Time Limit: 2 Seconds
Memory Limit: 65536 KB
On its trip, a train has to pass a lot of points (American English: switches)
and signals. The train��s track depends on the status of points and signals.
The responsible operator on the signal box does not handle them separately,
but tells the signal box the start and destination signal of the train��s journey.
The box then determines the correct status of points and signals and brings
them into the right position.
Figure 9: Schematic view of points and signals on a sample train track.
Figure 9 shows a sample scenario in which railway tracks are shown
as solid lines and signals are drawn as triangles (this is also the first scenario
of the sample input). Signals have a sense of direction: they are only valid
for the direction in which the triangle points (e.g., signal A is valid for
trains running from left to right, see also Figure 10). Points are located where
railway tracks meet (e.g., at points W1, W2, etc.). Points have a front side
(i.e., the side from which a train can take alternative directions) and a back
side and can be in two positions, named + and -. If a train comes from the front
side, it leaves the point at the + or - leg, dependent on the point��s position
(see Figure 11). If the train comes from one of the the back legs, it leaves
it at the front leg. Even then the point has to be put into the right state,
otherwise it gets damaged!
Figure 10: Signal valid for trains running from left to right
Figure 11: Point with two possible positions
Your task is to implement an automatic signal box, i.e., write a program which
finds the correct position of points and signals for a given start and destination.
The signal box should follow these rules:
A journey can only start and end at a signal. Both signals have to be in the
During a journey a train must not change its direction.
The journey consists of a sequence of signal and point settings. A signal is
only taken into account for the journey if it has the right direction. A point
along the way is always taken into account.
If there is more than one possible track from the start signal
to the destination signal, the correct one is determined by the following scheme:
Consider a set of path selection rules. These are given as a triple (x, y, z)
of point identifiers x and y, and a position z. A selection rule has the following
If there are alternative paths starting at point x and ending
at point y where x is approached from the front and y from the back, then consider
only paths in which x is in position z (z is either + or -).
If no such rule exists for a given point x, the - position must be chosen.
The sample in- and output demonstrate the application of the rules. Furthermore,
you can make the following assumptions:
The track plan is acyclic.
Within a path, each element is only used once or not at all.
If for a given point x several rules (x, y, z) exist, they will agree on the
position to be chosen.
The first line contains the number of scenarios.
In the first line of the input for every scenario, you are given two signal
identifiers for the departure and the destination, separated by a single blank.
The following line contains the number n of elements (points and signals) in
the track plan. You can assume 1<=n<=200 and that each element has a unique
identifier of at most 20 alphanumeric characters. The identifier "XXX"
is given to track ends.
There are signal and point elements, given in the following format:
Points are specified by a line "W I F M P" where W stands for "Weiche"
(German for point), I is the identifier of the point, F identifies the front
element of the point, and M and P give the identifiers of the back elements
of the point depending on whether it is in minus or plus position.
Signals are specified by a line "S I F B" where S stands for "Signal"
(German for signal), I is the identifier of the signal, and F and B give the
identifiers of the front and back elements of the signal.
The direction for which the signal is valid is from front to back.
The following line contains the number p, 0<=p<=100, of path selection
rules, followed by another p lines of the rules themselves. A rule is of the
form "FW X Y Z" where FW is the identifier of "Fahrstrabenwahl-Regel"
(German for path selection rule), X, Y and Z are the elements of the rule as
The output for every scenario begins with a line containing ��Scenario #i:��,
where i is the number of the scenario starting at 1.
For every scenario print out the elements on the path from departure to destination
in the order they are passed by the train. However, print the signals first,
followed by the points. Every element of the path must be on a line by itself.
Elements of the path are signal and point identifiers (the first and the last
signal identifiers must also be printed). For every point you should also give
the correct position of the point as either + or - on the same line, separated
from the point identifier by a single blank. If there is no possible path, print
Terminate each scenario by a blank line.
S AB A XXX
S A AB W1
W W1 A W2 P3
W W2 W1 P1 P2
S P1 N1 W2
S N1 P1 W11
W W11 F N1 W12
S F AC W11
S AC F XXX
S P2 N2 W2
S N2 P2 W12
W W12 W11 N3 N2
S P3 N3 W1
S N3 P3 W12
FW W1 W11 +
FW W11 W1 -
S S1 S2 XXX
S S2 S1 XXX
S S1 XXX W1
S S2 W1 XXX
S S3 XXX W2
S S4 W2 XXX
W W1 S1 S2 W2
W W2 S4 W1 S3
S S1 XXX W1
S S2 W4 XXX
S S3 W1 W2
S S4 W3 W4
W W1 S1 W2 S3
W W2 W3 W1 S3
W W3 W2 W4 S4
W W4 S2 W3 S4
FW W1 W2 +
Source: Northwestern Europe 2001