Replaced Vector3D with integer as key to map.

Chunks of voxel data are stored in a map, and it is quite common to need to search the map for a particular chunk. The key type used to be a Vector3D (i.e. the position of the chunk in 3D space) which makes conceptual sense but is relatively slow. Using a Vector3D as a key seems to have overhead, probably in terms of copying and performing comparisons. It seems to be significantly faster to use an integer as a key, so we now take the 3D position and pack it into a single integer by bitshifting.

Naturally this reduces the range of positions we can store - a 32-bit int can only encode 3 x 10-bit values, which means our volume can only be 1024 chunks in each direction (with a chunk often being 32x32x32 voxels). This should still be large enough for most uses, but an upcoming change will allow 64-bit keys to be used (at least on 64-bit builds) which then allows 21 bits of precision  per component. This is so large that it's almost infinite for any practical purposes.

include/PolyVox/PagedVolume.h

@@ -281,18 +281,30 @@ namespace PolyVox
 		PagedVolume& operator=(const PagedVolume& rhs);
 
 	private:	
-		Chunk* getChunk(int32_t uChunkX, int32_t uChunkY, int32_t uChunkZ) const;
+		Chunk* getChunk(int32_t key) const;
 
-		// Storing these properties individually has proved to be faster than keeping
-		// them in a Vector3DInt32 as it avoids constructions and comparison overheads.
-		// They are also at the start of the class in the hope that they will be pulled
-		// into cache - I've got no idea if this actually makes a difference.
-		mutable int32_t m_v3dLastAccessedChunkX = 0;
-		mutable int32_t m_v3dLastAccessedChunkY = 0;
-		mutable int32_t m_v3dLastAccessedChunkZ = 0;
+		uint32_t posToChunkKey(int32_t iXPos, int32_t iYPos, int32_t iZPos) const
+		{
+			// Bit-shifting of signed integer values has various issues with undefined or implementation-defined behaviour.
+			// Therefore we cast to unsigned to avoid these (we only care about the bit pattern anyway, not the actual value).
+			// See http://stackoverflow.com/a/4009922 for more details.
+			const uint32_t uXPos = static_cast<uint32_t>(iXPos);
+			const uint32_t uYPos = static_cast<uint32_t>(iYPos);
+			const uint32_t uZPos = static_cast<uint32_t>(iZPos);
+
+			const uint32_t xKey = ((uXPos >> m_uChunkSideLengthPower) & 0x3FF) << 20;
+			const uint32_t yKey = ((uYPos >> m_uChunkSideLengthPower) & 0x3FF) << 10;
+			const uint32_t zKey = ((uZPos >> m_uChunkSideLengthPower) & 0x3FF);
+
+			const uint32_t key = 0x80000000 | xKey | yKey | zKey;
+
+			return key;
+		}
+
+		mutable int32_t m_v3dLastAccessedChunkKey = 0;
 		mutable Chunk* m_pLastAccessedChunk = nullptr;
 
-		mutable std::unordered_map<Vector3DInt32, std::unique_ptr< Chunk > > m_mapChunks;
+		mutable std::unordered_map<uint32_t, std::unique_ptr< Chunk > > m_mapChunks;
 
 		mutable uint32_t m_uTimestamper = 0;
 
@@ -310,6 +322,15 @@ namespace PolyVox
 	};
 }
 
+/*#define POS_TO_CHUNK_KEY(x, y, z, uChunkSideLengthPower) \
+{ \
+	const int32_t chunkX = ((x >> uChunkSideLengthPower) & 0x3FF) << 20; \
+	const int32_t chunkY = ((y >> uChunkSideLengthPower) & 0x3FF) << 10; \
+	const int32_t chunkZ = ((z >> uChunkSideLengthPower) & 0x3FF); \
+	const int32_t key = (-2147483648) | chunkX | chunkY | chunkZ; \
+}*/
+
+
 #include "PolyVox/PagedVolume.inl"
 #include "PolyVox/PagedVolumeChunk.inl"
 #include "PolyVox/PagedVolumeSampler.inl"