Add popcount function

src/game/genetics/popcount.hpp

@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2020  Christopher J. Howard
+ *
+ * This file is part of Antkeeper source code.
+ *
+ * Antkeeper source code is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Antkeeper source code is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Antkeeper source code.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef ANTKEEPER_POPCOUNT_HPP
+#define ANTKEEPER_POPCOUNT_HPP
+
+namespace dna
+{
+
+/// Returns the number of set bits (*population count*) in @p x. Also known as the *Hamming weight*.
+template <typename T>
+constexpr int popcount(T x) noexcept;
+
+template <typename T>
+inline constexpr int popcount(T x) noexcept
+{
+	int n;
+	for (n = 0; x; ++n)
+		x &= x - 1;
+	return n;
+}
+
+} // namespace dna
+
+#endif // ANTKEEPER_POPCOUNT_HPP