Posted: May 13th, 2022
In this assignment, you are requested to write a simple word index program. The program will record the position of words in text files in a directory and then offer to retrieve the positions of a given word. The program should be named WordIndex. Your program will read a set of commands from a given file. The program should execute these commands sequentially. Such commands would first request to read all the words of the available text files and insert them into a map data structure. Then, search for a given word, add the words of a new file, remove a file from the list of words, or print a summary the word index. The command file name is given as argument on the command line. The directory containing the text files is called TextFiles. In this assignment you will implement and compare (experimentally) a linked list based and a hash table based map for storing these word positions.
Provisional and Indicative Timing Guidelines Step 1: Week 4 (after lecture on Maps) develop your linked list based map, implement and test the indexing and searching for a word Step 2: Week 5 (after lecture on Hash Tables) develop your hash table based map (you can first implement linear probing collision handling before later changing to a double hashing), implement and test the removing of files from the map, the sorting of the output, and the summary Step 3: Week 6 finalise your double hashing collision handling, run tests, run comparison of list-based and hash-table-based implementations, finalise your report 2 Implementation You are to implement the program based on a map, let us call it M. You will use map M for storing the words that appear in the text files with their positions (file names and line numbers).After storing all the words in M, the program should process the commands for searching for a word, adding a new file, removing an indexed file, display summary of the index. Notice that a word could occur several times in a file, the search result should display first the file where the word occurs the most, and all the lines in that file where it occurs by ascending order. 3 Input and output Your program will read its commands from one input file given as argument to the program. The input is composed of a sequence of lines where each line is a Word Index command. A class WordTxtReader which will read words one by one from a file and indicate its positions is provided. The class should be used both to read the commands and the words in the text files. See comments in the file WordTxtReader.java for the usage. Note that the class will discard any character which is neither a letter (a-z or A-Z) nor a number (0-9). The program will return a lowercase version of each word. The commands, their syntax and suggested outputs are as follows: Adding to the map the words of all the files addall Adds to the map the words and their positions in each files for all the files in the .txt in the TextFiles folder. An example of a TextFiles directory is given containing the text version of the lectures of the course (lect*.txt). Input: addall Suggested output: 27073 entries have been indexed from 16 files Searching for a word search nb word Search for a word in the text files that have been indexed. The output should be by order of most occurrence in a file, and should be limited to the number of file given as argument (nb). Example input: search 3 algorithms Suggested output: The word “algorithms” occurs 76 times in 7 files: 7 times in lect15.txt (lines 21, 86, 94, 95, 233, 278, 402) 6 times in lect00.txt (lines 16, 54, 121, 139, 147, 173) 5 times in lect02.txt (lines 57, 165, 212, 869, 1094) Adding to the map the words of a given file add filename Adds to the map the words and their positions in the given file. Note that the text file needs to be placed in TextFiles as the program will only read files from that directory. So for example, the command add file example.txt will read and insert to the map all the words and their positions in the file TextFiles/example.txt. Example input: add example.txt Suggested output: 15 entries have been indexed from file “example.txt” Removing in the map the word positions from a given file remove filename Removes from the map the positions of words from the given file. If a word has no more positions associated with it, it is removed from the map. Note that the file itself should not be removed from the TextFiles directory. Example input: remove example.txt Suggested output: 15 entries have been removed from file “example.txt” Overview of the Word Index overview Prints a summary of the number of indexed words, indexed positions and indexed files. Suggested output: Overview: number of words: 1304 number of positions: 954 number of files: 20 4 Classes and Interfaces WordIndex (class to complete) This is the class which contains the main program. You have to write most of the main program, the code in WordIndex.java currently reads the command line arguments (file names). The name must stay the same, WordIndex. In the version that you submit, you should be using the hash table map implementation. The linked list based implementation should be used only for comparison with the hash table implementation. IWordMap (interface provided) The interface your map should implement (both the linked list based map and the hash table based map). WordException (class to implement) This exception should be thrown by your map in case of unexpected conditions, see classes HashWordMap and ListWordMap for cases in which to throw this exception. WordTxtReader (class provided) This class provides a facility for reading words from a file. See section 3 for information. IPosition and WordPosition (interface and class provided) These interface and class represent a word position in a file. ListWordMap (class to implement) This class implements a map based on linked list. You can use Java’s build in LinkedList class by using java.util.LinkedList. This class must implement the provided IWordMap interface. You must implement the following public methods, and all the other methods which you might implement for this class must be private. Also any member variables must be private. public ListWordMap() Constructor for the class public void addPos(String word, IPosition pos) This method adds a new position to an entry of the map. It creates the entry if word is not already present in the map. public void removeWord(String word) throws WordException This method removes from the map the entry for word. Throws exception if word is not present in the map. public void removePos(String word, IPosition pos) throws WordException This method removes from the map position for word. Throws exception if word is not present in the map or if word is not associated to the given position. public Iterator words() This method returns an Iterator over all words in the map. The iteration1 is over objects of class String. public Iterator positions(String word) throws WordException This method returns an Iterator1 over all positions of word. The iteration is over objects of class IPosition. Throws exception if word is not present in the map. public int numberOfEntries() This method returns the number of entries stored in the map. IHashMonitor (interface provided) This is an interface your hash table based map should implement. HashWordMapProvidedTest (class provided) This contains unit test cases which we will use to test your hash table implementation. Compile and run it once you have implemented your HashWordMap class. It will run some tests on your hash table and will let you know which tests are passed/failed by your hash table. Read the source code of this class to understand what each test if doing and to fix your implementation in case of failed tests. To get the full score on the assignment, you must pass all the tests. HashWordMapProvidedExp (class provided) This program will run some experiments to display the number of probes depending on the max load factor of your hash table based map implementation. HashWordMap (class to implement) This class implements a map based on hash table, and should implement the provided IWordMap interface. It should also implement the IHashMonitor interface implement the methods needed by HashWordMapProvidedTest. You should use open addressing with double hashing strategy. Start with an initial hash table of size 13. Increase its size to the next prime number at least twice larger than the current array size (which is N) when the load factor gets larger than the maximum allowed load factor (maximum allowed load factor is to be given to the constructor to the hash table). You must design your hash function so that it produces few collisions. You should implement the following constructors. public HashWordMap() A constructor for the class which sets the maximum load factor to 0.5. public HashWordMap(float maxLoadFactor) A constructor for the class which allows to set the maximum load factor at construction time. You must implement the following public methods, and all other methods which you might implement for this class must be private. Any member variables must also be private. public void addPos(String word, IPosition pos) This method adds a new position to an entry of the map. It creates the entry if word is not already present in the map. public void removeWord(String word) throws WordException This method removes from the map the entry for word. Throws exception if word is not present in the map. public void removePos(String word, IPosition pos) throws WordException This method removes from the map position for word. Throws exception if word is not present in the map or if word is not associated to the given position. public Iterator words() This method returns an Iterator over all words in the map. The iteration1 is over objects of class String. public Iterator positions(String word) throws WordException This method returns an Iterator1 over all positions of word. The iteration is over objects of class IPosition. Throws exception if word is not present in the map. public int numberOfEntries() This method returns the number of entries stored in the map. public float getMaxLoadFactor() This method returns the maximum authorised load factor. public float getLoadFactor() This method returns the current load factor. public float averNumProbes() This method returns an average number of probes performed by your hash table so far. You should count the total number of operations performed by the hash table (each of find, insert, remove count as one operation, do not count any other operations) and also the total number of probes performed so far by the hash table. When averNumProbes() is called, it should return (float) numberOfProbes/numberOfOperations. As you decrease the maximum allowed load factor, the average number of probes should go down. When you run the HashWordMapProvidedTest program, it will run your hash table at different load factors and will print out the average probe numbers versus the running time. If you see that the average probe number goes up as the max load factor goes up, you are probably computing probes/implementing hash table correctly. You can implement any other methods that you want, but they must be declared as private methods. public int hashCode(String s) This method returns the hash code as an integer of a given string. You have to use the polynomial accumulation hash code for strings we talked about in class. Note that you can implement any auxiliary methods that you need to help you implement this method, but they must be declared as private methods. You should not use Java’s .hashCode method. ListWordMapTest and HashWordMapTest (classes to implement) These classes should contain jUnit 4 test cases2 that you used to test your ListWordMap class and HashWordMap class implementations. Tests that are focusing on the map methods can appear in both testing classes. Cases in HashWordMapTest could also test the behaviour of your hash table map with regards to the maximum and current load factors. You could combine the following strategies to write your test cases: • Write one test per branch of your program (testing that each if-branches or each loop-condition behaves as you expect), this is to ensure your test cases to cover most cases. • Write tests for borderline cases like when trying to remove an entry from an empty map, or adding an empty word. • When you notice that your program had bug, write a test case to test that you have properly fixed the problem. This will help you not repeat the bug later (this is called a non-regression test). 5 Hash Table vs. Linked List Map Implementation Text files containing Shakespeare’s work (from Project Gutenberg3) of different sizes (01-MND.txt, . . . , 06-KR3-MND.txt) are provided in a second TextFiles Shakespeare directory. Run your program with this directory using a command.txt file argument containing the commands addall and search 3 Hermia to check your Word Index programme. Get the running time using after each file’s words/positions are added to to the map with the Java method: System.currentTimeMillis(). Note that this method will return the current time, NOT the running time from the start of the program. Therefore, to get the total time (in milliseconds) your program took to complete, you should measure the current time at the very start of the program, then at the very end, and subtract the two. Since what changes between the different runs is the size of the text file, we should plot the running time vs. the size of the text file, that is the number of words in the text file. Count the number of words in each text file and plot, on the same chart, the number of words versus the running time for the list and for the hash based map implementations. The running time is essentially the time it takes to insert all the dictionary words, since the second command is only for searching the word Hermia. When we insert a word into a map, we also have to check if that word is already in the map. For a hash table, checking and inserting is expected to take a constant amount of time, and therefore inserting all elements in the map should take a linear time. For a linked list, inserting is constant amount of time, but checking if the element is already in the list is linear amount of time, and therefore inserting all elements in the map should take quadratic time. Thus hash table based implementation running time plot should resemble a linear function, linked list based implementation should resemble a quadratic function.
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