# CS141:    Assignment 3

Version 1.0  MAKE SURE YOU CHECK FOR UPDATES.

PART A. Written part.

1.  [20]  Assume a weighted  connected undirected graph \$G(V,E, w())\$ and assume
that all the weights of the edges are distinct integers (no two weights are equal).
Prove or disprove the following statement:
For any pair of nodes of the graph, the minimum cost path  between them is unique.

2. [20] Prove the following.
a.[10] What is the  minimum and maximum number of edges you can have in a graph?
Note: The edges should be expressed as a function of N.
b. [10]   A connected graph G(V,E), will have at least |V| - 1 edges.
Use induction and make sure you write clearly the  induction steps.

3. [20]  In the graph below,  you are at node  "e" and you want to create a tree to communicate
with your friends (the other black nodes: a,d, f, g) . The weights can be interpreted in  two different ways.

1.  Weight = Delay. Calculate the tree that minimizes the end to end delays
from you to all your friends. I.e. the delay from you to each one of your friends is the
minimum possible.

2. Weight = Cost in sweet green dollars. Calculate the tree that has minimum total cost.
NOTE: We are interested in creating a tree to make sure that you and your friends can communicate.

Explain which algorithm you will use, and show the creation of the tree one node at a time.
Note that you may need to modify the tree that you derive from known algorithms.
Show the final tree that you will use to communicate with your friends.
Show  in 1) the final end-to-end delay from e to every other node,  in 2) the total cost of the tree.

#### PART B. Programming part.  [100]

In the alien world of Faloutsonia,  there is a river, and the creatures cross this river daily.
The only way to cross is a boat with a guard and the boat has 3 positions for passengers.
There are two  species  that cross the river: the Xerons and the Omons.  The Xerons can eat
the Omons. (The Omons are vegetarians.)
The guard has to transfer Xerons and Omons across the river with the following rules:
* There must never be more Xerons than Omons in one side of the river without the guard,
because then the Xerons will eat the Omons.
Note: if the guard is present, she can make sure that none gets eaten.
Also, if Omons are equal to or more than the Xerons, then none gets eaten.
* The passengers in the boat  can not exceed the boat capacity
However, they can be less than the capacity or even zero. You can have passengers from either species.
While on the boat, the guard is present, so none gets eaten.

The fees: Every time the boat crosses the river the guard has to pay taxes to evil Michalorg,
supreme ruler of Faloutsonia. The tarif is as follows:
* empty boat:   C_e  dollars  (they are alien dollars with an exchange rate of 1:10000 to US\$)
* Xeron:            C_x  dollars  per head
* Ommons       C_o  dollars per head
The cost is additive: for a ride with 1 Xeron and two Omons, the cost is:
C_e + C_x +  2 * C_o
Naturally all costs are non-negative (positive or zero).

Every morning, the guard has to transport  X  Xerons, and O  Omons,
from the A side of the river to the B side of the river without anybody getting eaten and
with the minimum cost in taxes.

You are friends with the guard and you want  to write a C++ program to help
him find safe  and least costly way of boat rides  (sum of taxes payed should be minimum).

The input of the program is  the  number of Xerons, X, the number of Omons, O and the costs
C_e ,  C_x,  C_o.   Everybody is on the A side of the river.

The output should be:
a. [10] if there is a way to transfer everybody across (Print Yes or No).
b. [10] the minimum number of times that the boat has to cross the river
(total crossings in either direction).
c. [30] the exact sequence of states and moves of a safe for all trip.
The required output is like this:
A -> B:   X_a         O_a          move 1, 1
B -> A:   X_a-1    O_a - 1   move 1, 0
where  X_a  O_a is the number of creatures on side A  before you put anyone in the boat,
and "move 1, 1"  means that you intend to move one Xeron and one Omon from the X_a and O_a.
In the second line,  "move 1 0"  means that you intend to move 1 Xeron from B to A.

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Bonus 1 (+10%): Provide a clever way to terminate the search as early as possible.
Find it, describe it, explain why it is correct and implement it.

Bonus 2 (+10%):  Find the minimum number of rides required to transfer the creatures, independently
of the  associated cost.
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ATTENTION: YOUR PROGRAM SHOULD RUN IN LINUX ON hill.cs.

Your executable should be named "run" and it should be run as follows:

\$ run  filename

Now, filename is the name of a file that contains the problem information in the following
format:
X                                  // initial number of Xerons on side A
O                                   // initial number of Omons on side A
C_e                             // tax of empty boat
C_x                             // tax for each trasnported  Xeron
C_o                             // tax for each transported Omon
This file is can be assumed to be correct.

The other 50 points: are the report, the makefile, the programming style, programming efficiency etc.

THE REPORT [50].
a. How you model the problem
c. Main data structures, classes and methods
d. Why do you think your solution is correct (arguments, not proof)
e.  What is the complexity of your program.
The report should not be more than three pages (excluding the output of test cases).

Hints: 1. Think of how you should model the problem. How can you model a state? Probably a few numbers
are enough From one state, you can  go to a number of other states.  A graph may help.
2.  It is a good idea to get your computer to explore all possible moves, and find the best.
3. Make sure your program does not fall into infinite loops. How many times
do you want to consider or visit one state?
4. When do you stop searching for a solution? Do you want to revisit a state?

WHAT TO SUBMIT

Electronically:
c. a README file for instructions on how to run it (they should be as stated but write it nevertheless)
f. the report
e. the output of your program in the given test cases (to be completed).

A hard copy of your report  and your program and output should be given with the written  part of the assignment.
Put names,  email, section and lab, as in first assignment.

WARNING: A report that makes false claims about the program will be considered as cheating.
An output sample that is not generated by the submitted program will also be considered  as cheating.

Check:  http://www.cs.ucr.edu/~klick/    the links for linux (for makefiles) and programming for
stylistic and programming suggestions.