(svn r8876) -Fix

Replace tests with magic numbers by a simple extraction template for command parameters

src/cmd_helper.h

@@ -0,0 +1,28 @@
+/* $Id$ */
+
+#ifndef CMD_HELPER_H
+#define CMD_HELPER_H
+
+#include "direction.h"
+#include "macros.h"
+#include "road.h"
+
+
+template<uint N> static inline void ExtractValid();
+template<> static inline void ExtractValid<1>() {}
+
+
+template<typename T> struct ExtractBits;
+template<> struct ExtractBits<Axis>          { static const uint Count =  1; };
+template<> struct ExtractBits<DiagDirection> { static const uint Count =  2; };
+template<> struct ExtractBits<RoadBits>      { static const uint Count =  4; };
+
+
+template<typename T, uint N, typename U> static inline T Extract(U v)
+{
+	// Check if there are enough bits in v
+	ExtractValid<N + ExtractBits<T>::Count <= sizeof(U) * 8>();
+	return (T)GB(v, N, ExtractBits<T>::Count);
+}
+
+#endif