RotorBox.Rotor.swift

public struct RotorBox {
    
    public struct Rotor {
        
        let forward: Cipher
        let backward: Cipher
        var position: Token
        
        init(_ forward: @escaping Cipher, position: Token) {
            self.forward = forward
            // backwardはforwardのときと逆に動作する (FIXME)
            self.backward = { token in
                let tokens: [Token] = [.A, .B, .C, .D, .E, .F, .G, .H, .I, .J, .K, .L, .M, .N, .O, .P, .Q, .R, .S, .T, .U, .V, .W, .X, .Y, .Z]
                return tokens.filter{ forward($0) == token }.first!
            }
            self.position = position
        }
        
        mutating func rotate() {
            position = position.next()
        }
    }
    
    ...
}