Swift: String Extension to Convert Floating-Point Binary to Double

So far this is just some roughed out code based on this explanation. Feel free to comment, adapt and streamline. I have no doubts that it can be improved, but for a start it works. All comments welcome.

extension String {
func binaryToDecimal() -> Double {

    func binaryBeforePoint(binary:String) -> Int {
        let values = Array(binary.characters).reverse()
        var num = 0
        var div = 1
        for v in values {
            guard let n = Int(String(v)) where n == 1 || n == 0 else {fatalError("Contains invalid character.")}
            num += n * div
            div *= 2
        }
        return num
    }
    
    func binaryAfterPoint(binary:String) -> Double {
        let values = Array(binary.characters)
        var num:Double = 0
        var div:Double = 2
        for v in values {
            guard let d = Double(String(v)) where d == 1 || d == 0 else {fatalError("Contains invalid character.")}
            num += d/div
            div *= 2
        }
        return num
    }
    
    let binaryArray = Array(self.characters).split(".", maxSplit: 1, allowEmptySlices: false)
    
    
    if let beforePoint = binaryArray.first,
        afterPoint = binaryArray.last where binaryArray.count > 1 {
            return binaryAfterPoint(String(afterPoint)) + Double(binaryBeforePoint(String(beforePoint)))
    }
    else if self.containsString(".") == false {
        if let bValue = binaryArray.first {
            return Double(binaryBeforePoint(String(bValue)))
        }
        else {
            return 0
        }
        
    }
    else {
        if let bValue = binaryArray.first {
            return Double(binaryAfterPoint(String(bValue)))
        }
        else {
            return 0
        }
        
    }

}

}
"11011".binaryToDecimal() // 27
"11011.010".binaryToDecimal() // 27.25
".11011".binaryToDecimal() // 0.84375 

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