Rounding in Swift, adapted from: http://www.globalnerdy.com/2016/01/26/better-to-be-roughly-right-than-precisely-wrong-rounding-numbers-with-swift/

Double+Round.swift

extension Double {
    // Given a value to round and a factor to round to,
    // round the value to the nearest multiple of that factor.
    mutating func round(toNearest: Double) {
        self = (self / toNearest).rounded() * toNearest
    }
    
    // Given a value to round and a factor to round to,
    // round the value DOWN to the largest previous multiple
    // of that factor.
    mutating func roundDown(toNearest: Double) {
        self = floor(self / toNearest) * toNearest
    }
    
    // Given a value to round and a factor to round to,
    // round the value DOWN to the largest previous multiple
    // of that factor.
    mutating func roundUp(toNearest: Double) {
        self = ceil(self / toNearest) * toNearest
    }
    
    // Round the given value to a specified number
    // of decimal places
    mutating func round(toDecimalPlaces places: Int) {
        let divisor = pow(10.0, Double(places))
        self = (self * divisor).rounded() / divisor
    }
}