Turns CamelCase and lamaCase strings into snakecase aka underscorecase (Swift 3.0)

snake_cased.swift

// Swift 3.0
// snake_cased
import Foundation

extension String {
    mutating func snake_cased() {
        self = self.snake_case()
    }
    
    func snake_case() -> String {
        var result = ""
        var previousCharWasCapitalized = false
        
        for (index, char) in self.characters.enumerated() {
            var charStr = String(char)
            
            // remove (ignore) non-alphas
            if !charStr.isAlpha { continue }
            
            // If capital is found...
            if charStr == charStr.uppercased()
            {
                // ...lower case it...
                charStr = charStr.lowercased()
                
                // If it's not the first letter, nor follows another lowercased letter, prepend an underscore
                // (If it followed another operated-on letter, we'd get "JSON" -> "j_s_o_n" instead of "json")
                if
                    index != 0,
                    !previousCharWasCapitalized {
                    charStr = "_" + charStr
                }
                previousCharWasCapitalized = true
            }
                // If capital is not found, mark it for the next cycle, and move on.
            else { previousCharWasCapitalized = false }
            result += charStr
        }
        return result
    }
    
    var isAlpha: Bool {
        let alphaSet = CharacterSet.uppercaseLetters.union(.lowercaseLetters).union(.whitespacesAndNewlines)
        return self.rangeOfCharacter(from: alphaSet.inverted) == nil
    }
}

var testString = "authorizationCode"
assert(testString.snake_case() == "authorization_code")

testString.snake_cased()
assert(testString == "authorization_code")