/// See the #rcConfig documentation for more information on the configuration parameters.
/// 
/// @see rcAllocHeightfieldLayerSet, rcCompactHeightfield, rcHeightfieldLayerSet, rcConfig
bool rcBuildHeightfieldLayers(rcContext* ctx, rcCompactHeightfield& chf,
							  const int borderSize, const int walkableHeight,
							  rcHeightfieldLayerSet& lset)
{
	rcAssert(ctx);
	
	rcScopedTimer timer(ctx, RC_TIMER_BUILD_LAYERS);
	
	const int w = chf.width;
	const int h = chf.height;
	
	rcScopedDelete<unsigned char> srcReg((unsigned char*)rcAlloc(sizeof(unsigned char)*chf.spanCount, RC_ALLOC_TEMP));
	if (!srcReg)
	{
		ctx->log(RC_LOG_ERROR, "rcBuildHeightfieldLayers: Out of memory 'srcReg' (%d).", chf.spanCount);
		return false;
	}
	memset(srcReg,0xff,sizeof(unsigned char)*chf.spanCount);
	
	const int nsweeps = chf.width;
	rcScopedDelete<rcLayerSweepSpan> sweeps((rcLayerSweepSpan*)rcAlloc(sizeof(rcLayerSweepSpan)*nsweeps, RC_ALLOC_TEMP));
	if (!sweeps)
	{
		ctx->log(RC_LOG_ERROR, "rcBuildHeightfieldLayers: Out of memory 'sweeps' (%d).", nsweeps);
		return false;
	}
	
	
	// Partition walkable area into monotone regions.
	int prevCount[256];
	unsigned char regId = 0;

	//注意这里是三层循环:
	// for z 平面
	//		for x 平面
	//			for y 平面 (高度)
	// 最内层是对每个y平面的处理, 在每个y层面上根据span在x/z的连接性做region分配和合并, 也就是layer的意义: 按高度分层. 像是切片面包.
	// 从3d视角看是, 遍历x/z平面的每个cell, 依次检查当前cell与相邻cell在高度上的切片span是否有联通的, 如果有联通就把x/z平面相邻的cell上region赋值为相同id. 让x/z平面形成region.高度上
	for (int z = borderSize; z < h-borderSize; ++z)
	{
		// prevCount 记录的是当前x轴上的sweep和上一轮x循环(-z方向)的region相连的span数量.
		memset(prevCount,0,sizeof(int)*regId);
		//(按行扫描编号), 这个编号在y的循环体内, 也就是每扫描一行x则重置, 扫描完一行后后面会把sweepId变成regionId, 所以重置没问题.
		unsigned char nowSweepId = 0; 
		
		for (int x = borderSize; x < w-borderSize; ++x)
		{
			const rcCompactCell& c = chf.cells[x+z*w];
			
			for (int i = (int)c.index, ni = (int)(c.index+c.count); i < ni; ++i)
			{
				const rcCompactSpan& s = chf.spans[i];
				if (chf.areas[i] == RC_NULL_AREA) continue;

				unsigned char sweepId = 0xff;

				//(-1, 0)方向如果有连接
				// -x
				if (rcGetCon(s, 0) != RC_NOT_CONNECTED)
				{
					const int ax = x + rcGetDirOffsetX(0);
					const int ay = z + rcGetDirOffsetY(0);
					const int ai = (int)chf.cells[ax+ay*w].index + rcGetCon(s, 0);
					//如果连接的不是NULL_AREA且它的sweepId并不是未初始化状态(未设置, 默认值0xff) (sweepId存储在srcReg里)
					//那么把自己的sweepId也设置为相邻这个span的sweepId,因为是从左到右遍历, 所以-x是刚刚遍历过的,如果连接(x轴相邻的span)且有srcReg, 则设置为相同srcReg 
					if (chf.areas[ai] != RC_NULL_AREA && srcReg[ai] != 0xff) 
						sweepId = srcReg[ai];									
				}
				
				// 如果和左侧相邻span(-1, 0)没有连接, 或者连接的area是NULL, 或者sweepId无效, 则把自己的sweepId设置为新的id. (新分配一个扫描编号)
				if (sweepId == 0xff)
				{
					sweepId = nowSweepId++;
					sweeps[sweepId].nei = 0xff;
					sweeps[sweepId].ns = 0;
				}
				
				// 检查完-x方向. 再检查之前扫描过的z方向的邻居 (上一轮扫描过的)
				// 如果相连且sweepId不是0xff, 则判断是不是刚刚x方向新加的sweepId(还没邻居), 如果是则把z方向的这个邻居设置成自己的邻居
				// 如果当前邻居是z方向的这个span, 则把ns++, 把邻居sweepId记录的数量也加1(prevCount[nrSweepId]++)
				// 如果当前邻居不是z方向这个span, 说明和-z这一行有两个邻居, 则把邻居置为无效值 
				// (0, -1) x/z平面的下面 -> -z, 注意源码是 x/y 平面, 这里原本注释写的 -y
				if (rcGetCon(s,3) != RC_NOT_CONNECTED)
				{
					const int ax = x + rcGetDirOffsetX(3);
					const int ay = z + rcGetDirOffsetY(3);
					const int ai = (int)chf.cells[ax+ay*w].index + rcGetCon(s, 3);
					const unsigned char nrRegId = srcReg[ai];
					if (nrRegId != 0xff)
					{
						// Set neighbour when first valid neighbour is encoutered.
						if (sweeps[sweepId].ns == 0)
							sweeps[sweepId].nei = nrRegId;
						
						if (sweeps[sweepId].nei == nrRegId)
						{
							// Update existing neighbour
							sweeps[sweepId].ns++;
							prevCount[nrRegId]++;
						}
						else
						{
							// This is hit if there is nore than one neighbour.
							// Invalidate the neighbour.
							sweeps[sweepId].nei = 0xff;
						}
					}
				}
				
				srcReg[i] = sweepId;
			}
		}
		
		// Create unique ID.
		for (int i = 0; i < nowSweepId; ++i)
		{
			/// 如果邻居设置了, 而且邻居连接我的数量和我数量相同则说明我们是完全相临的, 可以合并, 否则意味着我的邻居可能还有其他sweepId和他相连. 
			/// 类似下面, A先扫描完, 形成了一个完整连续的region=1, 再遍历B时, prevCount[1] = 4, (A行3个1和1个2), 但是sweeps[1] = 3, (B行3个1)
			/// 所以此时B行里的1和A行里的1不能合并了. 要给B行的1分配新的regionId
			/// 
			///   <--- -x方向(左)
			///                             |
			/// B: [1] [1] [1]     [2]      |     -> 此处的1, 2都还是sweepId, 代表从左到右的扫描分割序号.
			/// A: [1] [1] [1] [1] [1]      |     -> 此时的1已经是regionId了.
			///                           -z方向(下)
			/// 
			/// 
			/// B: [1] [1]               |
			/// A: [1] [1] [1]           |     -> 这种情况可以合并,  prevCount[A1].nei = 2, sweeps[B1].ns = 2
			/// 
			/// B: [1] [1] [1] [1]       |
			/// A: [1] [1] [1]           |     -> 这种情况也可以合并,  prevCount[A1].nei = 3, sweeps[B1].ns = 3, (B第四个[1]因为和下面无连接, 所以两边都不计数)
			/// 
			/// If the neighbour is set and there is only one continuous connection to it,
			/// the sweep will be merged with the previous one, else new region is created.
			if (sweeps[i].nei != 0xff && prevCount[sweeps[i].nei] == (int)sweeps[i].ns)
			{
				sweeps[i].id = sweeps[i].nei;
			}
			else
			{
				if (regId == 255)
				{
					ctx->log(RC_LOG_ERROR, "rcBuildHeightfieldLayers: Region ID overflow.");
					return false;
				}
				sweeps[i].id = regId++;
			}
		}
		
		// 之前srcReg里记录的是sweepId, 现在改回regionId
		// Remap local sweep ids to region ids.
		for (int x = borderSize; x < w-borderSize; ++x)
		{
			const rcCompactCell& c = chf.cells[x+z*w];
			for (int i = (int)c.index, ni = (int)(c.index+c.count); i < ni; ++i)
			{
				if (srcReg[i] != 0xff)
					srcReg[i] = sweeps[srcReg[i]].id;
			}
		}
	}

	// Allocate and init layer regions.
	const int nregs = (int)regId;
	rcScopedDelete<rcLayerRegion> regs((rcLayerRegion*)rcAlloc(sizeof(rcLayerRegion)*nregs, RC_ALLOC_TEMP));
	if (!regs)
	{
		ctx->log(RC_LOG_ERROR, "rcBuildHeightfieldLayers: Out of memory 'regs' (%d).", nregs);
		return false;
	}
	memset(regs, 0, sizeof(rcLayerRegion)*nregs);
	for (int i = 0; i < nregs; ++i)
	{
		regs[i].layerId = 0xff;
		regs[i].ymin = 0xffff;
		regs[i].ymax = 0;
	}
	
	// Find region neighbours and overlapping regions.
	for (int z = 0; z < h; ++z) //遍历 z
	{
		for (int x = 0; x < w; ++x) //遍历 x/z 平面
		{
			const rcCompactCell& c = chf.cells[x+z*w];
			
			//记录y方向的区域id和数量
			unsigned char lregs[RC_MAX_LAYERS];
			int nlregs = 0;
			
			for (int i = (int)c.index, ni = (int)(c.index+c.count); i < ni; ++i) //遍历 y 方向 span
			{
				const rcCompactSpan& s = chf.spans[i];
				const unsigned char regionId = srcReg[i];
				if (regionId == 0xff) continue; //跳过没有区域的span
				
				regs[regionId].ymin = rcMin(regs[regionId].ymin, s.y);
				regs[regionId].ymax = rcMax(regs[regionId].ymax, s.y);
				
				// Collect all region layers.
				if (nlregs < RC_MAX_LAYERS)
					lregs[nlregs++] = regionId;
				
				// Update neighbours
				// 遍历4个方向, 记录邻居区域信息 (和自己不同区域)
				for (int dir = 0; dir < 4; ++dir)
				{
					if (rcGetCon(s, dir) != RC_NOT_CONNECTED)
					{
						const int ax = x + rcGetDirOffsetX(dir);
						const int ay = z + rcGetDirOffsetY(dir);
						const int ai = (int)chf.cells[ax+ay*w].index + rcGetCon(s, dir);
						const unsigned char nrReg = srcReg[ai]; 
						if (nrReg != 0xff && nrReg != regionId) //邻居的region 和 自己不一样
						{
							// Don't check return value -- if we cannot add the neighbor
							// it will just cause a few more regions to be created, which
							// is fine.
							addUnique(regs[regionId].neis, regs[regionId].nneis, RC_MAX_NEIS, nrReg);
						}
					}
				}
				
			}
			
			// 两层遍历高度(y)方向的区域 (两两检查), 
			// Update overlapping regions.
			for (int i = 0; i < nlregs-1; ++i)
			{
				for (int j = i+1; j < nlregs; ++j)
				{
					if (lregs[i] != lregs[j])
					{
						rcLayerRegion& ri = regs[lregs[i]];
						rcLayerRegion& rj = regs[lregs[j]];

						//在两个region的layers里记录该region在x/z平面上重叠的其他高度的regionId. 用于索引高度上的不同层.
						if (!addUnique(ri.layers, ri.nlayers, RC_MAX_LAYERS, lregs[j]) ||
							!addUnique(rj.layers, rj.nlayers, RC_MAX_LAYERS, lregs[i]))
						{
							ctx->log(RC_LOG_ERROR, "rcBuildHeightfieldLayers: layer overflow (too many overlapping walkable platforms). Try increasing RC_MAX_LAYERS.");
							return false;
						}
					}
				}
			}
			
		}
	}
	
	// Create 2D layers from regions.
	unsigned char layerId = 0;
	
	static const int MAX_STACK = 64;
	unsigned char stack[MAX_STACK];
	int nstack = 0;
	
	for (int i = 0; i < nregs; ++i)
	{
		rcLayerRegion& root = regs[i];
		// Skip already visited.
		if (root.layerId != 0xff)
			continue;

		// Start search.
		// 分配 layerId
		root.layerId = layerId;
		root.base = 1;
		
		nstack = 0;
		stack[nstack++] = (unsigned char)i; //region序号入栈
		
		while (nstack)
		{
			// Pop front
			rcLayerRegion& reg = regs[stack[0]];
			nstack--;
			for (int j = 0; j < nstack; ++j) //移除stack第一个元素.
				stack[j] = stack[j+1];
			
			const int nneis = (int)reg.nneis;
			for (int j = 0; j < nneis; ++j)
			{
				const unsigned char nei = reg.neis[j];
				rcLayerRegion& nrReg = regs[nei];
				// Skip already visited.
				if (nrReg.layerId != 0xff)
					continue;
				// Skip if the neighbour is overlapping root region.
				// 跳过 邻居是x/z重叠的不同高度的region
				if (contains(root.layers, root.nlayers, nei))
					continue;
				// Skip if the height range would become too large.
				// 如果两个区域加起来的高度落差太大 跳过 (因为高度差不大的情况下会合并layer, 但是合并太多后会导致layer上下表面的高差越来越大, 这时候就要打断合并了)
				const int ymin = rcMin(root.ymin, nrReg.ymin);
				const int ymax = rcMax(root.ymax, nrReg.ymax);
				if ((ymax - ymin) >= 255)
					 continue;

				if (nstack < MAX_STACK)
				{
					// Deepen 邻居入栈
					stack[nstack++] = (unsigned char)nei;
					
					// Mark layer id
					// 将邻居的layerId设置为自己的layerId. 合并成一个layer
					nrReg.layerId = layerId;
					// Merge current layers to root.
					// 将邻居的高度layers也合并到自己的layers, (合并成一个layer了, 高度重叠区域信息也要合并).
					for (int k = 0; k < nrReg.nlayers; ++k)
					{
						if (!addUnique(root.layers, root.nlayers, RC_MAX_LAYERS, nrReg.layers[k]))
						{
							ctx->log(RC_LOG_ERROR, "rcBuildHeightfieldLayers: layer overflow (too many overlapping walkable platforms). Try increasing RC_MAX_LAYERS.");
							return false;
						}
					}
					root.ymin = rcMin(root.ymin, nrReg.ymin); // 更新合并后的layer上下表面.
					root.ymax = rcMax(root.ymax, nrReg.ymax);
				}
			}
		}
		
		layerId++;
	}
	
	// Merge non-overlapping regions that are close in height.
	// 合并高度上差异不大, 而且没有重叠的区域, 楼梯, 坡等 
	const unsigned short mergeHeight = (unsigned short)walkableHeight * 4;
	
	for (int i = 0; i < nregs; ++i)
	{
		rcLayerRegion& ri = regs[i];
		if (!ri.base) continue; //只需要查询layer的 base region
		
		unsigned char newId = ri.layerId;
		
		for (;;)
		{
			unsigned char oldId = 0xff;
			
			for (int j = 0; j < nregs; ++j)  //双层遍历 region 两两计算
			{
				if (i == j) continue;
				rcLayerRegion& rj = regs[j];
				if (!rj.base) continue;
				
				// Skip if the regions are not close to each other.
				// 两个区域的上下表面+合并高差 不重叠, 则无法合并
				if (!overlapRange(ri.ymin,ri.ymax+mergeHeight, rj.ymin,rj.ymax+mergeHeight))
					continue;
				// Skip if the height range would become too large.
				const int ymin = rcMin(ri.ymin, rj.ymin);
				const int ymax = rcMax(ri.ymax, rj.ymax);
				if ((ymax - ymin) >= 255) //合并后高差太大 跳过
				  continue;
						  
				// Make sure that there is no overlap when merging 'ri' and 'rj'.
				bool overlap = false;
				// Iterate over all regions which have the same layerId as 'rj'
				for (int k = 0; k < nregs; ++k)
				{
					if (regs[k].layerId != rj.layerId)
						continue;
					// Check if region 'k' is overlapping region 'ri'
					// Index to 'regs' is the same as region id.
					// 和j相同layerId的区域, 判断是否和ri有重叠, 如果有重叠说明合并regionI 和 regionJ 后会导致用一个region在x/z平面出现重叠. 所以此时要break. 不能合并
					if (contains(ri.layers,ri.nlayers, (unsigned char)k))
					{
						overlap = true;
						break;
					}
				}
				// Cannot merge of regions overlap.
				if (overlap)
					continue;
				
				// Can merge i and j.
				oldId = rj.layerId;
				break;
			}
			
			// Could not find anything to merge with, stop.
			if (oldId == 0xff)
				break;
			
			// Merge
			for (int j = 0; j < nregs; ++j)
			{
				rcLayerRegion& rj = regs[j];
				if (rj.layerId == oldId)
				{
					rj.base = 0;
					// Remap layerIds.
					rj.layerId = newId;
					// Add overlaid layers from 'rj' to 'ri'.
					// 合并之后, 同样也需要 将邻居的高度layers也合并到自己的layers, (合并成一个layer了, 高度重叠区域信息也要合并).
					for (int k = 0; k < rj.nlayers; ++k)
					{
						if (!addUnique(ri.layers, ri.nlayers, RC_MAX_LAYERS, rj.layers[k]))
						{
							ctx->log(RC_LOG_ERROR, "rcBuildHeightfieldLayers: layer overflow (too many overlapping walkable platforms). Try increasing RC_MAX_LAYERS.");
							return false;
						}
					}

					// Update height bounds.
					ri.ymin = rcMin(ri.ymin, rj.ymin); // 更新合并后的layer上下表面.
					ri.ymax = rcMax(ri.ymax, rj.ymax);
				}
			}
		}
	}
	
	// 合并后layerId不连续了, 所以这里要重新remap下, 保持layerId连续
	// Compact layerIds
	unsigned char remap[256];
	memset(remap, 0, 256);

	// Find number of unique layers.
	layerId = 0;
	for (int i = 0; i < nregs; ++i)
		remap[regs[i].layerId] = 1;
	for (int oldLayerId = 0; oldLayerId < 256; ++oldLayerId)
	{
		if (remap[oldLayerId])
			remap[oldLayerId] = layerId++;
		else
			remap[oldLayerId] = 0xff;
	}
	// Remap ids.
	for (int i = 0; i < nregs; ++i)
		regs[i].layerId = remap[regs[i].layerId]; //从remap里查询oldLayerId对应的新layerId, 并赋值
	
	// No layers, return empty.
	if (layerId == 0)
		return true;
	
	// Create layers.
	rcAssert(lset.layers == 0);
	
	const int lw = w - borderSize*2;
	const int lh = h - borderSize*2;

	// Build contracted bbox for layers.
	float bmin[3], bmax[3];
	rcVcopy(bmin, chf.bmin);
	rcVcopy(bmax, chf.bmax);
	bmin[0] += borderSize*chf.cs;
	bmin[2] += borderSize*chf.cs;
	bmax[0] -= borderSize*chf.cs;
	bmax[2] -= borderSize*chf.cs;
	
	lset.nlayers = (int)layerId;
	
	lset.layers = (rcHeightfieldLayer*)rcAlloc(sizeof(rcHeightfieldLayer)*lset.nlayers, RC_ALLOC_PERM);
	if (!lset.layers)
	{
		ctx->log(RC_LOG_ERROR, "rcBuildHeightfieldLayers: Out of memory 'layers' (%d).", lset.nlayers);
		return false;
	}
	memset(lset.layers, 0, sizeof(rcHeightfieldLayer)*lset.nlayers);

	
	// Store layers.
	for (int i = 0; i < lset.nlayers; ++i)
	{
		unsigned char curId = (unsigned char)i;

		rcHeightfieldLayer* layer = &lset.layers[curId];

		const int gridSize = sizeof(unsigned char)*lw*lh; //体素x/z空间size, 二维数组长度

		layer->heights = (unsigned char*)rcAlloc(gridSize, RC_ALLOC_PERM);
		if (!layer->heights)
		{
			ctx->log(RC_LOG_ERROR, "rcBuildHeightfieldLayers: Out of memory 'heights' (%d).", gridSize);
			return false;
		}
		memset(layer->heights, 0xff, gridSize);

		layer->areas = (unsigned char*)rcAlloc(gridSize, RC_ALLOC_PERM);
		if (!layer->areas)
		{
			ctx->log(RC_LOG_ERROR, "rcBuildHeightfieldLayers: Out of memory 'areas' (%d).", gridSize);
			return false;
		}
		memset(layer->areas, 0, gridSize);

		layer->cons = (unsigned char*)rcAlloc(gridSize, RC_ALLOC_PERM);
		if (!layer->cons)
		{
			ctx->log(RC_LOG_ERROR, "rcBuildHeightfieldLayers: Out of memory 'cons' (%d).", gridSize);
			return false;
		}
		memset(layer->cons, 0, gridSize);
		
		// Find layer height bounds.
		int hmin = 0, hmax = 0; //上下表面高度 (体素单位)
		for (int j = 0; j < nregs; ++j)
		{
			if (regs[j].base && regs[j].layerId == curId)
			{
				hmin = (int)regs[j].ymin;
				hmax = (int)regs[j].ymax; //此处应该可以break ?
			}
		}

		layer->width = lw;
		layer->height = lh;
		layer->cs = chf.cs;
		layer->ch = chf.ch;
		
		// Adjust the bbox to fit the heightfield.
		rcVcopy(layer->bmin, bmin);
		rcVcopy(layer->bmax, bmax);
		layer->bmin[1] = bmin[1] + hmin*chf.ch; //体素高度转坐标高度
		layer->bmax[1] = bmin[1] + hmax*chf.ch;
		layer->hmin = hmin;
		layer->hmax = hmax;

		// Update usable data region.
		layer->minx = layer->width;
		layer->maxx = 0;
		layer->miny = layer->height;
		layer->maxy = 0;
		
		// Copy height and area from compact heightfield. 
		for (int z = 0; z < lh; ++z)
		{
			for (int x = 0; x < lw; ++x)
			{
				const int cx = borderSize+x;
				const int cz = borderSize+z;
				const rcCompactCell& c = chf.cells[cx+cz*w];
				for (int j = (int)c.index, nj = (int)(c.index+c.count); j < nj; ++j)
				{
					const rcCompactSpan& span = chf.spans[j];
					// Skip unassigned regions.
					if (srcReg[j] == 0xff)
						continue;
					// Skip of does nto belong to current layer.
					unsigned char lid = regs[srcReg[j]].layerId;
					if (lid != curId)
						continue;
					
					// Update data bounds.
					layer->minx = rcMin(layer->minx, x);
					layer->maxx = rcMax(layer->maxx, x);
					layer->miny = rcMin(layer->miny, z);
					layer->maxy = rcMax(layer->maxy, z);
					
					// Store height and area type.
					const int idx = x+z*lw;
					layer->heights[idx] = (unsigned char)(span.y - hmin);
					layer->areas[idx] = chf.areas[j];
					
					// Check connection.
					unsigned char portal = 0;
					unsigned char con = 0;
					for (int dir = 0; dir < 4; ++dir)
					{
						if (rcGetCon(span, dir) != RC_NOT_CONNECTED)
						{
							const int ax = cx + rcGetDirOffsetX(dir);
							const int ay = cz + rcGetDirOffsetY(dir);
							const int ai = (int)chf.cells[ax+ay*w].index + rcGetCon(span, dir);
							unsigned char alid = srcReg[ai] != 0xff ? regs[srcReg[ai]].layerId : 0xff;
							// Portal mask
							if (chf.areas[ai] != RC_NULL_AREA && lid != alid)
							{
								portal |= (unsigned char)(1<<dir);
								// Update height so that it matches on both sides of the portal.
								const rcCompactSpan& as = chf.spans[ai];
								if (as.y > hmin)
									layer->heights[idx] = rcMax(layer->heights[idx], (unsigned char)(as.y - hmin));
							}
							// Valid connection mask
							// 相邻的同layer的span连接信息记录在 cons的低4位. (上下左右)
							if (chf.areas[ai] != RC_NULL_AREA && lid == alid)
							{
								const int nx = ax - borderSize;
								const int ny = ay - borderSize;
								if (nx >= 0 && ny >= 0 && nx < lw && ny < lh)
									con |= (unsigned char)(1<<dir);
							}
						}
					}
					
					layer->cons[idx] = (portal << 4) | con; //相邻的不同layer的信息记录在cons的高4位.
				}
			}
		}
		
		if (layer->minx > layer->maxx)
			layer->minx = layer->maxx = 0;
		if (layer->miny > layer->maxy)
			layer->miny = layer->maxy = 0;
	}
	
	return true;
}