1. Butterworth / Bessel / Chebyshev Filters (Highpass, Bandpass,Bandstop, low pass)

It's C code is given in the link itself.

Low pass filters

1. Moving Average filter
2. Median average filter
3. kalman filter

let kFilteringFactor = 0.1 
var kAccelerationX = 0.0 
if (arr_motionSensor.count == 0) || flagRestartFilter{
    kAccelerationX = motionSensor.acceleration
}
else{
    // Applying Low pass kalman Filter of low frequency applied on acceleration
    kAccelerationX = (motionSensor.acceleration * kFilteringFactor) + (kAccelerationX * (1.0 - kFilteringFactor))
}