Statistics.swift

struct Statistics {
    private(set) var count: Double = 0

    private(set) var sum: Double = 0

    private(set) var sumOfSquares: Double = 0

    var average: Double {
        sum / count
    }

    var variance: Double {
        sumOfSquares/count - pow(average, 2)
    }

    var standardDeviation: Double {
        sqrt(variance)
    }

    @discardableResult
    mutating func add(_ newValue: Double) -> (void: (), outlier: Bool) {
        count += 1
        sum += newValue
        sumOfSquares += pow(newValue, 2)

        return (
            void: (),
            outlier: newValue > average + standardDeviation * 3 || newValue < average - standardDeviation * 3 // this should use IQR
        )
    }
}