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CS1332 Homework 5: Pattern Matching Solution
CS 1332 Section C
1 Important
1. All submitted code must compile under JDK 11. This includes unused code, so don’t submit extra files that don’t compile. Any compile errors will result in a 0.
2. Do not include any package declarations in your classes.
3. Do not change any existing class headers, constructors, instance/global variables, or method signatures.For example, do not add throws to the method headers since they are not necessary.
5. Do not use anything that would trivialize the assignment. (e.g. don’t import/use java.util.ArrayListfor an ArrayList assignment. Ask if you are unsure.)
6. Always consider the efficiency of your code. Even if your method is O(n), traversing the structure multiple times is considered inefficient unless that is absolutely required (and that case is very rare).
7. You must submit your source code - the .java files. Do not submit compiled code - the .class files.
8. Only the last submission will be graded. Make sure your last submission has all required files. Resubmitting voids prior submissions.
2 Style and Formatting
It is important that your code is not only functional, but written clearly and with good programming style. Your code will be checked against a style checker. The style checker is provided to you and it located on Canvas. A point is deducted for every style error that occurs. Please double check before you submit that your code is in the appropriate style so that you don’t lose any unnecessary points!
2.1 Javadocs
Javadocs should be written for any private helper methods that you create. They should follow a style similar to the existing javadocs on the assignment. Any javadocs you write must be useful and describe the contract, parameters, and return value of the method. Random or useless javadocs added only to appease checkstyle will lose points.
2.2 Vulgar/Obscene Language
Any submission that contains profanity, vulgar, or obscene language will receive an automatic zero on the assignment. This policy applies to all aspects of your code, such as comments, variable names, and javadocs.
1
3 FORBIDDEN STATEMENTS
2.3 Exceptions
When throwing exceptions, you must include a message by passing in a String as a parameter. The message must be useful and tell the user what went wrong. ”Error”, ”Oof - Bad things are happening”, and ”FAIL” are not good messages. Additionally, the name of an exception itself is not a good message.
2.4 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.
3 Forbidden Statements
• package
• System.arraycopy()
• clone()
• assert()
• Arrays class
• Thread class
• Collections class
• Collection.toArray()
• Refleciton APIs
• Inner or nested classes
• Lambda Expressions
• Method References (using the :: operator to obtain a reference to a method)
4 JUnits
5 DELIVERABLES
5 Deliverables
You must submit all of the following file(s) to the corresponding assignment on Gradescope. Make sure all file(s) listed below are in each submission, as only the last submission is graded. Make sure the filename(s) matches the filename(s) below, and that only the following file(s) are present.
1. PatternMatching.java
5.1 Pattern Matching
For all of the 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 and just return an empty list since there cannot be any occurrences of the pattern in the text.
5.1.1 Character Comparator
CharacterComparator is a comparator that takes in two characters and compares them. This allows you to see how many times you have called compare(); besides this functionality, its return values are what you’d expect a properly implemented compare() method to return. You must use this comparator as the number of times you call compare() with it will be used when testing your assignment.
If you do not use the passed in comparator, this will cause tests to fail and will significantly lower your grade on this assignment.
You must implement the algorithms as they were taught in class. We are expecting exact comparison counts for this homework. If you are getting fewer comparison counts than expected, it means one of two things, either you implemented the algorithm wrong (most likely) or you are using an optimization not taught in the class (unlikely).
5.1.2 KMP Failure Table
The Knuth-Morris-Pratt (KMP) algorithm relies on using the prefix 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). This is an array of length m where each index will correspond to the character at that index in the pattern. Each index i of the failure table should contain the length of the longest proper prefix that matches a proper suffix of pattern[0, ..., i]. A proper prefix/suffix does not equal the string itself. There are different ways of calculating the failure table, but we are expecting the specific format described below.
For any string pattern, have a pointer i starting at the first letter, a pointer j starting at the second letter, and an array called table that is the length of the pattern. First, set index 0 of table to 0. Then, while j is still a valid index within pattern:
1. 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.
5 DELIVERABLES
2. If the characters pointed to by i and j do not match:
(a) If i is not at 0, then change i to table[i - 1]. Do not increment j or write any value to the table.
(b) 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:
5.1.3 KMP 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:
1. If the mismatch occurs at index 0 of the pattern, then shift the pattern by 1.
2. 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, if the whole pattern is ever matched, instead of shifting the pattern over by 1 to continue searching for more matches, the pattern should be shifted so that the pattern at index failure[j-1], where j is at pattern.length, aligns with the index after the match in the text. KMP treats a match as a “mismatch” on the character immediately following the match.
5.1.4 Boyer-Moore Last Occurrence Table
6 GRADING
5.1.5 Boyer-Moore Searching Algorithm
The shifting rule considers the character in the text at which the comparison process failed (assuming that a failure occurred). If the last occurrence of that character is to the left in the pattern, shift so that the pattern occurrence aligns with the mismatched text occurrence. If the last occurrence of the mismatched character does not occur to the left in the pattern, shift the pattern over by one (to prevent the pattern from moving backwards). In addition, if the mismatched character does not exist in the pattern at all (no value in last table) then pattern shifts completely past this point in the text.
For finding multiple occurrences, if you find a match, shift the pattern over by one and continue searching.
5.1.6 Rabin-Karp
Note: You must use the exact rolling hash function specified in the javadocs. You are not allowed to use Math.pow() for the intial hash calculation, nor are you allowed to use it for updating the text hash.This is because exponentiating a number is not an O(1) operation, so creating your own custom power method is also inefficient.
6 Grading
Here is the grading breakdown for the assignment. There are various deductions not listed that are incurred when breaking the rules listed in the PDF and in other various circumstances. Note that for this assignment efficiency is explicitly mentioned as contributing to your point total. In contrast to past assignments, unimplemented methods will result in losing efficiency points.
• Pattern Matching − 80 pts
• Efficiency − 10 pts
• Checkstyle − 10 pts
• Total: 100 pts
1 Important
1. All submitted code must compile under JDK 11. This includes unused code, so don’t submit extra files that don’t compile. Any compile errors will result in a 0.
2. Do not include any package declarations in your classes.
3. Do not change any existing class headers, constructors, instance/global variables, or method signatures.For example, do not add throws to the method headers since they are not necessary.
5. Do not use anything that would trivialize the assignment. (e.g. don’t import/use java.util.ArrayListfor an ArrayList assignment. Ask if you are unsure.)
6. Always consider the efficiency of your code. Even if your method is O(n), traversing the structure multiple times is considered inefficient unless that is absolutely required (and that case is very rare).
7. You must submit your source code - the .java files. Do not submit compiled code - the .class files.
8. Only the last submission will be graded. Make sure your last submission has all required files. Resubmitting voids prior submissions.
2 Style and Formatting
It is important that your code is not only functional, but written clearly and with good programming style. Your code will be checked against a style checker. The style checker is provided to you and it located on Canvas. A point is deducted for every style error that occurs. Please double check before you submit that your code is in the appropriate style so that you don’t lose any unnecessary points!
2.1 Javadocs
Javadocs should be written for any private helper methods that you create. They should follow a style similar to the existing javadocs on the assignment. Any javadocs you write must be useful and describe the contract, parameters, and return value of the method. Random or useless javadocs added only to appease checkstyle will lose points.
2.2 Vulgar/Obscene Language
Any submission that contains profanity, vulgar, or obscene language will receive an automatic zero on the assignment. This policy applies to all aspects of your code, such as comments, variable names, and javadocs.
1
3 FORBIDDEN STATEMENTS
2.3 Exceptions
When throwing exceptions, you must include a message by passing in a String as a parameter. The message must be useful and tell the user what went wrong. ”Error”, ”Oof - Bad things are happening”, and ”FAIL” are not good messages. Additionally, the name of an exception itself is not a good message.
2.4 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.
3 Forbidden Statements
• package
• System.arraycopy()
• clone()
• assert()
• Arrays class
• Thread class
• Collections class
• Collection.toArray()
• Refleciton APIs
• Inner or nested classes
• Lambda Expressions
• Method References (using the :: operator to obtain a reference to a method)
4 JUnits
5 DELIVERABLES
5 Deliverables
You must submit all of the following file(s) to the corresponding assignment on Gradescope. Make sure all file(s) listed below are in each submission, as only the last submission is graded. Make sure the filename(s) matches the filename(s) below, and that only the following file(s) are present.
1. PatternMatching.java
5.1 Pattern Matching
For all of the 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 and just return an empty list since there cannot be any occurrences of the pattern in the text.
5.1.1 Character Comparator
CharacterComparator is a comparator that takes in two characters and compares them. This allows you to see how many times you have called compare(); besides this functionality, its return values are what you’d expect a properly implemented compare() method to return. You must use this comparator as the number of times you call compare() with it will be used when testing your assignment.
If you do not use the passed in comparator, this will cause tests to fail and will significantly lower your grade on this assignment.
You must implement the algorithms as they were taught in class. We are expecting exact comparison counts for this homework. If you are getting fewer comparison counts than expected, it means one of two things, either you implemented the algorithm wrong (most likely) or you are using an optimization not taught in the class (unlikely).
5.1.2 KMP Failure Table
The Knuth-Morris-Pratt (KMP) algorithm relies on using the prefix 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). This is an array of length m where each index will correspond to the character at that index in the pattern. Each index i of the failure table should contain the length of the longest proper prefix that matches a proper suffix of pattern[0, ..., i]. A proper prefix/suffix does not equal the string itself. There are different ways of calculating the failure table, but we are expecting the specific format described below.
For any string pattern, have a pointer i starting at the first letter, a pointer j starting at the second letter, and an array called table that is the length of the pattern. First, set index 0 of table to 0. Then, while j is still a valid index within pattern:
1. 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.
5 DELIVERABLES
2. If the characters pointed to by i and j do not match:
(a) If i is not at 0, then change i to table[i - 1]. Do not increment j or write any value to the table.
(b) 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:
5.1.3 KMP 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:
1. If the mismatch occurs at index 0 of the pattern, then shift the pattern by 1.
2. 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, if the whole pattern is ever matched, instead of shifting the pattern over by 1 to continue searching for more matches, the pattern should be shifted so that the pattern at index failure[j-1], where j is at pattern.length, aligns with the index after the match in the text. KMP treats a match as a “mismatch” on the character immediately following the match.
5.1.4 Boyer-Moore Last Occurrence Table
6 GRADING
5.1.5 Boyer-Moore Searching Algorithm
The shifting rule considers the character in the text at which the comparison process failed (assuming that a failure occurred). If the last occurrence of that character is to the left in the pattern, shift so that the pattern occurrence aligns with the mismatched text occurrence. If the last occurrence of the mismatched character does not occur to the left in the pattern, shift the pattern over by one (to prevent the pattern from moving backwards). In addition, if the mismatched character does not exist in the pattern at all (no value in last table) then pattern shifts completely past this point in the text.
For finding multiple occurrences, if you find a match, shift the pattern over by one and continue searching.
5.1.6 Rabin-Karp
Note: You must use the exact rolling hash function specified in the javadocs. You are not allowed to use Math.pow() for the intial hash calculation, nor are you allowed to use it for updating the text hash.This is because exponentiating a number is not an O(1) operation, so creating your own custom power method is also inefficient.
6 Grading
Here is the grading breakdown for the assignment. There are various deductions not listed that are incurred when breaking the rules listed in the PDF and in other various circumstances. Note that for this assignment efficiency is explicitly mentioned as contributing to your point total. In contrast to past assignments, unimplemented methods will result in losing efficiency points.
• Pattern Matching − 80 pts
• Efficiency − 10 pts
• Checkstyle − 10 pts
• Total: 100 pts
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