# Homework 7: String Searching Solution

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## Description

String Searching

For this assignment you will be coding 3 di erent string searching algorithms: Boyer-Moore, Knuth-Morris-Pratt, and Rabin-Karp. There is information about all three in the interface and more information about Boyer-Moore and KMP in the book (also under resources on T-Square). If you implement any of the three algorithms in an unexpected manner (i.e. contrary to what the Javadocs and PDF specify), you may receive a 0.

For all of the search algorithms, make sure you check the simple failure cases as soon as possible. For example, if the pattern is longer than the text, don’t do any preprocessing on the pattern/text.

Do not use ANYTHING from the java.lang.Math class in any method for this assign-ment.

Knuth-Morris-Pratt

Failure Table Construction

The Knuth-Morris-Pratt (KMP) algorithm relies on using the pre x of the pattern to determine how much to shift the pattern by. The algorithm itself uses what is known as the failure table (also called failure function). There are di erent ways of calculating the failure table, but we are expecting one speci c format described below.

For any string pattern, have a pointer i starting at the rst letter, a pointer j starting at the second letter, a table called table that is the length of the pattern. Then, while j is still a valid index within pattern:

If the characters pointed to by i and j match, then write i + 1 to index j of the table and increment i and j.

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Homework 7: String Searching Due: See T-Square

If the characters pointed to by i and j do not match:

{ If i is not at 0, then change i to table[i – 1]. Do not increment j or write any value to the table.

{ If i is at 0, then write i to index j of the table. Increment only j.

For example, for the string abacab, the failure table will be:

 a b a c a b 0 0 1 0 1 2

For the string ababac, the failure table will be:

 a b a b a c 0 0 1 2 3 0

For the string abaababa, the failure table will be:

 a b a a b a b a 0 0 1 1 2 3 2 3

For the string aaaaaa, the failure table will be:

 a a a a a a 0 1 2 3 4 5

Searching Algorithm

For the main searching algorithm, the search acts like a standard brute-force search for the most part, but in the case of a mismatch:

If the mismatch occurs at index 0 of the pattern, then shift the pattern by 1.

If the mismatch occurs at index j of the pattern and index i of the text, then shift the pattern such that index failure[j-1] of the pattern lines up with index i of the text, where failure is the failure table. Then, continue the comparisons at index i of the text (or index failure[j-1] of the pattern). Do not restart at index 0 of the pattern.

In addition, when a match is found, instead of shifting the pattern over by 1 to continue searching for more matches, the pattern should be shifted over by failure[j-1], where j is at pattern.length.

CharSequence

CharSequence is an interface that is implemented by String, StringBuffer, StringBuilder and many others. We have also included a class, SearchableString, that implements CharSequence. You may use any class that implements CharSequence while testing your code. SearchableString allows you to see how many times you have called charAt(). We will be looking at the number of times you call charAt() while grading.

Do not use any method except charAt() and length(); all other methods will either throw an exception or will return invalid data. In addition, do not attempt to circumvent the retrictions we placed in the SearchableString class.

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Homework 7: String Searching Due: See T-Square

Here is the grading breakdown for the assignment. There are various deductions not listed that are incurred when breaking the rules listed in this PDF, and in other various circumstances.

 Methods: kmp 15pts buildFailureTable 10pts boyerMoore 15pts buildLastTable 10pts rabinKarp 15pts generateHash 5pts updateHash 5pts Other: Checkstyle 10pts E ciency 15pts Total: 100pts

A note on JUnits

We have provided a very basic set of tests for your code, in StringSearchingStudentTests.java. These tests do not guarantee the correctness of your code (by any measure), nor does it guarantee you any grade. You may additionally post your own set of tests for others to use on the Georgia Tech GitHub as a gist. Do NOT post your tests on the public GitHub. There will be a link to the Georgia Tech GitHub as well as a list of JUnits other students have posted on the class Piazza.

If you need help on running JUnits, there is a guide, available on T-Square under Resources, to help you run JUnits on the command line or in IntelliJ.

Style and Formatting

It is important that your code is not only functional but is also written clearly and with good style. We will be checking your code against a style checker that we are providing. It is located in T-Square, under Resources, along with instructions on how to use it. We will take o a point for every style error that occurs. If you feel like what you wrote is in accordance with good style but still sets o the style checker please email Grayson Bianco (gbianco6@gatech.edu) with the subject header of \CheckStyle XML”.

Javadoc any helper methods you create in a style similar to the existing Javadocs. Like the existing Javadocs, the Javadocs for your helper method(s) must describe well what the method does, what each parameter means (if any), and what the returned value is (if any). If a method is overridden or implemented from a superclass or an interface, you may use @Override instead of writing Javadocs.

Exceptions

When throwing exceptions, you must include a message by passing in a String as a parameter. The mes-sage must be useful and tell the user what went wrong. \Error”, \BAD THING HAPPENED”, and \fail” are not good messages. The name of the exception itself is not a good message.

For example:

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Homework 7: String Searching Due: See T-Square

throw new IllegalArgumentException(“Cannot insert null data into data structure.”);

Generics

If available, use the generic type of the class; do not use the raw type of the class. For example, use new LinkedList<Integer>() instead of new LinkedList(). Using the raw type of the class will result in a penalty.

Forbidden Statements

You may not use these in your code at any time in CS 1332.

Anything from the java.lang.Math class

break may only be used in switch-case statements

continue package

System.arraycopy() clone()

assert()

Arrays class Array class Objects class Stack class

Collections class

Collection.toArray()

Re ection APIs

Inner, nested, or anonymous classes

Debug print statements are ne, but nothing should be printed when we run them. We expect clean runs – printing to the console when we’re grading will result in a penalty. If you use these, we will take o points.

Provided

The following le(s) have been provided to you. There are several, but you will edit only one of them.

1. StringSearching.java

This is the class in which you will implement the di erent string searching algorithms. Feel free to add private static helper methods but do not add any new public methods, new classes, instance variables, or static variables.

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Homework 7: String Searching Due: See T-Square

1. StringSearchingStudentTests.java

This is the test class that contains a set of tests covering the basic operations on the StringSearching class. It is not intended to be exhaustive and does not guarantee any type of grade. Write your own tests to ensure you cover all edge cases.

1. SearchableString.java

This is a wrapper class around a String object. It counts the number of times charAt() has been called and disables some other unnecessary operations. Do not modify this le.

Deliverables

You must submit all of the following le(s). Please make sure the lename matches the lename(s) below, and that only the following le(s) are present. T-Square does not delete les from old uploads; you must do this manually. Failure to do so may result in a penalty.

1. StringSearching.java

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