Replace rand() by a true uniform distribution generator

src/base/utils/random.cpp

@@ -0,0 +1,53 @@
+/*
+ * Bittorrent Client using Qt and libtorrent.
+ * Copyright (C) 2017  Mike Tzou
+ *
+ * This program 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 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program 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 this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ *
+ * In addition, as a special exception, the copyright holders give permission to
+ * link this program with the OpenSSL project's "OpenSSL" library (or with
+ * modified versions of it that use the same license as the "OpenSSL" library),
+ * and distribute the linked executables. You must obey the GNU General Public
+ * License in all respects for all of the code used other than "OpenSSL".  If you
+ * modify file(s), you may extend this exception to your version of the file(s),
+ * but you are not obligated to do so. If you do not wish to do so, delete this
+ * exception statement from your version.
+ *
+ */
+
+#include "random.h"
+
+#include <cassert>
+#include <chrono>
+#include <random>
+
+// on some platform `std::random_device` may generate the same number sequence
+static bool hasTrueRandomDevice{ std::random_device{}() != std::random_device{}() };
+static thread_local std::mt19937 generator{
+    hasTrueRandomDevice
+    ? std::random_device{}()
+    : (std::random_device::result_type) std::chrono::system_clock::now().time_since_epoch().count()
+};
+
+uint32_t Utils::Random::rand(const uint32_t min, const uint32_t max)
+{
+    // better replacement for `std::rand`, don't use this for real cryptography application
+    // min <= returned_value <= max
+    assert(min <= max);
+
+    // new distribution is cheap: http://stackoverflow.com/a/19036349
+    std::uniform_int_distribution<uint32_t> uniform(min, max);
+    return uniform(generator);
+}