74 KLOC for a decoder? That's ridiculous. Use invokedynamic. Yes, people more typically associate invokedynamic with interpreter implementations or whatever, but it's actually perfect for this use case. Generate the right code on demand and let the JVM cache it so that subsequent invocations are just as fast as if you'd written it by hand.

Jesus Christ this isn't 2005 anymore and people need to learn to use the real power of the JVM. It's stuff like this that sets it apart