Information Processing Letters 114 (2014) 130–136
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Information Processing Letters
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Optimum sweeps of simple polygons with two guards
Xuehou Tan
a,b,∗
,BoJiang
b
a
School of Information Science and Technology, Tokai University, 4-1-1 Kitakaname, Hiratsuka 259-1292, Japan
b
School of Information Science and Technology, Dalian Maritime University, Linghai Road 1, Dalian, China
article info abstract
Article history:
Received 15 March 2013
Received in revised form 30 October 2013
Accepted 1 November 2013
Available online 6 November 2013
Communicated by R. Uehara
Keywords:
Computational geometry
Visibility
Ray shooting
Polygon sweep problem
Two-guard problem
A polygon P admits a sweep if two mobile guards can detect an unpredictable, moving
target inside P , no matter how fast the target moves. Two guards move on the polygon
boundary and are required to always be mutually visible. The objective of this study is to
find an optimum sweep such that the sum of the distances travelled by the two guards
in the sweep is minimized. We present an O
(n
2
) time and O (n) space algorithm for
optimizing this metric, where n is the number of vertices of the given polygon. Our result
is obtained by reducing this problem to finding a shortest path between two nodes in a
graph of size O
(n).
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Motivated by the relations to the well-known Art Gallery
and Watchman Route problems, much attention has re-
cently been devoted to the problem of detecting an un-
predictable, moving target in an n-sided polygon P by a
group of mobile guards. Both the target and the guards are
modeled by points that can continuously move in P .The
goal of the guards is to eventually “see” the target, or to
verify that no target is present in the polygon, no matter
how fast the target moves. Many types of polygon shapes
and the visibilities of the guards have been studied in the
literature [3–8,10,11].
In this paper, we focus on the two-guard model studied
in [3,6],
in which two (point) guards move on the poly-
gon boundary, always remaining mutually visible. The goal
is to sweep P with two guards so that at any instant,
the line segment connecting the guards partitions P into
a “cleared” region (not containing the target) and an “un-
explored” region (it may contain the target). In the end,
we would like to know whether the whole polygon P is
clearedorthetargetisdetected,ifitiseverpossible.This
*
Corresponding author.
E-mail address: tan@wing.ncc.u-t
okai.ac.jp (X. Tan).
target-finding model has obvious advantages for safety and
communication between the guards.
Icking and Klein were the first to study the problem of
sw
eeping corridors with two guards, which was called the
two-guard problem [6].Asimplepolygonwithanentrance
s and an exit t on its boundary is called a corridor.Two
guards move on the boundary of the corridor P ,startingat
s , and finally force the target out of P through t.Theygave
an O
(n log n) time algorithm for determining whether a
corridor can be swept with two guards [6]. Later, a linear-
time algorithm was presented by Heffernan [5]. Tseng et al.
gave an O
(n log n) time algorithm to determine whether
there is a pair of vertices in P that allows a sweep. This
result has also been improved to O
(n) [1].
The problem of sweeping simple polygons with two
guar
ds was further studied in [10,11]. Since neither the
entrance nor the exit on the polygon boundary is given,
the starting point (on the polygon boundary) of any sweep
schedule may be visited by the target for the second or
more time, i.e., recontamination is generally necessary for
the problem of sweeping simple polygons with a chain of
guards [3,4]. This makes it more difficult and challenging.
A linear-time algorithm has been presented for determin-
ing whether a polygon can be swept with two guards
by checking on several forbidden configurations [11].
0020-0190/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ipl.2013.11.002