This allows using someone else's data to compare runtimes, behavior, etc. without having to recompile. Since it's patched into the function that all days use to read, it's incompatible with running all days, which I feel is a reasonable compromise and behavior expectation.
The Mode() check is how the internet says you can test if you should even try to look at stdin, and the Size() check ensures that there's actually data to be read instead of just an open stdin handle (running in VSCode with a debugger seems to keep the stdin handle open, for example, so it passes the Mode() check and then hangs when trying to read since there's nothing to actually read).
This makes it slightly easier to adjust VSCode's launch.json to hop around debugging different days. Not much, but a little. And every little bit helps!
