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Fixed Hash Map

Introduction

This project was created by Aryik Bhattacharya for the coding challenge portion of the KPCB fellows program. The problem statement was:

Using only primitive types, implement a fixed-size hash map that associates string keys with arbitrary data object references (you don't need to copy the object). Your data structure should be optimized for algorithmic runtime and memory usage. You should not import any external libraries, and may not use primitive hash map or dictionary types in languages like Python or Ruby.

Building and Testing

To test the project, enter mvn clean test on the command line. To package the project into a .jar, enter mvn package on the command line. The built jar file will be located at target/fixedhasmap-1.0.0.jar.

Dependencies

The project is built using Maven with the standard Maven project structure (src/main/* and src/test/*). The tests are written using Spock, a testing/specification framework with a highly expressive syntax that is compatible with most IDEs, build tools, and CI servers due to its jUnit runner. You do not need to do anything to obtain spock as Maven handles dependency management.

You must have Maven installed to test or package the project. If you do not have Maven, you can still compile the source class using javac but the test file is not guaranteed to compile because the dependencies (groovy and spock) may not be installed on your machine. Maven is open source and readily available on all operating systems. To install Maven, either use your favorite package manager (homebrew, apt-get, etc.), or download it in one of many formats here.

Hash Map Information

The important class for this project is FixedHashMap. FixedHashMap's default initializer creates a hash map with a size of 10. The other constructor takes an integer that represents the desired size of the hash map. The map itself is a an array with a length equal to the smallest power of 2 that is smaller than the specified size. The map uses Java's inbuilt hashCode() function for hashing. Hashing collisions are dealt with by creating a linked list of Node objects. Each Node simply contains the key, value, and a reference to the next Node in the list.

Accesses and insertions are in constant time in the average case. In the worst case, which occurs when every key inserted into the map has the same hash, accesses and insertions are in O(n) time as the linked list has to be traversed. The map uses O(n) space. The map supports the following functions:

  • boolean set(String key, Object value)
  • Object get(String key)
  • Object delete(String key)
  • float load()

Tests

The test suite was written with a focus on covering edge cases and all possible execution paths. Thanks to Spock's expressive syntax, it should be fairly clear what each test does.