Key combinations on poll-based input

Question

So assume you have an input system that is based on polling

void update()
{
    if( Keyboard[ 'A' ] )
        // A is down
}

Say you want to be able to recognize 3 to 8 length key combinations (like down, down-forward, forward, A for hado-ken)

How would you best create a generic (easily modifiable/programmable) key combination input system on polled input?

Answer 1

Fundamentally, you can't. Key combinations require an ordering, and an ordering requires events.

What you can do is turn polling into events by comparing the key states each frame and generating post-hoc keyup/keydown events from the differences. It's not as reliable because you lose the timestamping and other intra-frame ordering that "native" event systems provide, but it will allow you to detect and order at least one key per frame.

Once you have those ordered events, you can use a finite state machine or other tool to match them against your move lists and execute the correct one.

Answer 2

One way is to store the current and previous input states, and compare them every time that you poll the input.

For each key that can be pressed, store an object that has a timestamp of the last time that the key switched from a down state to an up state.

Update these objects by doing this at every poll:

void update(){
    some_key_has_been_pressed = false;
    foreach(key in keys){
        if(previous_input[key].Down && current_input[key].Up){
            keys[key].up_timestamp = current_time();
        }

        if(current_input[key].Down){
            keys[key].down_timestamp = current_time();
            some_key_has_been_pressed = true;
        }
    }
}

Now you can pattern match your combos against the contents of keys.

Have a combo object for each combo, and call update() on each combo object at each poll.

A combo object's update() method will pattern match the combo, checking if all necessary conditions for the combo are satisfied at this poll. I.e. all keys timestamps for the combo so far are in order, and no other key that would break the combo has been pressed this frame. For each condition met, increment a counter in the combo object to the next condition to check. When all conditions are met in a combo, call the method that the combo should perform. If some_key_has_been_pressed == true but the key that is the next condition for the combo has not been pressed, then reset the combo's satisfied condition counter to 0.

The above is my preferred method, as it is simple to implement, easy to maintain, efficient, and highly modular.

However for another good method, check out the XNA input sequence sample, which is written in C#, and the logic is likely transferrable to the language you're using.

Answer 3

Make a stack of the last X key events, for each key event add an object with the key and time of the press/release, then check if the last members of the stack match a special pattern and if those keys were pressed fast enough to count as a combination. Finally remove the oldest object from the stack.