Project 5: Competitive Spelling Bee

Projects are designed to test your mastery of course material as well as your programming skills; think of them as “take-home exams” and don’t communicate with anyone about possible solutions. This project focuses on the implementation, testing, benchmarking, and application of hash tables.


This project is a little more open-ended than what you’re probably used to for this course. We no longer break things down into separate problems below, the entire project is the problem now. This also means that the “rubric” for the project is the grading breakdown above, and it all applies.

Your “one” task for this project is to take the simple spell checker we give you and to turn it into the fastest, most memory-efficient spell checker in the course, subject to the constraints detailed below. You are expected to do this by (once again) implementing the Map interface, this time using one of several hash table techniques (your choice, see below).

Important: The 20% for Performance will be awarded by ranking your submissions according to both time and space required by your spell checker. Less time is better, less space is better, and both are considered separately for 10% each. So one submission might win in both categories, or different submissions might win in each, it all depends. Submissions that are tied (perform almost the same in our benchmarks) will receive the same rank (in the respective dimension); it’s theoretically possible (although highly unlikely) for all submissions to be tied.

Note that your spell checker must produce correct results in order to take part in the performance tournament! If that wasn’t the case, winning in the tournament would be trivial: Just submit a solution that does nothing. Obviously that’s not okay…

The Spell Checker

The good news is that you don’t actually have to write the spell checker: We hand you for free! Here’s a quick example for how it works:

$ java Spell
Error: Need path to dictionary file!
$ java Spell ogden.dict
I know how to program.

As you can see Spell expects the name of a dictionary file as the sole command line argument. It reads that dictionary file into a Map and then proceeds to read words from standard input. It tries to find each word in the Map it generated and it prints out the words it doesn’t know, assuming they are spelling errors. (So input without spelling errors results in no output whatsoever.) Obviously the ogden.dict dictionary is not very complete (indeed it only contains 852 words, not nearly enough for a decent spell checker) so it thinks know and program are spelling errors.

You can find on Piazza. We have also posted a tarball spell.tar.gz that contains three different dictionaries, three different example texts to check, and the expected errors that should be output for some of those combinations.

You can determine how well Spell is doing using the familiar xtime script (which we’ll also use for ranking, albeit in a slightly modified form). Here are two examples for benchmarking:

$ ./xtime java Spell ogden.dict <democracy.txt >/dev/null
3.08 seconds 190416 kilobytes java Spell ogden.dict
$ ./xtime java Spell scowl-small.dict <sanity.txt >/dev/null
67.45 seconds 211520 kilobytes java Spell scowl-small.dict

All these files are available to you, and you should probably start by trying to figure out why (before you add your new, improved Map implementation) is only useful for tiny dictionary files.

Note that you must use the program as provided on Piazza. The only change you’re allowed to make is to replace new SimpleMap with new HashMap for your new Map implementation. (We will use our own for grading, so if you made changes to yours, those changes will “disappear” for grading.)

The Hash Table

You are supposed to improve the performance of by replacing its use of SimpleMap with a new HashMap class you write. Obviously your has to implement the Map interface, but beyond that you have quite a few options for how to proceed:

We’re actually happy to help those adventurous folks who stroll all the way into universal hashing territory: You can find on Piazza, a class that implements two common constructions for universal hash functions. Just be warned that the code is pretty new and might contain a bug or three. (You can’t blame us later if your stuff doesn’t work: It’s “free code” so it comes with “no warranties” of any kind. Don’t use it if you’re worried about it breaking.)

As is the “big piece of code” you have to write and submit this week, you obviously need “all the usual pieces” for it. In particular:

And that’s it. Yes, you’re really on your own for figuring out what kind of hash table you should implement. Just a word of caution: The code in Peter’s lecture notes is not exactly great. You should probably only consider it as a starting point if you really have no idea what else to do.

All critical map operations, except insert, must run in O(1) expected time (or better); insert can run in O(1) amortized time in case you have to grow the bucket array table to keep the load factor down.

Got Extra Hash Tables?

Depending on just how serious you are about those Performance points, you may well end up writing several different hash tables over the course of this project. However, you have to pick one of those as your and submit it under that name. Presumably you’ll pick the best one, but if you want to submit any of the others you wrote (not required, strictly optional) make sure you give them different names that express what kind of hash table you used. (So, for example, if in addition to your “winning” you also implemented a hash table using separate chaining, call that one or something similar. Whichever one you call is the one we’ll run against everybody else’s code.)

Iterative Development

You cannot know how fast your HashMap is until it’s actually written. You cannot improve your HashMap until you can tell how fast it is. So the worst mistake you can make is to “think about it” for days without writing any code. (Thinking ahead is good in principle, thinking ahead for too long is the problem here.)

We recommend you start right now by writing the simplest HashMap you can think of and making that work. For example you could write one based on separate chaining but with a fixed array size. Are you going to win with that? No! But at least now you’ve started playing the game.

You want your test cases and benchmarks in place before you keep going. Make sure that your test cases are complete and that your benchmarks tell you how well the various Map operations work for that first version of HashMap. You should probably save a backup as soon as you get done with the first round.

From then on, it’s “try to improve things” followed by “see if the tests still pass” followed by “benchmark to see if things actually got better” followed by either “Whoops, that was a bad idea, let’s undo that” or “Yay, I made progress, let’s save a backup of the new version” and so on and so forth. We predict that there will be a correlation between how well you do and how often you “went around” this iterative development cycle.

What Classes Are Allowed?

You may use java.util.ArrayList and java.util.LinkedList if you wish. You may not use fancier Java collection classes, in particular you may not use java.util.HashMap to implement your HashMap.

You may use the SimpleArray and NodeList classes we handed out earlier in the semester if you wish. You may not use other Set or Map implementations to hack your HashMap though.

If you want to use some other data structure class, better ask on Piazza first! You don’t want to find out minutes before the deadline that you used something that’s not okay…

What about the README?

You should use your README file to explain how you approached this project, what you did in what order, how it worked out, how your plans changed, etc. Try to summarize all the different ways you developed, evaluated, and improved your HashMap over time. If you don’t have a story to tell here, you probably didn’t do enough…

Random Hints


You must turn in a gzip-compressed tarball of your project; the filename should be cs226-assignment-number-jhed.tar.gz with number replaced by the number of this project (see above) and jhed replaced by your JHED identifier. (For example, Peter would use cs226-assignment-3-pfroehl2.tar.gz for his submission of Project 3.) The tarball should contain no derived files whatsoever (i.e. no .class files, no .html files, etc.), but should allow building all derived files. Include a plain text README file (not README.txt or README.docx or whatnot) that briefly explains what your programs do and contains any other notes you want us to check out before grading; your answers to written problems should be in this file as well. Finally, make sure to include your name and email address in every file you turn in (well, in every file for which it makes sense to do so anyway)!


For reference, here is a short explanation of the grading criteria; some of the criteria don’t apply to all problems, and not all of the criteria are used on all projects.

Packaging refers to the proper organization of the stuff you hand in, following both the guidelines for Deliverables above as well as the general submission instructions for projects.

Style refers to Java programming style, including things like consistent indentation, appropriate identifier names, useful comments, suitable javadoc documentation, etc. Many aspects of this are enforced automatically by Checkstyle when run with the configuration file available on Piazza. Style also includes proper modularization of your code (into interfaces, classes, methods, using public, protected, and private appropriately, etc.). Simple, clean, readable code is what you should be aiming for.

Testing refers to proper unit tests for all of the data structure classes you developed for this project, using the JUnit 4 framework as introduced in lecture. Make sure you test all (implied) axioms that you can think of and all exception conditions that are relevant.

Performance refers to how fast/with how little memory your program can produce the required results compared to other submissions.

Functionality refers to your programs being able to do what they should according to the specification given above; if the specification is ambiguous and you had to make a certain choice, defend that choice in your README file.

If your programs cannot be built you will get no points whatsoever. If your programs cannot be built without warnings using javac -Xlint:all we will take off 10% (except if you document a very good reason; no, you cannot use the @SuppressWarnings annotation either). If your programs fail miserably even once, i.e. terminate with an exception of any kind, we will take off 10% (however we’ll also take those 10% off if you’re trying to be “excessively smart” by wrapping your whole program into a universal try-catch).