diff --git a/CardParser.swift b/CardParser.swift
index e732ea5..d5b9348 100644
--- a/CardParser.swift
+++ b/CardParser.swift
@@ -10,199 +10,186 @@ import Foundation
 //MARK: - CardType
 
 enum CardType {
-    case amex
-    case diners
-    case discover
-    case jcb
-    case masterCard
-    case visa
-
-    static let allValues: [CardType] = [.visa, .masterCard, .amex, .diners, .discover, .jcb]
-
-    private var validationRequirements: ValidationRequirement {
-        let prefix: [PrefixContainable], length: [Int]
-
-        switch self {
-        /* // IIN prefixes and length requriements retreived from https://en.wikipedia.org/wiki/Bank_card_number on June 28, 2016 */
-
-        case .amex:         prefix = ["34", "37"]
-                            length = [15]
-
-        case .diners:       prefix = ["300"..."305", "309", "36", "38"..."39"]
-                            length = [14]
-
-        case .discover:     prefix = ["6011", "65", "644"..."649", "622126"..."622925"]
-                            length = [16]
-
-        case .jcb:          prefix = ["3528"..."3589"]
-                            length = [16]
-
-        case .masterCard:   prefix = ["51"..."55", "2221"..."2720"]
-                            length = [16]
-
-        case .visa:         prefix = ["4"]
-                            length = [13, 16, 19]
-
-        }
-
-        return ValidationRequirement(prefixes: prefix, lengths: length)
+  case amex
+  case diners
+  case discover
+  case jcb
+  case masterCard
+  case visa
+  
+  static let allValues: [CardType] = [.visa, .masterCard, .amex, .diners, .discover, .jcb]
+  
+  private var validationRequirements: ValidationRequirement {
+    let prefix: [PrefixContainable], length: [Int]
+    
+    switch self {
+      /* // IIN prefixes and length requriements retreived from https://en.wikipedia.org/wiki/Bank_card_number on June 28, 2016 */
+      
+    case .amex:         prefix = ["34", "37"]
+    length = [15]
+      
+    case .diners:       prefix = ["300"..."305", "309", "36", "38"..."39"]
+    length = [14]
+      
+    case .discover:     prefix = ["6011", "65", "644"..."649", "622126"..."622925"]
+    length = [16]
+      
+    case .jcb:          prefix = ["3528"..."3589"]
+    length = [16]
+      
+    case .masterCard:   prefix = ["51"..."55", "2221"..."2720"]
+    length = [16]
+      
+    case .visa:         prefix = ["4"]
+    length = [13, 16, 19]
+      
     }
-
-    var segmentGroupings: [Int] {
-        switch self {
-        case .amex:      return [4, 6, 5]
-        case .diners:    return [4, 6, 4]
-        default:         return [4, 4, 4, 4]
-        }
+    
+    return ValidationRequirement(prefixes: prefix, lengths: length)
+  }
+  
+  var segmentGroupings: [Int] {
+    switch self {
+    case .amex:      return [4, 6, 5]
+    case .diners:    return [4, 6, 4]
+    default:         return [4, 4, 4, 4]
     }
-
-    var maxLength: Int {
-        return validationRequirements.lengths.max() ?? 16
-    }
-
-    var cvvLength: Int {
-        switch self {
-        case .amex: return 4
-        default: return 3
-        }
+  }
+  
+  var maxLength: Int {
+    return validationRequirements.lengths.max() ?? 16
+  }
+  
+  var cvvLength: Int {
+    switch self {
+    case .amex: return 4
+    default: return 3
     }
+  }
+  
+  func isValid(_ accountNumber: String) -> Bool {
+    return validationRequirements.isValid(accountNumber) && CardType.luhnCheck(accountNumber)
+  }
+  
+  func isPrefixValid(_ accountNumber: String) -> Bool {
+    return validationRequirements.isPrefixValid(accountNumber)
+  }
+  
+}
 
+fileprivate extension CardType {
+  
+  struct ValidationRequirement {
+    let prefixes: [PrefixContainable]
+    let lengths: [Int]
+    
     func isValid(_ accountNumber: String) -> Bool {
-        return validationRequirements.isValid(accountNumber) && CardType.luhnCheck(accountNumber)
+      return isLengthValid(accountNumber) && isPrefixValid(accountNumber)
     }
-
+    
     func isPrefixValid(_ accountNumber: String) -> Bool {
-        return validationRequirements.isPrefixValid(accountNumber)
+      guard prefixes.count > 0 else { return true }
+      return prefixes.contains { $0.hasCommonPrefix(with: accountNumber) }
     }
-
-}
-
-fileprivate extension CardType {
-
-    struct ValidationRequirement {
-        let prefixes: [PrefixContainable]
-        let lengths: [Int]
-
-        func isValid(_ accountNumber: String) -> Bool {
-            return isLengthValid(accountNumber) && isPrefixValid(accountNumber)
-        }
-
-        func isPrefixValid(_ accountNumber: String) -> Bool {
-            guard prefixes.count > 0 else { return true }
-            return prefixes.contains { $0.hasCommonPrefix(with: accountNumber) }
-        }
-
-        func isLengthValid(_ accountNumber: String) -> Bool {
-            guard lengths.count > 0 else { return true }
-            return lengths.contains { accountNumber.length == $0 }
-        }
+    
+    func isLengthValid(_ accountNumber: String) -> Bool {
+      guard lengths.count > 0 else { return true }
+      return lengths.contains { accountNumber.count == $0 }
     }
-
-    // from: https://gist.github.com/cwagdev/635ce973e8e86da0403a
-    static func luhnCheck(_ cardNumber: String) -> Bool {
-        var sum = 0
-        let reversedCharacters = cardNumber.characters.reversed().map { String($0) }
-        for (idx, element) in reversedCharacters.enumerated() {
-            guard let digit = Int(element) else { return false }
-            switch ((idx % 2 == 1), digit) {
-            case (true, 9): sum += 9
-            case (true, 0...8): sum += (digit * 2) % 9
-            default: sum += digit
-            }
-        }
-        return sum % 10 == 0
+  }
+  
+  // from: https://gist.github.com/cwagdev/635ce973e8e86da0403a
+  static func luhnCheck(_ cardNumber: String) -> Bool {
+    var sum = 0
+    let reversedCharacters = cardNumber.reversed().map { String($0) }
+    for (idx, element) in reversedCharacters.enumerated() {
+      guard let digit = Int(element) else { return false }
+      switch ((idx % 2 == 1), digit) {
+      case (true, 9): sum += 9
+      case (true, 0...8): sum += (digit * 2) % 9
+      default: sum += digit
+      }
     }
-
+    return sum % 10 == 0
+  }
+  
 }
 
 //MARK: - CardState
 
 enum CardState {
-    case identified(CardType)
-    case indeterminate([CardType])
-    case invalid
+  case identified(CardType)
+  case indeterminate([CardType])
+  case invalid
 }
 
 extension CardState: Equatable {}
 func ==(lhs: CardState, rhs: CardState) -> Bool {
-    switch (lhs, rhs) {
-    case (.invalid, .invalid): return true
-    case (let .indeterminate(cards1), let .indeterminate(cards2)): return cards1 == cards2
-    case (let .identified(card1), let .identified(card2)): return card1 == card2
-    default: return false
-    }
+  switch (lhs, rhs) {
+  case (.invalid, .invalid): return true
+  case (let .indeterminate(cards1), let .indeterminate(cards2)): return cards1 == cards2
+  case (let .identified(card1), let .identified(card2)): return card1 == card2
+  default: return false
+  }
 }
 
 extension CardState {
-
-    init(fromNumber number: String) {
-        if let card = CardType.allValues.first(where: { $0.isValid(number) }) {
-            self = .identified(card)
-        }
-        else {
-            self = .invalid
-        }
+  
+  init(fromNumber number: String) {
+    if let card = CardType.allValues.first(where: { $0.isValid(number) }) {
+      self = .identified(card)
     }
-
-    init(fromPrefix prefix: String) {
-        let possibleTypes = CardType.allValues.filter { $0.isPrefixValid(prefix) }
-        if possibleTypes.count >= 2 {
-            self = .indeterminate(possibleTypes)
-        }
-        else if possibleTypes.count == 1, let card = possibleTypes.first {
-            self = .identified(card)
-        }
-        else {
-            self = .invalid
-        }
+    else {
+      self = .invalid
     }
-
+  }
+  
+  init(fromPrefix prefix: String) {
+    let possibleTypes = CardType.allValues.filter { $0.isPrefixValid(prefix) }
+    if possibleTypes.count >= 2 {
+      self = .indeterminate(possibleTypes)
+    }
+    else if possibleTypes.count == 1, let card = possibleTypes.first {
+      self = .identified(card)
+    }
+    else {
+      self = .invalid
+    }
+  }
+  
 }
 
 //MARK: - PrefixContainable
 
 fileprivate protocol PrefixContainable {
-
-    func hasCommonPrefix(with text: String) -> Bool
-
+  
+  func hasCommonPrefix(with text: String) -> Bool
+  
 }
 
 extension ClosedRange: PrefixContainable {
-
-    func hasCommonPrefix(with text: String) -> Bool {
-        //cannot include Where clause in protocol conformance, so have to ensure Bound == String :(
-        guard let lower = lowerBound as? String, let upper = upperBound as? String else { return false }
-
-        let trimmedRange: ClosedRange<String> = {
-            let length = text.length
-            let trimmedStart = lower.prefix(length)
-            let trimmedEnd = upper.prefix(length)
-            return trimmedStart...trimmedEnd
-        }()
-
-        let trimmedText = text.prefix(trimmedRange.lowerBound.characters.count)
-        return trimmedRange ~= trimmedText
-    }
-
+  
+  func hasCommonPrefix(with text: String) -> Bool {
+    //cannot include Where clause in protocol conformance, so have to ensure Bound == String :(
+    guard let lower = lowerBound as? String, let upper = upperBound as? String else { return false }
+    
+    let trimmedRange: ClosedRange<Substring> = {
+      let length = text.count
+      let trimmedStart = lower.prefix(length)
+      let trimmedEnd = upper.prefix(length)
+      return trimmedStart...trimmedEnd
+    }()
+    
+    let trimmedText = text.prefix(trimmedRange.lowerBound.count)
+    return trimmedRange ~= trimmedText
+  }
+  
 }
 
 extension String: PrefixContainable {
-
-    func hasCommonPrefix(with text: String) -> Bool {
-        return hasPrefix(text) || text.hasPrefix(self)
-    }
-
+  
+  func hasCommonPrefix(with text: String) -> Bool {
+    return hasPrefix(text) || text.hasPrefix(self)
+  }
+  
 }
-
-fileprivate extension String {
-
-    func prefix(_ maxLength: Int) -> String {
-        return String(characters.prefix(maxLength))
-    }
-
-    var length: Int {
-        return characters.count
-    }
-
-}
-