如何计算两个矩形之间的距离？ (上下文：Lua中的一个游戏)

(1) check if the rects intersect. If so,the distance between them is 0.
(2) If not,think of r2 as the center of a telephone key pad,#5.
(3) r1 may be fully in one of the extreme quadrants (#1,#3,#7,or #9). If so
    the distance is the distance from one rect corner to another (e.g.,if r1 is
    in quadrant #1,the distance is the distance from the lower-right corner of
    r1 to the upper-left corner of r2).
(4) Otherwise r1 is to the left,right,above,or below r2 and the distance is
    the distance between the relevant sides (e.g.,if r1 is above,the distance
    is the distance between r1's low y and r2's high y).