zip_file_replace command is compressing the data when the compression is set to store uncompressed

[dillof]

Discussed in #483

^{Originally posted by KRaggett February 14, 2025}
Hi,
I am investigating using libzip to create and modify files containing chunks of data. In my test case my chunks of data are vectors of doubles. I want to create an archive file and at a later date be able to read one of the files from the archive, modify it and replace it with updated data.

I create a zipfile with one directory and within this directory are 10 files containing my data.

I create the file using the following code and it correctly contains all the data uncompressed. (I have removed all error checking for brevity.)

void writeLibzipFile(const std::filesystem::path& zipFilePath, const std::string& folderName, const std::vector<std::vector<double>>& dataBuffers, int numDataPackets)
{
   auto zipFile = zipFilePath;
   zipFile += ".zip";

   int errorp;
   zip_t* archive = zip_open(reinterpret_cast<const char*>(zipFile.u8string().c_str()), ZIP_CREATE | ZIP_EXCL | ZIP_FL_ENC_UTF_8, &errorp);

   // Add the directory to the zip archive
   zip_dir_add(archive, folderName.c_str(), ZIP_FL_ENC_UTF_8);

   // Add files to the directory from the buffer of data
   for (size_t i = 0; i < numDataPackets; ++i) 
   {
      std::string fileName = folderName + "/data" + std::to_string(i) + ".bin";

      zip_source_t* source = zip_source_buffer(archive, dataBuffers[i].data(), dataBuffers[i].size() * sizeof(double), 0);

      // add the file to the zip file
      zip_uint64_t fileIndex = zip_file_add(archive, fileName.c_str(), source, ZIP_FL_ENC_UTF_8);

      // set the file compression to uncompressed
      zip_set_file_compression(archive, fileIndex, ZIP_CM_STORE, 0);
   }
   // Write any changes to the archive
   zip_close(archive);
}

I then open this file and replace the 3rd file of data (data2.bin) with new data using the following code:

void replaceExistingDataset(const std::filesystem::path& inputFilepath, std::string_view datasetName, const std::vector<double>& newDataPacket)
{
   std::filesystem::path zipfile = inputFilepath;

   int zipError;
   // open the existing zip file
   zip_t* archive = zip_open(reinterpret_cast<const char*>(zipfile.u8string().c_str()), ZIP_FL_ENC_UTF_8, &zipError);

   // Read the stats from the zip file
   int fileInZip = zip_get_num_entries(archive, 0);
   zip_stat_t zipStat;
   zip_stat_init(&zipStat);
   
   // create a source buffer from the new data
   zip_source_t* source = zip_source_buffer(archive, newDataPacket.data(), newDataPacket.size() * sizeof(double), 0);

   for (zip_uint64_t fileIndex = 0; fileIndex < fileInZip; ++fileIndex)
   {
      zip_stat_index(archive, fileIndex, 0, &zipStat);

      if (zipStat.name == datasetName)
      {
         // Replace the data in the file with the new data
         zip_file_replace(archive, zipStat.index, source, ZIP_FL_ENC_UTF_8);

         // set the file compression to uncompressed
         zip_set_file_compression(archive, zipStat.index, ZIP_CM_STORE, 0);
      }
   }
   // Write any changes to the archive
   zip_close(archive);
}

When I open the archive in Windows explorer the updated file in the archive has been compressed.

I have tried replacing the compression type with ZIP_CM_DEFAULT; if I do this just in the file replace code then it makes no difference, if I set it in both the file create and the file replace code then when the file is replaced all the files in my archive are compressed.

I have verified the index I am passing to the zip_file_replace() and zip_set_file_compression() functions is the same and the correct index for the file being replaced.
I don't get any errors returned from any of the calls to the libzip functions.

If anyone knows what I am doing wrong or if I am missing a step which will allow me to replace files in an existing archive and set the compression to uncompressed I would really appreciate your help.
Thanks, Kate