grains: template version

cpp/grains/grains.h

@@ -3,12 +3,57 @@
 #define EXERCISM_RUN_ALL_TESTS
 
 
-namespace grains {
+#include <limits>
+#include <stdexcept>
 
-typedef unsigned long long ulong_t;
-typedef unsigned int       index_t;
 
-ulong_t square(index_t index);
-ulong_t total();
+template <size_t   number_of_squares = 64,
+          typename ulong_t = unsigned long long>
+class custom_grains
+{
+    typedef typename std::numeric_limits<ulong_t> limits_t;
 
-}
+    static_assert(std::is_unsigned<ulong_t>::value,
+            "ulong_t must be unsigned");
+
+    static_assert(number_of_squares <= limits_t::digits,
+            "ulong_t is too short");
+
+    struct total_as_max
+    {
+        static inline ulong_t total()
+        {
+            return limits_t::max();
+        }
+    };
+
+    struct total_by_shift
+    {
+        static inline ulong_t total()
+        {
+            return ulong_t(1) << number_of_squares - 1;
+        }
+    };
+
+    typedef typename std::conditional<
+        number_of_squares == limits_t::digits,
+        total_as_max,
+        total_by_shift>::type   total_policy_t;
+
+public:
+
+    static ulong_t square(size_t index)
+    {
+        if (index < 1 || index > number_of_squares)
+            throw std::domain_error("invalid square number");
+
+        return ulong_t(1) << (index - 1);
+    }
+
+    static ulong_t total()
+    {
+        return total_policy_t::total();
+    }
+};
+
+typedef custom_grains<> grains;