Exponential Moving Average

ExponentialMovingAverage.swift

func exponentialMovingAverage(currentAverage: Double, newValue: Double, smoothing: Double) -> Double {
    return smoothing * newValue + (1 - smoothing) * currentAverage
}

// Usage:
// var a = 3
// a = exponentialMovingAverage(a, 8, 0.5)


// Swift 2
struct ExponentialMovingAverage {
    var currentValue: Double
    let smoothing: Double

    init(initialValue: Double = 0, smoothing: Double = 0.5) {
        self.currentValue = initialValue
        self.smoothing = smoothing
    }
    
    mutating func update(next: Double) -> Double {
        currentValue = smoothing * next + (1 - smoothing) * currentValue
        return currentValue
    }
}

extension ExponentialMovingAverage: CustomStringConvertible, CustomDebugStringConvertible {
    var description: String { return "average = \(currentValue)" }
    var debugDescription: String { return "<ExponentialMovingAverage smoothnig = \(smoothing), currentValue = \(currentValue)" }
}

extension ExponentialMovingAverage: Equatable {}
func ==(lhs: ExponentialMovingAverage, rhs: ExponentialMovingAverage) -> Bool {
    return lhs.smoothing == rhs.smoothing && lhs.currentValue == rhs.currentValue
}

// Usage
var average = ExponentialMovingAverage(initialValue: 3)
average.update(5)
average.update(5)
average

Works with Swift 4.2 as well. Thank you