Rotor class

Enigma.xcodeproj/project.pbxproj

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+		C0DA3ABF1B5AB49200D8D68E /* Rotor.swift in Sources */ = {isa = PBXBuildFile; fileRef = C0DA3ABE1B5AB49200D8D68E /* Rotor.swift */; };
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 				C0DA3A9C1B5AACB300D8D68E /* Info.plist */,
+				C0DA3ABE1B5AB49200D8D68E /* Rotor.swift */,
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+				C0DA3ABF1B5AB49200D8D68E /* Rotor.swift in Sources */,
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 				C0DA3A911B5AACB300D8D68E /* AppDelegate.swift in Sources */,
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Enigma/Rotor.swift

@@ -0,0 +1,50 @@
+//
+//  Rotor.swift
+//  Enigma
+//
+//  Created by Eryn Wells on 2015-07-18.
+//  Copyright © 2015 Eryn Wells. All rights reserved.
+//
+
+import Foundation
+
+
+class Rotor {
+    enum Error: ErrorType {
+        /** Thrown when the initializer is given an invalid series. */
+        case InvalidSeries
+        case InvalidCharacter
+    }
+
+    static let alphabet: [Character] = Array("ABCDEFGHIJKLMNOPQRSTUVWXYZ".characters)
+
+    /** The position of first letter in `series` in the `alphabet`. */
+    var position: Int
+    /** The series of characters that this rotor cycles through. */
+    let series: [Character]!
+
+    convenience init(series: String) throws {
+        try self.init(series: Array(series.characters))
+    }
+
+    init(series: [Character]) throws {
+        self.position = 0
+        guard series.count == Rotor.alphabet.count else {
+            self.series = nil
+            throw Error.InvalidSeries
+        }
+        self.series = series
+    }
+
+    func advance() {
+        position = position.successor() % Rotor.alphabet.count
+    }
+
+    func encode(c: Character) throws -> Character {
+        let offset: Int! = Rotor.alphabet.indexOf(c)
+        guard offset != nil else {
+            throw Error.InvalidCharacter
+        }
+        return series[(offset + position) % series.count]
+    }
+}