1065 - Pool

Time Limit : 1 Second

Memory Limit : 128 MB

Submission: 24

Solved: 8

Billiards, also commonly known as “pool,” is a popular game in North America. The game is played on a
rectangular table with six pockets—one at each corner and one in the middle of each of the two longer sides
of the table. The object of the game is to strike a cue ball so that it collides with other balls, knocking them
into the pockets.
The surface of our pool table measures 108” by 54”, and to facilitate computation, we place it on the
Cartesian plane with its southwest corner situated at (0, 0), and its northeast corner at (108, 54). Therefore,
the centers of the 6 pockets, numbered 1 through 6, will have coordinates of (0, 0), (54, 0), (108, 0), (0, 54),
(54, 54), and (108, 54), respectively. The billiard balls are spherical and measure 2” in diameter.

Given the location of the cue ball, a target ball, and a number of other balls on the table, your task for this
problem is to write a program to determine whether or not you can successfully make a particular pool shot.
The cue ball can be struck in any direction in a straight line. We consider collisions between the balls to
be perfectly elastic, so that the target ball will always travel in a straight line, away from the point on its
surface contacted by the cue ball.
A shot is considered possible if the cue ball can be struck so that it collides directly with the target ball, in
turn sending the target ball directly into a pocket. Neither ball should collide with any other balls, bounce
off the edges of the table (cushions), nor should their centers cross the boundaries of the table. In other
words, you are not to consider any bank shots, combination shots, spin shots, or any other trick shots in
this problem. Note that the difference between the incoming angle of the cue ball and the outgoing angle of
the target ball must be greater than 90◦ . The target ball is considered to land in a pocket when its center
coincides with the center of that pocket.

The input test file will contain multiple cases. The first line of each test case contains four real numbers,
xc yc xt yt , where (xc , yc ) is the location of the cue ball and (xt , yt ) is the location of the target ball. The
second line of the test case contains an integer n (where 0 ≤ n ≤ 14), the number of additional balls on the
table, followed by n pairs of real numbers, x1 y1 . . . xn yn , where (xi , yi ) is the location of the ith additional
(possibly obstructing) ball. No two balls overlap, and all balls are strictly in the interior of the table; in
particular, all provided coordinates obey 3 < x < 105 and 3 < y < 51.
Input is terminated with a single line containing only the number 0; do not process this line.
For each test case, output on a single line the number(s) of the pocket(s) into which the target ball can be
shot, sorted in ascending numerical order. Separate pocket numbers with a single space. If there are no
pockets for which a clear shot exists, output the words “no shot”.
sample input
30 27 70 24
80 27 40 27
2 38 28 38 26
81.0 27.0 54.5 27.0
1 54.0 33.3
81.0 27.0 54.5 25.0
1 54.0 33.3
sample output
3 6
no shot
1 4
1 2 4
Stanford Local 2007
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