Minimization of the Maximum Distance between the Two Guards Patrolling 49
algorithm can automatically find the starting and ending vertices such that the
maximum distance over all pairs of the starting and ending points is minimized,
provided that the given polygon is straight walkable.
1
The rest of this paper is organized as follows. In Section 2 of this paper, we give
basic definitions used in the paper. In Section 3, we show that an optimum straight
walk can be decomposed into a sequence of basic motions, called the atomic walks.
The atomic walks are so defined that their solutions in the min-max metric can
easily be found. In Section 4, we introduce a data structure that records all possi-
ble atomic walks in the given polygon and a transition relation among the atomic
walks. By applying Dijkstra’s algorithm to the obtained diagram, we can then give
our O(n
2
log n) time algorithm for finding an optimum straight walk such that the
maximum distance between the two guards is minimized. Finally, we pose some
open problems for further research in Section 5.
2 Preliminary
Let P denote a simple polygon of n vertices and ∂P the boundary of P .Justfor
convenience, we assume that P is in a general position in the plane. That is, no
three vertices of P are collinear, and no three edge extensions have a common
point. Two points p, q ∈ P are visible from each other if the line segment
connecting them, denoted by
pq, does not intersect the exterior of P .Wedenote
by |xy| the length of
xy.
Let g
1
, g
2
be two point guards on ∂P.Letg
1
(t)andg
2
(t) denote the positions
of g
1
and g
2
on ∂P at time t;werequirethatg
1
(t)andg
2
(t): [0, ∞) → ∂P be two
continuous functions. A point p ∈ P is said to be detected or illuminated at t if p
is contained in the line segment
g
1
(t)g
2
(t). A configuration of g
1
and q
2
at time
t is a pair of the points g
1
(t)andg
2
(t) such that the line segment g
1
(t)g
2
(t) lies
in the interior of P . Assume that the initial positions of two guards are located
at a same vertex of P . The configuration of g
1
and q
2
at a time thus divides
P into a clear region that does not contain the target, and an uncleared (or a
contaminated) region that may contain the target. Any line segment
g
1
(t)g
2
(t)
at a time t is called a walk segment. The point g
1
(t)isthewalk partner of g
2
(t),
and vice versa.
A walk instruction can be given by a pair of functions g
1
(t), g
2
(t) such that
either of g
1
(t)andg
2
(t) specifies an algebraic path, i.e., an edge of P along
which the guard g
1
or g
2
moves. More specifically, the following two types of
walk instructions are considered: Two guards g
1
and g
2
move along segments of
single edges such that (i) no intersections occur among all line segments
g
1
(t)g
2
(t)
during the movement, or (ii) any two segments
g
1
(t)g
2
(t) intersect each other.
(Probably, one guard stands still, while the other moves.) See [11].
Let the area of P be one, and let P (t) denote the fraction of the clear area
at time t. Initially, P (0) = 0. We say a walk schedule exists for P , or equally, P
1
Without loss of generality, we assume that a walk schedule always starts (ends) at
a polygon vertex.