Introduces a joystick analogue to the shared keyboard interface, and implements it for the Vic-20.

Inputs/Joystick.cpp

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+//
+//  Joystick.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 14/10/2017.
+//  Copyright © 2017 Thomas Harte. All rights reserved.
+//
+
+#include "Joystick.hpp"

Inputs/Joystick.hpp

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+//
+//  Joystick.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 14/10/2017.
+//  Copyright © 2017 Thomas Harte. All rights reserved.
+//
+
+#ifndef Joystick_hpp
+#define Joystick_hpp
+
+#include <vector>
+
+namespace Inputs {
+
+/*!
+	Provides an intermediate idealised model of a simple joystick, allowing a host
+	machine to toggle states, while an interested party either observes or polls.
+*/
+class Joystick {
+	public:
+		virtual ~Joystick() {}
+	
+		enum class DigitalInput {
+			Up, Down, Left, Right, Fire
+		};
+
+		// Host interface.
+		virtual void set_digital_input(DigitalInput digital_input, bool is_active) = 0;
+		virtual void reset_all_inputs() = 0;
+};
+
+}
+
+#endif /* Joystick_hpp */