Common: Add xorshift-based PRNG class

src/common/CMakeLists.txt

@@ -67,6 +67,7 @@ add_library(common
   timer.cpp
   timer.h
   types.h
+  xorshift_prng.h
 )
 
 target_include_directories(common PRIVATE "${CMAKE_CURRENT_SOURCE_DIR}/..")

src/common/common.vcxproj

@@ -50,6 +50,7 @@
     <ClInclude Include="types.h" />
     <ClInclude Include="minizip_helpers.h" />
     <ClInclude Include="windows_headers.h" />
+    <ClInclude Include="xorshift_prng.h" />
   </ItemGroup>
   <ItemGroup>
     <ClCompile Include="assert.cpp" />

src/common/common.vcxproj.filters

@@ -53,6 +53,7 @@
     <ClInclude Include="sha256_digest.h" />
     <ClInclude Include="thirdparty\aes.h" />
     <ClInclude Include="task_queue.h" />
+    <ClInclude Include="xorshift_prng.h" />
   </ItemGroup>
   <ItemGroup>
     <ClCompile Include="small_string.cpp" />

src/common/xorshift_prng.h

@@ -0,0 +1,79 @@
+// SPDX-FileCopyrightText: 2025 Connor McLaughlin <stenzek@gmail.com>
+// SPDX-License-Identifier: CC0-1.0
+
+#pragma once
+
+#include "types.h"
+
+class XorShift128PlusPlus
+{
+public:
+  struct State
+  {
+    u64 s0;
+    u64 s1;
+  };
+
+  static constexpr State GetInitialState(u64 seed)
+  {
+    u64 x = seed;
+    return State{SplitMix64(x), SplitMix64(x)};
+  }
+
+  constexpr XorShift128PlusPlus() : m_state(GetInitialState(0)) {}
+  constexpr XorShift128PlusPlus(u64 seed) : m_state(GetInitialState(seed)) {}
+
+  ALWAYS_INLINE const State& GetState() const { return m_state; }
+  State* GetMutableStatePtr() { return &m_state; }
+  void SetState(State& state) { m_state = state; }
+
+  void Reset(u64 seed) { m_state = GetInitialState(seed); }
+
+  u64 Next()
+  {
+    // https://xoroshiro.di.unimi.it/xoroshiro128plusplus.c
+    u64 s0 = m_state.s0;
+    u64 s1 = m_state.s1;
+    u64 result = RotateLeft64(s0 + s1, 17) + s0;
+
+    s1 ^= s0;
+    m_state.s0 = RotateLeft64(s0, 49) ^ s1 ^ (s1 << 21); // a, b
+    m_state.s1 = RotateLeft64(s1, 28);                   // c
+
+    return result;
+  }
+
+  ALWAYS_INLINE u64 NextRange(u64 n)
+  {
+    // This constant should be folded.
+    const u64 max_allowed_value = (UINT64_C(0xFFFFFFFFFFFFFFFF) / n) * n;
+    for (;;)
+    {
+      const u64 x = Next();
+      if (x > max_allowed_value)
+        continue;
+
+      return x % n;
+    }
+  }
+
+  template<typename T>
+  ALWAYS_INLINE T NextRange(T low, T high)
+  {
+    return low + static_cast<T>(NextRange(static_cast<u64>(high - low)));
+  }
+
+private:
+  static constexpr u64 SplitMix64(u64& x)
+  {
+    // https://xoroshiro.di.unimi.it/splitmix64.c
+    u64 z = (x += 0x9e3779b97f4a7c15);
+    z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9;
+    z = (z ^ (z >> 27)) * 0x94d049bb133111eb;
+    return z ^ (z >> 31);
+  }
+
+  static ALWAYS_INLINE u64 RotateLeft64(const u64 x, int k) { return (x << k) | (x >> (64 - k)); }
+
+  State m_state;
+};