Improve partition box key hashing

src/Partition.h

@@ -43,7 +43,6 @@ typedef std::unordered_map<ID, Ball *> DictOfBalls;
 typedef std::vector<Ball *> VectorOfBalls;
 typedef std::vector<StaticCollidable *> VectorOfStaticCollidables;
 typedef std::pair<ID, Ball*> DictEntry;
-typedef std::unordered_map<Key,Box *,std::hash<size_t>> MapOfBoxes;
 typedef std::vector<Box *> VectorOfBoxes;
 typedef std::back_insert_iterator< VectorOfBoxes > BoxVectorInsertor;
 
@@ -80,12 +79,6 @@ class Key
         if (iz > k.iz) return false;
         return n<k.n;
     }
-    //hash key
-    operator size_t () const {
-        return std::hash<int64_t>()((ix<<3) + (iy<<2) + (iz<<1) + n);
-    }
-
-
     int64_t ix;
     int64_t iy;
     int64_t iz;
@@ -108,6 +101,27 @@ class Key
     }
 };
 
+struct KeyHash
+{
+    size_t operator()(const Key& key) const noexcept
+    {
+        size_t seed = 0;
+        const auto combine = [&seed](int64_t value)
+        {
+            const size_t valueHash = std::hash<int64_t>{}(value);
+            seed ^= valueHash + static_cast<size_t>(0x9e3779b9U) + (seed << 6) + (seed >> 2);
+        };
+
+        combine(key.ix);
+        combine(key.iy);
+        combine(key.iz);
+        combine(key.n);
+        return seed;
+    }
+};
+
+typedef std::unordered_map<Key, Box*, KeyHash> MapOfBoxes;
+
 
 class Partition
 {
@@ -204,4 +218,4 @@ class Partition
     BoxAllocator mBoxAllocator;
 };
 
-#endif
+#endif

tests/CMakeLists.txt

@@ -12,6 +12,7 @@ target_sources(DestinyTest PRIVATE
     TestAABB.cpp
     TestBoxShape.cpp
     TestCollision.cpp
+    TestPartition.cpp
     TestPlane.cpp
     TestTriangle.cpp
     TestVector3d.cpp

tests/TestPartition.cpp

@@ -0,0 +1,44 @@
+// Copyright © 2026 CCP ehf.
+
+#include "StdAfx.h"
+
+TEST(PartitionKeyHash, SeparatesCoordinatesThatPreviouslyCollided)
+{
+    const Key first(1, 0, 0, 16);
+    const Key second(0, 2, 0, 16);
+
+    // The previous linear hash produced 24 for both keys:
+    // (1 << 3) + 16 == (2 << 2) + 16.
+    EXPECT_NE(KeyHash{}(first), KeyHash{}(second));
+}
+
+TEST(PartitionKeyHash, RetainsEveryDistinctBoxKey)
+{
+    MapOfBoxes boxes;
+    constexpr int gridSize = 16;
+
+    for (int64_t ix = 0; ix < gridSize; ++ix)
+    {
+        for (int64_t iy = 0; iy < gridSize; ++iy)
+        {
+            for (int64_t iz = 0; iz < gridSize; ++iz)
+            {
+                boxes.emplace(Key(ix, iy, iz, gridSize), nullptr);
+            }
+        }
+    }
+
+    EXPECT_EQ(boxes.size(), static_cast<size_t>(gridSize * gridSize * gridSize));
+
+    for (int64_t ix = 0; ix < gridSize; ++ix)
+    {
+        for (int64_t iy = 0; iy < gridSize; ++iy)
+        {
+            for (int64_t iz = 0; iz < gridSize; ++iz)
+            {
+                const auto found = boxes.find(Key(ix, iy, iz, gridSize));
+                ASSERT_NE(found, boxes.end());
+            }
+        }
+    }
+}