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#pragma once
#include <stdint.h>
// 4J added - Storage for block & sky light data. Lighting data is normally stored as 4-bits per tile, in a DataLayer class of 16384 bytes ( 128 x 16 x 16 x 0.5 )
// This class provides more economical storage for such data by taking into consideration that it is quite common for large parts of the lighting data in a level to
// be very compressible (large amounts of 0 for block lights, 0 and 15 for sky lights).
// We are aiming here to balance performance (lighting data is accessed very frequently) against size.
// Details of storage method:
// 1. Lighting is split into horizontal planes, of which there are 128, and each taking up 128 bytes (16 x 16 x 0.5)
// 2. Each of these layers has a permanently allocated index in this class (planeIndices).
// 3. Data for allocatedPlaneCount planes worth of data is allocated in the data array ( allocatedPlaneCount * 128 bytes )
// 4. If a plane index for a layer is < 128, then the data for that layer is at data[ index * 128 ]
// 5. If a plane index for a layer is 128, then all values for that plane are 0
// 6. If a plane index for a layer is 129, then all values for that plane are 15
// This class needs to be thread safe as there are times where chunk (and light) data are shared between server & main threads. Light values are queried
// very regularly so this needs to be as light-weight as possible.
// To meet these requirements, this class is now implemented using a lock-free system, implemented using a read-copy-update (RCU) type algorithm. Some details...
// (1) The storage details for the class are now packed into a single int64_t, which contains both a pointer to the data that is required and a count of how many planes worth
// of storage are allocated. This allows the full storage to be updated atomically using compare and exchange operations (implemented with InterlockedCompareExchangeRelease64).
// (2) The data pointer referenced in this int64_t points to an area of memory which is 128 + 128 * plane_count bytes long, where the first 128 bytes stoere the plane indices, and
// the rest of the data is variable in size to accomodate however many planes are required to be stored
// (3) The RCU bit of the algorithm means that any read operations don't need to do any checks or locks at all. When the data needs to be updated, a copy of it is made and updated,
// then an attempt is made to swap the new data in - if this succeeds then the old data pointer is deleted later at some point where we know nothing will be reading from it anymore.
// This is achieved by putting the delete request in a queue which means it won't actually get deleted until 2 game ticks after the last time its reference existed, which should give
// us a large margin of safety. If the attempt to swap the new data in fails, then the whole write operation has to be attempted again - this is the only time there is really a
// high cost for this algorithm and such write collisions should be rare.
//#define LIGHT_COMPRESSION_STATS
class SparseLightStorage_SPU
{
private:
// unsigned char planeIndices[128];
unsigned char* m_pData;
// unsigned char *data;
// unsigned int allocatedPlaneCount;
static const int ALL_0_INDEX = 128;
static const int ALL_15_INDEX = 129;
public:
SparseLightStorage_SPU(unsigned char* data) : m_pData(data) {}
unsigned char* getDataPtr() { return m_pData; }
inline int get(int x, int y, int z) // Get an individual lighting value
{
unsigned char *planeIndices, *data;
getPlaneIndicesAndData(&planeIndices, &data);
if( planeIndices[y] == ALL_0_INDEX )
{
return 0;
}
else if ( planeIndices[y] == ALL_15_INDEX )
{
return 15;
}
else
{
int planeIndex = x * 16 + z; // Index within this xz plane
int byteIndex = planeIndex / 2; // Byte index within the plane (2 tiles stored per byte)
int shift = ( planeIndex & 1 ) * 4; // Bit shift within the byte
int retval = ( data[ planeIndices[y] * 128 + byteIndex ] >> shift ) & 15;
return retval;
}
}
inline void getPlaneIndicesAndData(unsigned char **planeIndices, unsigned char **data)
{
*planeIndices = m_pData;
*data = m_pData + 128;
}
};
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