Simple implementation of low-pass filter

LowPassFilterSignal.swift

struct LowPassFilterSignal {
    /// Current signal value
    var value: Double
    
    /// A scaling factor in the range 0.0..<1.0 that determines
    /// how resistant the value is to change
    let filterFactor: Double

    /// Update the value, using filterFactor to attenuate changes
    mutating func update(newValue: Double) {
        value = filterFactor * value + (1.0 - filterFactor) * newValue
    }
}

It's probably the naming that has made you confused. The previous computed value plays the significant role in creation of a new one, hence should be multiplied by higher factor (in our case, where α is picked as a very small number closer to 0, that factor is (1-α)).
Thats the whole point of this filter, to not let new value significantly peak over the current one.

So next to (1-α), where α should number between 0 and 1 but closer to 0, you should have previously computed value.
(1-α) * smoothedValueFromPreviousStep + α * newRawValue

If you look at the equation bellow, you might get better understand where s the motivation coming from.
value * (1-α) + value * α = (1 - α + α) * value = 1 * value = value

Finally, if you take an opposite approach in defining α, where it still resides in [0,1] but closer to 1, then you can revert the naming.