[aligned_data_shards] + copy_len, 0, blocksize - copy_len);
        }
      }

      // Add a payload buffer describing the shard buffer
      payload_buffers.emplace_back(std::begin(shards), shards.size());

      if (fecpercentage != 0) {
        // Point into our allocated buffer for the parity shards
        for (auto x = 0; x < parity_shards; ++x) {
          shards_p[data_shards + x] = (uint8_t *) &shards[(parity_shard_offset + x) * blocksize];
        }