rust: add solution for year 2017, day 18 #123
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I thought it was a little too complicated, and also not well-suited for part 2. I think I can rewrite it and make it simpler.
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This was actually pretty fun. I thought that I was going to need to have multithreading when I first encountered part 2 (which was two years ago), but after studying things like distributed systems, it appears that my thinking has changed a bit.
The solution is based on the fact that we are able to choose our own "scheduling" of the programs. So I chose an alternating approach, where each process runs until it blocks (by not being able to receive anything, or by running to the end of the program) and produces some output. That output is then fed to the other program. As soon as one program blocks and has produced no output, the two programs are deadlocked or terminated.
For example, consider (without loss of generality) that program 0 just finished running and is now blocked or terminated, and it is time for program 1 to run. If the output from program 1 is empty, program 0 will never progress: if it is blocked waiting for input, it will never receive one, since program 1 has not produced any output; if it is terminated, it will never progress. Therefore, the two programs are now deadlocked and/or terminated.