Port a game's rules before its physics, driven by a recorded run

What to know

A game's rules (laps, checkpoints, the order, the end, the results) can be ported and proved exact before its physics, if the core takes the actors' motion as an input and a recording of the original supplies it. In POD the race rules (crates/pod-race) replay a recorded race and end with every word of the eight race records, the order and the results screen exactly the original's.

What made it work:

• Cut the core where the original cuts. POD's physics step moves a car and then runs the gate test on the move. The core's step takes each car's move (the point before and after) and runs the gate test and the rules; drawing, sound and HUD leave as events. The same entry point serves the physics port later.
• Keep the original's memory layout where the results are compared. The race record is kept as its 47 words, pointers stored as the game's addresses, so the final records compare byte for byte with the recording.
• Measure the arithmetic first (re-emulate): the gate test, the order updates and the time format ran under Unicorn on 20,000 inputs each before the replay; the replay then only tests how they combine.
• Say what the replay cannot test. The recording sees about one frame in eight, so the replay rebuilds frames at the clocks the game counted each gate at, which it takes from the recording. What it tests is everything else: which crossings count, where each time lands, places, flags, best laps, the finishing order and the printed results.
• Compare at every sample, not only at the end, with a window for reads that straddle frames (replay-sample-straddles-frames), and check the test can fail (a one-unit change to a split was caught).

What it turned up: a second writer of the order table that the trace had missed (trace-claims-need-an-address-scan), and a wrong inference about the FPU mode (x87-magic-add-rounding).