using System;

namespace ClassicalSharp {

	/// <summary> Represents a fixed size map of blocks. Stores the raw block data,
	/// heightmap, dimensions and various metadata such as environment settings. </summary>
	public sealed partial class Map {
		
		int CalcHeightAt( int x, int maxY, int z, int index ) {
			int mapIndex = ( maxY * Length + z ) * Width + x;
			for( int y = maxY; y >= 0; y-- ) {
				byte block = mapData[mapIndex];
				if( info.BlocksLight[block] ) {
					heightmap[index] = (short)(y - 1);
					return y - 1;
				}
				mapIndex -= oneY;
			}
			
			heightmap[index] = -10;
			return -10;
		}
		
		void UpdateHeight( int x, int y, int z, byte oldBlock, byte newBlock ) {
			bool didBlock = info.BlocksLight[oldBlock];
			bool nowBlocks = info.BlocksLight[newBlock];
			if( didBlock == nowBlocks ) return;
			
			int index = (z * Width) + x;
			int height = heightmap[index];
			if( height == short.MaxValue ) {
				// We have to calculate the entire column for visibility, because the old/new block info is
				// useless if there is another block higher than block.y that blocks sunlight.
				CalcHeightAt( x, maxY, z, index );
			} else if( y > height ) {
				if( nowBlocks ) {
					heightmap[index] = (short)(y - 1);
				} else {
					// Part of the column is now visible to light, we don't know how exactly how high it should be though.
					// However, we know that if the old block was above or equal to light height, then the new light height must be <= old block.y
					CalcHeightAt( x, y, z, index );
				}
			}
		}
		
		// Equivalent to
		// for x = startX; x < startX + 18; x++
		//    for z = startZ; z < startZ + 18; z++
		//       CalcHeightAt(x, maxY, z) if height == short.MaxValue
		// Except this function is a lot more optimised and minimises cache misses.
		internal unsafe void HeightmapHint( int startX, int startZ, byte* mapPtr ) {
			int x1 = Math.Max( startX, 0 ), x2 = Math.Min( Width, startX + 18 );
			int z1 = Math.Max( startZ, 0 ), z2 = Math.Min( Length, startZ + 18 );
			int xCount = x2 - x1, zCount = z2 - z1;
			int* skip = stackalloc int[xCount * zCount];
			
			int elemsLeft = InitialHeightmapCoverage( x1, z1, xCount, zCount, skip );
			if( !CalculateHeightmapCoverage( x1, z1, xCount, zCount, elemsLeft, skip, mapPtr ) ) {
				FinishHeightmapCoverage( x1, z1, xCount, zCount, skip );
			}
		}
		
		unsafe int InitialHeightmapCoverage( int x1, int z1, int xCount, int zCount, int* skip ) {
			int elemsLeft = 0, index = 0, curRunCount = 0;
			for( int z = 0; z < zCount; z++ ) {
				int heightmapIndex = ( z1 + z ) * Width + x1;
				for( int x = 0; x < xCount; x++ ) {
					int height = heightmap[heightmapIndex++];
					skip[index] = 0;
					if( height == short.MaxValue ) {
						elemsLeft++;
						curRunCount = 0;
					} else {
						skip[index - curRunCount]++;
						curRunCount++;
					}
					index++;
				}
				curRunCount = 0; // We can only skip an entire X row at most.
			}
			return elemsLeft;
		}
		
		unsafe bool CalculateHeightmapCoverage( int x1, int z1, int xCount, int zCount, int elemsLeft, int* skip, byte* mapPtr ) {
			int prevRunCount = 0;
			for( int y = maxY; y >= 0; y-- ) {
				if( elemsLeft <= 0 ) return true;
				int mapIndex = ( y * Length + z1 ) * Width + x1;
				int heightmapIndex = z1 * Width + x1;
				
				for( int z = 0; z < zCount; z++ ) {
					int baseIndex = mapIndex;
					int index = z * xCount;
					for( int x = 0; x < xCount; ) {
						int curRunCount = skip[index];
						x += curRunCount; mapIndex += curRunCount; index += curRunCount;
						
						if( x < xCount && info.BlocksLight[mapPtr[mapIndex]] ) {
							heightmap[heightmapIndex + x] = (short)( y - 1 );
							elemsLeft--;
							skip[index] = 0;
							int offset = prevRunCount + curRunCount;
							int newRunCount = skip[index - offset] + 1;
							
							// consider case 1 0 1 0, where we are at 0
							// we need to make this 3 0 0 0 and advance by 1
							int oldRunCount = ( x - offset + newRunCount ) < xCount ? skip[index - offset + newRunCount] : 0;
							if( oldRunCount != 0 ) {
								skip[index - offset + newRunCount] = 0;
								newRunCount += oldRunCount;
							}
							skip[index - offset] = newRunCount;
							x += oldRunCount; index += oldRunCount; mapIndex += oldRunCount;
							prevRunCount = newRunCount;
						} else {
							prevRunCount = 0;
						}
						x++; mapIndex++; index++;
					}
					prevRunCount = 0;
					heightmapIndex += Width;
					mapIndex = baseIndex + Width; // advance one Z
				}
			}
			return false;
		}
		
		unsafe void FinishHeightmapCoverage( int x1, int z1, int xCount, int zCount, int* skip ) {
			for( int z = 0; z < zCount; z++ ) {
				int heightmapIndex = ( z1 + z ) * Width + x1;
				for( int x = 0; x < xCount; x++ ) {
					int height = heightmap[heightmapIndex];
					if( height == short.MaxValue )
						heightmap[heightmapIndex] = -10;
					heightmapIndex++;
				}
			}
		}
	}
}