# Two Guys Arguing

## mario math

Posted in java, mario, math by youngnh on 08.13.09

To calculate Mario’s velocity, with KEY_RIGHT pressed down continuously for n frames:

let (1.2 if KEY_SPEED is held down as well)

at frame one, then, your velocity

frame 2:
frame 3:
frame 4:

which, if you multiply through, looks like this:

Which makes it a bit easier to generalize Mario’s velocity at any frame n:

we know from paying attention in high school wikipedia that:

however, our sequence only goes to n-1 frames. It’s missing the last term:

subtracting xn from both sides

which we could have maybe figured out faster if we had remembered this identity:

replacing the lower m with 0 and upper n with n-1 we get:

which multiplied by is what we found earlier.

and so mario’s velocity after n frames of acceleration is:

which can be calculated in constant time by this Java function:

public float velocityAfter(float accel, int nFrames) {
return accel * ((1 - Math.pow(0.89, nFrames)) / 0.11);
}


Whew.

Thanks to Heath Borders and my co-blogger Ben for working this out on a whiteboard with me.

Tagged with: , , ,

### One Response

1. viv said, on 04.18.11 at 1:42 am

for frame 2, why v = a * 0.89 + a

If initial vel. is ‘a’ and accelrn. is ‘a’
then after time t,
v = a + a * t

After time t1 after t,
v2 = v + a*t1
i.e. v2 = a + a*t + a*t1 = a (1 + t + t1)
v3 = v2 + a*t2 = a(1 + t + t1 + t2)

How come the factor of 0.89 and why v2, v3 are calculated differently? What have I missed?
Thanks.