Swift Challenge #3

swift_challenge_3

// Takes into account of multiple matching substrings and returns an array of String.Indexes

extension String {
    func findIndexOf(subString: String) -> [String.Index]?{
        
        let selfLength = countElements(self)
        let subStringLength = countElements(subString)
        
        if subStringLength == 0 || selfLength == 0 || selfLength < subStringLength {
            return nil
        }

        let selfArray = Array(self)
        let subStringArray = Array(subString)

        var indexes: [Int] = []
        for (i, letter) in enumerate(selfArray[0...selfLength-subStringLength]){
            if equal(subStringArray, selfArray[i..<subStringLength+i]){
                indexes.append(i)
            }
        }

        return indexes.count != 0 ? indexes.map{advance(self.startIndex, $0)} : nil
    }
}