# CS 202 Assignment 3 Hash Tables solution

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## Description

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In this assignment, you are required to implement different types of hash
tables.

The course policy about plagiarism is as follows:
1. Students must not share actual program code with other students.
2. Students must be prepared to explain any program code they submit.
3. Students cannot copy code from the Internet.
4. Students must indicate any assistance they received.
5. All submissions are subject to automated plagiarism detection.
Students are strongly advised that any act of plagiarism will be reported
to the Disciplinary Committee

In the first task of this assignment, you are expected to implement (a) the
polynomial hash code from the textbook (Section 9.2.3) and (b) bitwise
hash code function as shown below:
str = s1s2s3…sn-1sn
e.g., (in case of str = “Hello”, ‘H’ is s1, ‘e’ is s2 … ‘o’ is s5)
Initialize bitwise_hash = 0

For every si in str
bitwise_hash ^= (bitwise_hash << 5) + (bitwise_hash >> 2) + si
along with the division method compression function for both (a) and (b).
The value of the parameter a in the polynomial hash code should be
configurable.

The specifics of the first hash table are as follows:
The first hash table will use chaining, where you will be required to use the
LinkedList from previous assignments. This HashTable will be created with
a fixed size. It should support the insert, deletion and lookup commands.
The constructor should take the size of the table as a parameter. Use hash
functions implemented in the Task 1.

Now you will try out the same hash function with a different hash table,
which should use open addressing with linear probing. This Hash Table will
initially be created with a small size; it must support resizing along with
insert, deletion and lookup. Use hash functions implemented in the Task 1.
Bonus: Also implement this with quadratic probing.

As you have seen in the implementations of linear probing and chaining,
the issue of collisions was addressed by storing both the colliding values,
but these techniques increase the look up time. So, in order to improve this,
in this task you will be implementing double hashing as discussed in the
class using the two functions you made in Task1.

Double hashing cannot completely eliminate collisions. To obtain full credit
in this task, you will have to devise and implement a method to handle the
case when both functions result in a collision.

One method to adopt, for example, would be the following:
Index = h(key) + i*d(key), where h() is the first hash function, d() is the
second hash function and i is an integer zero onwards (0,1,2,3……)
Hence to compute the index to insert the value at, use the above formula
but keep the value of i as 0. If you get a collision then use 1 as the value of
i. If you get a collision again, use 2 as the value for i and so on.

In this task you are going to implement a cache by using your
implementation of hash tables. For this part, use the implementation which
you think is best (you can also modify that). The details of this part are up
to you to decide. Use your best knowledge and optimize your cache.

Your code should read a space separated sequence of codes
(numbers) from files secret1.txt, secret2.txt, secret3.txt and retrieve the
words corresponding to the codes from dictionary.txt and print them on the
screen. For example, given a sequence “599454 34247 69702 85130”, the
screen should show “THIS COURSE IS LOVE”. Please see the file
dictionary.txt for clarification.

You should cache the words once they are fetched. Next time they
appear in the sequence, retrieve the corresponding word from the cache
instead of dictionary.txt. You need to use hash table with size 1000 for your
cache. Note that your cache will be limited in size so you cannot store
every word in that and you will have to implement any policy for
accommodating new values in the cache when it is already full. We would
recommend using the policy LFU, Least Frequently Used (refer to
https://en.wikipedia.org/wiki/Least_frequently_used for details).

Your program should run two modes, one in which cache is used and
the other one in which cache is not used. Observe the time taken in
decoding secrets (numbers) in both modes and print them on screen. You
will be graded on the basis of your implementation and understanding of
this part. You can use any previous implementations for this part and can
include the header files.

* https://en.wikipedia.org/wiki/Cache_(computing)
(This is an open-ended question as you can try to optimize this as you like
but do not forget to explain your approach in a separate doc file.)

### Deliverables:

You are required to submit the following:
1. Implementation of the hash table with chaining
2. Implementation of the hash table with open addressing (linear
probing)
3. Implementation of the hash table with open addressing (double
hashing)
4. Implementation of the Task 5 in cache.cpp file with a brief doc file
explaining your implementation and the time taken for reading
sequences from the secret files with cache and without cache.
Note: In order to compile the test cases, you will be required to give the
following flags: