grains: with std::conditional

cpp/grains/grains.cpp

@@ -1,18 +1,56 @@
 #include "grains.h"
 
+#include <limits>
+#include <stdexcept>
+
+
 namespace grains {
 
+using namespace std;
+
+
+namespace {
+    
 static const index_t number_of_squares = 64;
 
+template <typename T>
+struct total_as_max
+{
+    static inline T total()
+    {
+        return numeric_limits<T>::max();
+    }
+};
+
+template <typename T>
+struct total_by_shift
+{
+    static inline T total()
+    {
+        return T(1) << number_of_squares - 1;
+    }
+};
+
+static_assert(number_of_squares <= numeric_limits<ulong_t>::digits,
+              "grains::ulong_t is too small for grains::total");
+
+typedef conditional<number_of_squares == numeric_limits<ulong_t>::digits,
+                    total_as_max<ulong_t>,
+                    total_by_shift<ulong_t>>::type total_policy_t;
+}
+
 
 ulong_t square(index_t index)
 {
-    return 1ULL << (index - 1);
+    if (index < 1 || index > number_of_squares)
+        throw domain_error("invalid square number");
+
+    return ulong_t(1) << (index - 1);
 }
 
 ulong_t total()
 {
-    return (1ULL << number_of_squares) - 1;
+    return total_policy_t::total();
 }
 
 }