File sync logic and associated UI indicators

[kphurley]

• I don't think the files are large enough to have memory concerns, so we can probably just download them, store in memory and compare contents
• This should be useful for this, which appears to read byte-by-byte: https://github.com/rook2pawn/node-filecompare#readme
• We should not assume filenames, so I think the logic should be: download the files to sync to a queue in memory, pop a file off of the queue and search each file in the user's directory for the file, if found, exclude it from the list to sync
• It should be able to get zip archives, and read the raw pgn contents. This approach from stack overflow looks like it will work:
  https://stackoverflow.com/questions/10359485/how-to-download-and-unzip-a-zip-file-in-memory-in-nodejs.
  Here's the zip lib mentioned: https://github.com/cthackers/adm-zip.

[kphurley]

This requires a way for the main process to communicate with the renderer process. It seems like the preferred way to do this is with contextBridge. See this issue in the electron repo with examples: electron/electron#9920 (comment)

[kphurley]

Ok, I at least got it sending from renderer to main, so we just need to get it working in reverse. Looks like we will

• Set the state as "synching"
• Send the URLs over to the main process
• Download the contents at those URLs and extract them to a data structure in memory
• Compare each file with what's there already in the indicated directory
• Send back any urls not found
• Set the UI state to either data available or up to date as appropriate

[kphurley]

It seems like adm-zip only works if the file is already on disk. I wonder now if we should just maintain a local cache of all of the zips on disk regardless, and just use the UI as a way to get the desired files to the location specified.