InputCommon: Extract ControlReference from ControllerInterface

Better separation of concerns. Relegates `ControllerInterface` to
enumerating input controls, and the new `ControlReference` deals with
combining inputs and configuration expression parsing.

Source/Core/InputCommon/ControlReference/ExpressionParser.h

@@ -0,0 +1,72 @@
+// Copyright 2013 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <memory>
+#include <string>
+#include "InputCommon/ControllerInterface/Device.h"
+
+namespace ciface
+{
+namespace ExpressionParser
+{
+class ControlQualifier
+{
+public:
+  bool has_device;
+  Core::DeviceQualifier device_qualifier;
+  std::string control_name;
+
+  ControlQualifier() : has_device(false) {}
+  operator std::string()
+  {
+    if (has_device)
+      return device_qualifier.ToString() + ":" + control_name;
+    else
+      return control_name;
+  }
+};
+
+class ControlFinder
+{
+public:
+  ControlFinder(const Core::DeviceContainer& container_, const Core::DeviceQualifier& default_,
+                const bool is_input_)
+      : container(container_), default_device(default_), is_input(is_input_)
+  {
+  }
+  std::shared_ptr<Core::Device> FindDevice(ControlQualifier qualifier);
+  Core::Device::Control* FindControl(ControlQualifier qualifier);
+
+private:
+  const Core::DeviceContainer& container;
+  const Core::DeviceQualifier& default_device;
+  bool is_input;
+};
+
+class ExpressionNode;
+class Expression
+{
+public:
+  Expression() : node(nullptr) {}
+  Expression(ExpressionNode* node);
+  ~Expression();
+  ControlState GetValue();
+  void SetValue(ControlState state);
+  int num_controls;
+  ExpressionNode* node;
+};
+
+enum ExpressionParseStatus
+{
+  EXPRESSION_PARSE_SUCCESS = 0,
+  EXPRESSION_PARSE_SYNTAX_ERROR,
+  EXPRESSION_PARSE_NO_DEVICE,
+};
+
+ExpressionParseStatus ParseExpression(const std::string& expr, ControlFinder& finder,
+                                      Expression** expr_out);
+}
+}