CRC8 of the custom HID input

Unity Engine
,
1

Where would be the best place to validate a CRC8 sum of the HID response?

I Have a HidController script that reads values from custom HID as well as binds InputActions to events. These events should not be fired in case of wrong HID response (wrong CRC8).

2

Implement IInputStateCallbackReceiver. Inspect the event when you get it in OnStateEvent, and don’t commit it via InputState.Change if the CRC is wrong.

1 Like
3

Thanks for the reply! It looks like the correct approach, but I still need a help: How to get the bytes of the response from the InputEventPtr? Should I use unsafe mode?

4

Most likely yes. If you have a struct that defines the state for your devices, you can get it from the event like this

var state = StateEvent.GetState<MyStateStruct>(eventPtr);

But for computing a CRC this will likely not be of much use. StateEvent.From (or FromUnchecked if you already know it’s a state event) gives you a StateEvent. From there you get to the state via the state property.

You can compute a CRC directly from that memory or, if you need a byte[ ] array for a managed method, set aside a buffer you use over and over and just copy into.

private byte[] m_Buffer;

public unsafe void OnStateEvent(InputEventPtr eventPtr)
{
    var stateEventPtr = StateEvent.FromUnchecked(eventPtr);
    var stateSize = (int)stateEventPtr->stateSizeInBytes;
    if (m_Buffer == null)
        m_Buffer = new byte[stateSize];
    fixed (byte* bufferPtr = m_Buffer)
        UnsafeUtility.Memcpy(bufferPtr, stateEventPtr->state, stateSize);
    var crc = MyCRCMethod(m_Buffer);
5

Thanks a lot!

I did this (no unsafe!):

 public void OnStateEvent(InputEventPtr eventPtr)
    {
        var o = (byte[])this.ReadValueFromEventAsObject(eventPtr);
        var crc = Crc8.Calculate(o);
        if (crc == 0)
            InputState.Change(this, eventPtr);
        else
            Debug.LogWarning("CRC Failed!");
    }

And I am it testing now.

But

var state = StateEvent.GetState<MyStateStruct>(eventPtr);

is what I’ll try in a second as well.

6

Ok, it seems to work! :slight_smile:
Last question: what about the GetStateOffsetForEvent ? I’m not sure what should I do here (if anything).

7

Simply set offset to 0 and return true.

1 Like
8

One thing I just caught, note that you also need to commit the new state yourself via InputState.Change. OnStateEvent basically says “if there’s a state event for me, don’t handle it; I’ll handle it myself”.

9

Sorry, you lost me. Is it enough that I call InputState.Change for valid CRC ?

10

Pseuo-logic

void OnStateEvent(eventPtr)
{
    if (CRCCheckSucceeds(eventPtr))
        InputState.Change(this, eventPtr);
}
11

Doh, oversight on my end. Ignore me. You’re good.

1 Like