Conceptually, how does replay work in a game?

Question

I was kind of curious as to how replay might be implemented in a game.

Initially, I thought that there would be just a command list of every player/ai action that was t

Answer 1

Technically you should write your engine to be deterministic, that is no randomness. Assuming a character in the game is aiming at the arm of an opponent, and fires a weapon, then the same amount of damage should be applied to the opponent in all cases.

Assuming a bomb detonates at location X, the particles produced by that explosion should always result in the same visual result. If you need randomness, create a set of random numbers, select a seed value when the game is played, and save that seed value in the replay.

In general having randomness in a game is a bad idea. Even for things like multiplayer, you can't have half your players able to see around an explosion while the others can't simply because they didn't get the right random value.

Make everything deterministic, and you should be fine.