CSE143—Computer Programming II
Programming Assignment #8 (bonus)
due: Friday, 6/2/06, 11 pm
There is an extra credit option for this
assignment that is worth a measly 2 points.
In other words, this isn’t intended as an opportunity for you to
increase your grade. It is intended as
an extra coding exercise for those who are interested in exploring how to make
their Huffman program behave better.
There will be no partial credit.
If you do the extra credit option, you
are still required to complete the standard HuffmanTree and to submit it along
with your HuffmanNode. So if you work on
this, do so only after you have completed and submitted the standard
assignment. To keep things clear, for
this part of the assignment you should create a class called HuffmanTree2. You can copy your HuffmanTree class and
modify it appropriately to get the initial version of this class.
The main goal of this variation is to
eliminate the code file. When you use a
utility like zip, you don’t expect it to produce two output files (a code file
and a binary file). You expect it to
produce one file. That’s what we’ll do
in this variation. To do so, we’ll have
to be able to include information in the binary file about the tree and its
structure.
In the original version we had three
programs: MakeCode, Encode and Decode.
For this version there are two main programs: Encode2 and Decode2.
In all, you will have to include the following three new
methods in your class along with the other methods we had in HuffmanTree:
|
Method |
Description |
|
HuffmanTree2(BitInputStream input) |
Constructs a Huffman tree from the given input stream. Assumes that the standard bit representation has been used for the tree. |
|
void assign(String[] codes) |
Assigns codes for each character of the tree. Assumes the array has null values before the method is called. Fills in a String for each character in the tree indicating its code. |
|
void writeHeader(BitOutputStream output) |
Writes the current tree to the output stream using the standard bit representation. |
In the original HuffmanTree we had a
method called write that would write the codes to an output file. Here the Encode2 program does the actual
encoding. It first reads the file and
computes the frequencies. Then it calls
your constructor to create an appropriate HuffmanTree. It has to have some way to find out what
codes your tree has come up with so that it can encode the characters of the
file. It does so by calling the assign
method in your class passing it an array of Strings that are all null. Your method will replace the null’s with
codes for the characters included in the tree.
The Encode2 program also calls the method
writeHeader in your class. The idea is
to write to the bit stream a representation of the tree that can be used to
reconstruct it later. As we did with the
QuestionTree in assignment 7, we can print the tree using a preorder traversal. For a branch node, we write a 0 indicating
that it is a branch. We don’t need to
write anything more, because the branch nodes contain no data. For a leaf node, we will write a 1. Then we need to write the ASCII value of the
character stored at this leaf. There are
many ways to do this. We basically need
to write some bits that can be read later to reconstruct the character
value. The value will require up to 9
bits to write (it would be 8 if it weren’t for our pseudo-eof character).
We need to decide on a convention for
writing an integer in 9 bits that we can reverse later when we read it back
in. Below are the two methods we would
like you to use to do this. They are
inverses of each other in that read9 will recreate what write9 writes to the
stream:
// pre : 0 <= n < 512
// post: writes a 9-bit representation of n to the
given output stream
private void write9(BitOutputStream output, int n) {
for (int i
= 0; i < 9; i++) {
output.writeBit(n % 2);
n /=
2;
}
}
// pre : an integer n has been encoded using write9
or its equivalent
// post: reads 9 bits to reconstruct the original
integer
private int read9(BitInputStream input) {
int
multiplier = 1;
int sum =
0;
for (int i
= 0; i < 9; i++) {
sum +=
multiplier * input.readBit();
multiplier *= 2;
}
return
sum;
}
Encode2 produces a binary file that first
has a header with information about the tree and then has the individual codes
for the characters of the file. The
Decode2 program has to use this information to reconstruct the original
file. It begins by calling the
constructor listed in the table above, asking your class to read the header
information and reconstruct the tree.
Once the tree has been reconstructed, the program calls your decode
method from the original assignment to reproduce the original file.
A collection of files necessary for this
part of the assignment will be on the handouts page called ass8-bonus.zip. It will include Encode2.java, Decode2.java
and examples of encoded input files called short.bonus and hamlet.bonus. Encode2 should produce exactly the same
output when used with your version of HuffmanTree2. There will be a separate turn-in for the
bonus in which you submit HuffmanNode.java and HuffmanTree2.java (you shouldn’t
need to make any changes to your node class for the bonus, but it will be
easier for us to grade if you submit both when you turn it in).