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Comment by trjordan | Hacker News Reader

Is an input always valid? "Firing once" is an input: you have to validate that it's been long enough since last firing that you should act on it (limit the rate). "Moving left" is an input, but by how much? Can I send "Left, 9999999" and replicate a teleport hack by pre-calculating these inputs?

You could bound the inputs to a simple stream of enums (FIRE, LEFT, RIGHT, etc.), but it wouldn't be crazy to batch it up a little bit and send some magnitude data as well (FIRE 2, LEFT 50, etc.). It's a little harder to validate, but depending on the specifics of implementation, that could be a win.

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"Moving left" is NOT an input. An input is "The controller is pushed to the left" (with perhaps a numeric value of how far it is pushed to the left). "Firing once" is similarly NOT an input. "The fire button is down" and "The fire button is up" are inputs. That is how well implemented first person shooters have worked since the Quake days (I think Quake 2 actually).

The point of the article is actually that the server doesn't validate anything. There's nothing to validate. The only data it gets from the client is the state of the controls. Then it runs the real simulation. The client only ran a prediction of what it thought the server would do. Sometimes, it's wrong and corrects itself when it gets new values from the server.

No, it's more like "move left" and you move left at your velocity (95, let's say) for that one frame. Next frame you can send any move, but it will only move you 95 spaces in that direction, no more.

It's the same as just holding down a button on your keyboard - the game engine moves you according to the rules. Now you're just transmitting the fact that the player is facing NW and sent a move left command. You're just running the game on the server like you would on the user's machine.

Conceptually it's actually simpler - you're just running the same thing on the server and sometimes backtrack in the client when the connection is laggy.

Another way to look at it: the game simulation runs in a certain number of frames per second. Each frame you send to the server the state of your input devices (holding left arrow key, left mouse button is clicked, etc) and this is translated to moving, firing weapons, etc according to the rules of the game.

You can't teleport hack by sending "left 9999" because your inputs only serve to tell the server what direction you want to move towards. Your maximum movement speed depends on the game rules and should not be under your control.

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