libstrat: add StratosphereRandomUtils

include/stratosphere.hpp

@@ -39,4 +39,6 @@
 
 #include "stratosphere/title_ids.hpp"
 
-#include "stratosphere/on_crash.hpp"
+#include "stratosphere/on_crash.hpp"
+
+#include "stratosphere/random.hpp"

include/stratosphere/random.hpp

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2018-2019 Atmosphère-NX
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope 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, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+#include <switch.h>
+
+class StratosphereRandomUtils {
+    public:
+        static u32 GetRandomU32(u32 max);
+        static u64 GetRandomU64(u64 max);
+
+        template<typename T>
+        static T GetRandom(T max);
+};

source/random.cpp

@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2018-2019 Atmosphère-NX
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope 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, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <switch.h>
+#include <stratosphere.hpp>
+#include <random>
+
+/* Official HOS uses TinyMT. This is high effort. Let's just use XorShift. */
+/* https://en.wikipedia.org/wiki/Xorshift */
+class XorShiftGenerator {
+    public:
+        using result_type = uint32_t;
+        static constexpr result_type (min)() { return 0; }
+        static constexpr result_type (max)() { return UINT32_MAX; }
+        static constexpr size_t num_seed_dwords = 4;
+    private:
+        result_type random_state[num_seed_dwords];
+    public:
+
+        explicit XorShiftGenerator() {
+            /* Seed using process entropy. */
+            u64 val = 0;
+            for (size_t i = 0; i < num_seed_dwords; i++) {
+                if (R_FAILED(svcGetInfo(&val, 0xB, 0, i))) {
+                    std::abort();
+                }
+                this->random_state[i] = result_type(val);
+            }
+        }
+
+        explicit XorShiftGenerator(std::random_device &rd) {
+            for (size_t i = 0; i < num_seed_dwords; i++) {
+                this->random_state[i] = result_type(rd());
+            }
+        }
+
+        result_type operator()() {
+            result_type s, t = this->random_state[3];
+            t ^= t << 11;
+            t ^= t >> 8;
+            this->random_state[3] = this->random_state[2]; this->random_state[2] = this->random_state[1]; this->random_state[1] = (s = this->random_state[0]);
+            t ^= s;
+            t ^= s >> 19;
+            this->random_state[0] = t;
+            return t;
+        }
+
+        void discard(size_t n) {
+            for (size_t i = 0; i < n; i++) {
+                operator()();
+            }
+        }
+};
+
+static XorShiftGenerator g_rnd_generator;
+
+template<typename T>
+T StratosphereRandomUtils::GetRandom(T max) {
+    std::uniform_int_distribution<T> rnd(0, max);
+    return rnd(g_rnd_generator);
+}
+
+/* These are slightly biased, but I think that's totally okay. */
+u32 StratosphereRandomUtils::GetRandomU32(u32 max) {
+    return GetRandom<u32>(max);
+}
+
+u64 StratosphereRandomUtils::GetRandomU64(u64 max) {
+    return GetRandom<u64>(max);
+}
+