ControllerInterface: Input detection improvements.

Source/Core/InputCommon/ControllerInterface/Device.h

@@ -55,9 +55,22 @@ public:
   class Input : public Control
   {
   public:
-    // things like absolute axes/ absolute mouse position will override this
+    // Things like absolute axes/ absolute mouse position should override this to prevent
+    // undesirable behavior in our mapping logic.
     virtual bool IsDetectable() { return true; }
+
+    // Implementations should return a value from 0.0 to 1.0 across their normal range.
+    // One input should be provided for each "direction". (e.g. 2 for each axis)
+    // If possible, negative values may be returned in situations where an opposing input is
+    // activated. (e.g. When an underlying axis, X, is currently negative, "Axis X-", will return a
+    // positive value and "Axis X+" may return a negative value.)
+    // Doing so is solely to allow our input detection logic to better detect false positives.
+    // This is necessary when making use of "FullAnalogSurface" as multiple inputs will be seen
+    // increasing from 0.0 to 1.0 as a user tries to map just one. The negative values provide a
+    // view of the underlying axis. (Negative values are clamped off before they reach
+    // expression-parser or controller-emu)
     virtual ControlState GetState() const = 0;
+
     Input* ToInput() override { return this; }
   };
 
@@ -96,11 +109,7 @@ protected:
   {
   public:
     FullAnalogSurface(Input* low, Input* high) : m_low(*low), m_high(*high) {}
-    ControlState GetState() const override
-    {
-      return (1 + m_high.GetState() - m_low.GetState()) / 2;
-    }
-
+    ControlState GetState() const override;
     std::string GetName() const override { return m_low.GetName() + *m_high.GetName().rbegin(); }
 
   private: