NSNumberFormatter subclass to express weight down to the microgram

weightFormatter.swift

// http://stackoverflow.com/a/35959239/2227743

enum MassUnit: Double {
    case Microgram = 1e-6
    case Milligram = 1e-3
    case Gram = 1
    case Kilogram = 1e3
    
    static let allUnits: [MassUnit] = [.Microgram, .Milligram, .Gram, .Kilogram]
    
    var unitAbbreviation: String {
        get {
            switch self {
            case .Microgram: return "mcg"
            case .Milligram: return "mg"
            case .Gram: return "g"
            case .Kilogram: return "kg"
            }
            
        }
    }
}

class MyMassFormatter: NSNumberFormatter {
    func bestFitStringForWeightInGrams(weight: Double) -> String {
        var selectedString = self.stringFromNumber(weight)!
        var selectedUnit = MassUnit.Gram
        
        // Pick the unit that results in the shortest string
        for unit in MassUnit.allUnits {
            if let str = self.stringFromNumber(weight / unit.rawValue)
                where str.characters.count < selectedString.characters.count {
                selectedString = str
                selectedUnit = unit
            }
        }
        
        return selectedString + selectedUnit.unitAbbreviation
    }
}

let formatter = MyMassFormatter()
formatter.format = "0.######"

print(formatter.bestFitStringForWeightInGrams(0.000001))   // 1mcg
print(formatter.bestFitStringForWeightInGrams(0.005))      // 5mg
print(formatter.bestFitStringForWeightInGrams(2500))       // 2.5kg
print(formatter.bestFitStringForWeightInGrams(1234.5))     // 1234.5g