CSCI 241 - A3

Scott Wehrwein

Spring 2022

Overview

A3 consists of three phases.

  1. Implement a standard min-heap in Heap.java.

  2. Implement a hash table in HashTable.java.

  3. Augment your heap in Heap.java to implement efficient contains and changePriority methods.

Getting Started

The Github Classroom invitation link for this assignment is in Assignment 3 on Canvas. Begin by accepting the invitation and cloning a local working copy of your repository as you have for past assignments.

The Heap class depends on the AList class you wrote for Lab 4. Copy your AList.java from your lab 4 repository into your A3 repo’s lib/src/main/java/heap/ directory.

Unit Tests and Gradle

AListTest.java, from Lab 4, has been included. If your implementation fails any of the AList tests, fix that first.

You are provided with unit tests for each phase in A3Test.java. The test methods are numbered with three-digit numbers; the hundreds place indicates which phase the test pertains to. Test often, and make sure you pass any tests associated with a TODO item before moving on to the next one. You may find it helpful to run only the tests for the phase you’re currently working on using the –tests flag with a wildcard; for example to run only the phase 2 tests, you could enter

gradle test --tests "test2*"

Phase 1

In Phase 1, you’ll complete the implementation of a min-heap given in the skeleton repository’s Heap.java file. For details on how these operations work, see the slides from Lecture 12.

The tasks listed below are marked in Heap.java as TODO 1.0, TODO 1.1, and so on. Phase 3 involves further modifications to Heap.java. For now, ignore anything in the code marked as Phase 3 Only, or TODO 3.x.

  1. Read and understand the class invariant in the comment at the top of Heap.java. Ignore the Phase 3 parts for now. This specifies the properties the Heap must satisfy. All public methods are responsible for making sure that the class invariants are true before the method returns.

  2. Implement the add method, using the bubbleUp helper according to its specification.

  3. Implement the swap helper method, which you’ll use in both bubbling routines.

  4. Implement the bubbleUp helper method. Feel free to use private helper methods to keep this code clean.

  5. Implement peek. Recall that peek returns the minimum element without modifying the heap.

  6. Implement poll using bubbleDown, which you’ll implement next.

  7. Implement bubbleDown. You are highly encouraged to use one or more helper methods to keep this code clean. In fact, we’ve included a suggested private method smallerChild, along with its specification, that we used in our solution.

Phase 2

In Phase 2 you’ll implement a hash table with chaining for collision resolution. Although we could base it on our AList class, we need access to the internals of the growth process to handle growing and rehashing as needed. Similarly, for the underlying storage we could use an array of LinkedLists, but dealing with Java’s LinkedList machinery ends up being a bit of a headache—more so than simply writing a little linked list code to do the chaining by hand. For these reasons, you’ll complete a standalone hash table implementation in HashTable.java without using any tools from java.util. The following major design decisions have been made for you:

Here’s some sample code using the hash table and its output using my solution. The dump method shows the internal layout of the table: each bucket in the buckets array stores a reference to Pair objects, each of which stores a reference to the next Pair in the chain, or null.

Code:

HashTable<Integer,Integer> hm = new HashTable<Integer,Integer>(4);
hm.put(0,0);
hm.put(4,1);
hm.put(19,1);
hm.put(19,4);
hm.dump()

Output:

Table size: 3 capacity: 4
0: -->(4, 1)-->(0, 0)--|
1: --|
2: --|
3: -->(19, 4)--|

Your job in Phase 2 is to implement four methods: (get, put, containsKey, and remove). Details of how you implement them are left up to you - be sure to read the specification of each method carefully to be sure you’re implementing the specified behavior correctly, completely, and according to the given efficiency bounds.

Your tasks:

  1. To get familiar with the underlying storage mechanism, read the existing code in HashTable.java. In particular, read the javadoc comments above the class, the comments describing each field and its purpose, and the Pair inner class. Then take a look at the provided dump method, which may be useful for debugging.

  2. Implement get(K key).

  3. Implement put without worrying about load factor and array growth. Make sure that you replace the value if the key is already in the map, and insert a new key-value pair otherwise. After implementing get and put without growing the array, you should pass test210PutGet, test211PutGet, test212Put, test213Put, test230PutGet, and test231Put.

  4. Implement containsKey Your code should pass test240containsKey and test241containsKey.

  5. Implement remove. Your code should pass test250Remove and test251Remove.

  6. Finally, modify put to check the load factor and grow and rehash the array if the load factor exceeds 0.8 after the insertion. Use the createBucketArray helper method to create the new array. We’ve also included a method stub for a growIfNeeded private helper method with a suggested specification; you are welcome to implement and use this, but not required to. At this point your code should pass all Phase 2 tests. Note: If you’re not careful, put can have worst-case \(O(n^2)\) runtime.

Phase 3

In Phase 3, we turn back to the Heap class. Now that we have a working HashTable implementation, we can overlay the hash table on the heap in order to make the contains and changePriority operations efficient. This is a common strategy when building data structures in the real world: often a textbook data structure does not provide efficient runtimes for all the operations you need, so you can combine multiple textbook data structures to get more efficient operations at the cost of some extra bookkeeping and storage.

In the Phase 1 heap, finding a given value in the tree requires searching the whole tree, so the runtime is O(n). In Phase 3, we’ll use a HashTable to map from values to heap indices, which allows us to find any value in the heap in expected O(1) time using a hash table lookup. Keeping the HashTable up to date requires small changes throughout the Heap class to make sure that the HashTable stays consistent with the state of the heap - whenever the heap is modified, we need to update the HashTable to match.

One constraint imposed by the use of a HashTable is that each value can only map to a single index. This means that if we insert two entries with equal value into the table, we can’t differentiate between them and store both indices HashTable. To deal with this, we will simply add the requirement that all values stored in the Heap must be distinct. Note that two different values may still have equal priorities.

Your tasks are as follows:

  1. Update the add method to keep the map consistent with the state of the heap. Also be sure to throw an exception if a duplicate value is inserted—the map makes it possible to check this efficiently. For now, don’t worry about the map during bubbleUp—the next TODO will handle this. At this point, your code should pass test300Add.

  2. swap - update the swap method to keep the HashTable consistent with the heap. If you used swap to implement both bubbling routines, bubbleUp and bubbleDown You should now pass test310Swap and test315Add_BubbleUp.

  3. Update poll. Your code should now pass test330Poll_BubbleDown_NoDups and
    test340testDuplicatePriorities.

  4. Implement contains. Once again, the map makes this easy and efficient. You should pass test350contains.

  5. Implement changePriority by finding the value in question, updating its priority, and fixing the Heap property by bubbling it up or down. You should now pass test360ChangePriority.

At this point, your code should be correct! Check the following things before you submit:

  1. Each method adheres to the asymptotic runtime given in its specification, if any.

  2. Your code follows the style guidelines set out in the rubric and the syllabus.

  3. Your submission compiles and passes all tests on the command line in Linux without modification.

  4. All code is committed and pushed to your A3 GitHub repository.

Game Plan

Start small, test incrementally, and git commit often. Please keep track of the number of hours you spend on this assignment, as you will be asked to report it in A3 Survey. Hours spent will not affect your grade.

A suggested timeline for completing the assignment in a stress-free manner is given below:

  1. By Wednesday, 5/11: Complete TODO 1.0. Reading is hard, but it’s worth your while: don’t skip this step!

  2. By Saturday,/ 5/14: Complete TODO 1.1–1.6 (phase 1 heap functionality)

  3. By Wednesday, 5/18: Complete TODO 2.0–2.5 (phase 2 hash table functionality)

  4. By Sunday, 5/22: Complete TODO 3.1–3.5 (phase 3 heap functionality)

How and What to Submit

Submit the assignment by pushing your final changes to GitHub before the deadline, then submitting A3 Survey on Canvas.

Extra Credit

For up to 5 points of extra credit, complete the following enhancement in an extensions branch of your repository. One piece of important functionality that’s missing from our hash table implementation is the ability to iterate over its key-value pairs. Augment the base assignment’s hash table class so that it implements Iterable<HashTable<K,V>.Pair>. This requires implementing the iterator method to return an instance of an iterator class that you’ll need to implement as an inner class of the HashTable. You may find the Java documentation for Iterable<T> and Iterator<T> helpful.

If you complete any extra credit, please include a readme.txt file in your repository describing what you did, instructions for testing it (ideally by running some example code and/or unit tests that you’ve written) and any design decisions you made. Please also mention your enhancements in the A3 Survey on Canvas.

Rubric

Points are awarded for correctness and efficiency of your program, and points can be deducted for errors in commenting, style, clarity, and following assignment instructions. Correctness will be judged based on the unit tests provided to you. A3 is out of a total of 50 points.

Submission

Code : Correctness

Code : Efficiency

Clarity deductions