0.引言
记录一下，不然久了可能就看不懂了。
1.图解Neighbor Pruning & Forced Neighbors
case3中，方向为正方体对角线。
// nsz contains the number of neighbors for the four different types of moves:
// no move (norm 0): 26 neighbors always added
// 0 forced neighbors to check (never happens)
// 0 neighbors to add if forced (never happens)
// straight (norm 1): 1 neighbor always added
// 8 forced neighbors to check
// 8 neighbors to add if forced
// diagonal (norm sqrt(2)): 3 neighbors always added
// 8 forced neighbors to check
// 12 neighbors to add if forced
// diagonal (norm sqrt(3)): 7 neighbors always added
// 6 forced neighbors to check
// 12 neighbors to add if forced
static constexpr int nsz[4][2] = {{26, 0}, {1, 8}, {3, 12}, {7, 12}};
straight (norm 1): 即是case1，对垂直与屏幕的平面，从当前节点检测周围8个点是不是障碍物，如果是，则下一平面的8个点即为forced neighbors，将8个均加入openset，疑问：为什么不是将下一平面的9个点均加入？哦：1 neighbor always added。
diagonal (norm sqrt(3)): 即是case3,从小正方体看，"7 neighbors always added"容易理解(7个natural neighbors)，从底层图看，图中没画完全，再考虑障碍物对称情况，可能会有6个forced neighbors。“12 neighbors to add if forced”是最多12个neighbors加入openset，而不是13个。
diagonal (norm sqrt(2)): 即是case2，先看case3,再看case2其实更好理解，case2的顶层类似于case3的底层，于是有6个可能的forced neighbors，然后再看中间层的情况，同样是只画了一部分(画全了更不好理解了)，对称性加上去就可以看出有2个可能的forced neighbors，故，case2一共有8个可能的forced neighbors。方向只在中间层，“3 neighbors always added”很好理解。疑问：为什么是“12 neighbors to add if forced”而不是11？