Created: 2008-12-17 07:00
Updated: 2016-05-08 19:36


This is an implementation of the RIT CS4 puzzle solver project.


The RIT CS4 course is (or, at least, was) the first C++ course taught to Computer Science students at RIT (generally in the second year). The most common project used for this course is a puzzle solver project, which requires around 4 solutions over the 10-week period.

I've TA'ed for this course many times, and I've accumulated various solutions. I've placed a few up here, in the hopes that they may server as useful examples for other people who've already taken the course.

If I discover that the C++ version is being abused, I'll take it down.


(The most up-to-date version of this project can be found here: CS4 Projects page).

In general, the project is split into two parts:

  1. A generalized solver : the primary piece of the project is a solver that will perform a breadth-first search of a given problem space. It can solve puzzles that provide the following:
    • A starting position
    • A function for getting "next" or "neighbor" positions
    • A function for determining if a given position is the "end"
  2. Various puzzles : Puzzles implement the required functionally plus a minimal main method for starting from the command-line.

The general flow of the program is:

  1. The puzzle's main method takes care of configuring the puzzle as specified (either as arguments to the command line or as names of files specified on the command line - the project is also used to teach the basics of file/stream input/output in C++).
  2. The puzzle's main method starts up the solver
  3. The solver searches over the problem space until it finds a solution or runs out of unique and new configurations
  4. The solver prints out the path (a shortest path) to the solution.

The project is always taught in C++.

Common puzzles

Here are a few of the commonly-used puzzles:


This one is rather simple - you have a clock (generally, a 12-hour clock, but the number of hours can be specified), and you want to find the shortest "path" the hour hand can take to get from one hour to the next. The clock, of course, "wraps around" at the largest hour to the smallest.


A clock that has 12 hours on it. The current hour is 3, the desired hour is 6. The shortest path is: 3, 4, 5, 6.

VClock (Variable Clock)

This is very similar to Clock, except that the hand can step in increments of more than just +1/-1 (they are specified on the command line).


A "vclock" that has 12 hours on it. The current hour is 2, the desired hour is 9. The possible steps are +2, +1, and -3. The shortest path is 2 (-3) 11 (-3) 8 (+1) 9.


This is the "water bucket" puzzle that most people are familiar with - you have multiple buckets or containers of water, each which can hold a specific amount. By filling buckets up with water, pouring them into each other, and emptying buckets, your goal is to make one bucket contain the desired about of water (measured in hand-wavy "units").

The most common example is that you have two containers that hold 3 and 5 units, respectively. The goal amount is 4 units of water.

The shortest path is:

(0, 0) (0, 3) (3, 3) (1, 5) (1, 0) (0, 1) (3, 1) (0, 4)

(TODO: More examples to come)



Most of these puzzles have implementations in C++ that I wrote during the courses. As there are 3 puzzles taught each time, and the puzzles change up (some are there almost every time, like clock), I have a few that I've written. I don't remember which 3 puzzles were assigned the year I took the course.


The summer before my senior year, while on co-op for Microsoft, I wrote a version of the puzzle solver and a few of the puzzles in Lisp. I had two goals for this:

  1. To learn more about Lisp - I love the language, but I wanted to actually use it :)
  2. To see if Lisp really is seventy-gigabazillion-times slower than C++ - I heard this one a lot, mostly from people who suck at programming in both languages, so I wanted to see how true it was.

I also wrote it around the time I read Practical Common Lisp, so I included a tiny unit testing framework from that book.

I learned a few things from this:

  • It is amazing how easy it was to add unit testing. Even writing Java in the Eclipse environment (with JUnit) is still more hassle than what I did here. The upside (in Eclipse/JUnit) are the pretty red and green bars, but I found I could live without those :)
  • Related to the above bullet, I wrote a very small amount of code, and almost no code that wasn't directly related to exactly what I needed to do. At some level, this means very little of the overhead of fairly useless keywords (e.g., "how many times do I have to declare that this variable is an int?). At a more important level, it means that my code is very semantically compressed - I only have to write that which will bring me closer to my goal, and not 7,000 pounds of "public static void main blah blah blah open-bracket close-bracket".
  • As for the speed - the lisp solution (running on a few different lisps on Ubuntu at the time), after compiled (using (load (compile-file "foo.lisp"))), was just as fast as the C++ solution compiled (without much or any optimization, though). Compiling the C++ stuff with full optimization made it faster (though much bigger), but I'd still trade the milliseconds for the many other positives of Lisp.


This one I've just decided to do. Nothing here yet, but soon :)

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