I wrote this chess program as part of an interview for a scala programming 
position at Novus (www.novus.com).  They asked me to share my answer with them
via github.

I only had about a day to do it, so it is far from perfect.  It doesn't 
implement castling, en-passant, pawn promotion, there is no real UI, and there
is no AI.

UPDATE: I later implemented a breadth-first mini-max search for a simple AI 
player.

UPDATE: later implemented castling and pawn promotion.  Still no en-passant

UPDATE: I made this a bit more generic, and added a connect4 game that can be 
played by the same minimax AI player.  Sample usage below is a bit out of date
because the package name for the import has changed.  I might rename this 
project, so current github URL subject to change.


My Requirements:
====================================================================
1) You choose the programming language (Scala is a huge plus thou)
2) We care about:
    -class design (under Scala, it also means correct use of immutable
vs mutable class implementations)
    -idiomatic language use
     -code clarity
3) We don't care if the UI is "beautiful" or not.  But we do care if the
code is "beautiful"
4) It should run (as a text-based command line application, as a web
app, or use a GUI)
5) There is no need to implement advanced moves such as castling, en
passant, or promotion, but it should be easy to add these
6) I recommend implementing "check" and "check mate"
====================================================================

To play, just open an `sbt console` and make API calls directly.  The 
coordinates are indexed such that "a1" is (0,0), and "h8" is (7,7).

Example output from sbt is below.  The piece names begin with "w" or "b" to 
signify white or black pieces.  Sample usage can also be found in the unit 
tests.  Exceptions are thrown on invalid moves.

scala> brian@brian-laptop:~/unison/Projects/chess$ sbt clean compile test
[info] Set current project to default-64fda4 (in build file:/home/brian/unison/Projects/chess/)
[success] Total time: 0 s, completed Sep 4, 2012 10:42:48 AM
[info] Updating {file:/home/brian/unison/Projects/chess/}default-64fda4...
[info] Resolving org.scala-lang#scala-library;2.9.1 ...
[info] Resolving org.scalatest#scalatest_2.9.1;1.8 ...
[info] Done updating.
[info] Compiling 1 Scala source to /home/brian/unison/Projects/chess/target/scala-2.9.1/classes...
[success] Total time: 11 s, completed Sep 4, 2012 10:42:59 AM
[info] Compiling 1 Scala source to /home/brian/unison/Projects/chess/target/scala-2.9.1/test-classes...
[info] ChessSpec:
[info] A newly created default chess board 
[info] - should have appropriate pieces
[info] - should throw InvalidMoveException if an illegal move is attempted
[info] - should throw invalid move exception if an opponents piece is moved
[info] - should have 20 possible moves
[info] - should find a king in the right place
[info] a particular game of chess 
[info] - should detect check, but not check mate
[info] a foolsmate game 
[info] - should be detected as checkmate
[info] an empty board 
[info] - should be detected as a stalemate
[info] Passed: : Total 8, Failed 0, Errors 0, Passed 8, Skipped 0
[success] Total time: 4 s, completed Sep 4, 2012 10:43:03 AM
brian@brian-laptop:~/unison/Projects/chess$ 


brian@brian-laptop:~/unison/Projects/chess$ sbt console
[info] Set current project to default-64fda4 (in build file:/home/brian/unison/Projects/chess/)
[info] Starting scala interpreter...
[info] 
Welcome to Scala version 2.9.1.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_26).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import net.brianvaughan.scala.chess._
import net.brianvaughan.scala.chess._

scala> var board = BoardState.startingBoard
board: net.brianvaughan.scala.chess.BoardState = 

bRo|bKn|bBi|bQu|bKi|bBi|bKn|bRo|
bPa|bPa|bPa|bPa|bPa|bPa|bPa|bPa|
   |   |   |   |   |   |   |   |
   |   |   |   |   |   |   |   |
   |   |   |   |   |   |   |   |
   |   |   |   |   |   |   |   |
wPa|wPa|wPa|wPa|wPa|wPa|wPa|wPa|
wRo|wKn|wBi|wQu|wKi|wBi|wKn|wRo|

scala> board = board.movePiece(4,1,4,3)
board: net.brianvaughan.scala.chess.BoardState = 
bRo|bKn|bBi|bQu|bKi|bBi|bKn|bRo|
bPa|bPa|bPa|bPa|bPa|bPa|bPa|bPa|
   |   |   |   |   |   |   |   |
   |   |   |   |   |   |   |   |
   |   |   |   |wPa|   |   |   |
   |   |   |   |   |   |   |   |
wPa|wPa|wPa|wPa|   |wPa|wPa|wPa|
wRo|wKn|wBi|wQu|wKi|wBi|wKn|wRo|

scala> board = board.movePiece(1,6,1,5)
board: net.brianvaughan.scala.chess.BoardState = 
bRo|bKn|bBi|bQu|bKi|bBi|bKn|bRo|
bPa|   |bPa|bPa|bPa|bPa|bPa|bPa|
   |bPa|   |   |   |   |   |   |
   |   |   |   |   |   |   |   |
   |   |   |   |wPa|   |   |   |
   |   |   |   |   |   |   |   |
wPa|wPa|wPa|wPa|   |wPa|wPa|wPa|
wRo|wKn|wBi|wQu|wKi|wBi|wKn|wRo|

scala> board = board.movePiece(5,0,2,3)
board: net.brianvaughan.scala.chess.BoardState = 
bRo|bKn|bBi|bQu|bKi|bBi|bKn|bRo|
bPa|   |bPa|bPa|bPa|bPa|bPa|bPa|
   |bPa|   |   |   |   |   |   |
   |   |   |   |   |   |   |   |
   |   |wBi|   |wPa|   |   |   |
   |   |   |   |   |   |   |   |
wPa|wPa|wPa|wPa|   |wPa|wPa|wPa|
wRo|wKn|wBi|wQu|wKi|   |wKn|wRo|

scala> board = board.movePiece(3,6,3,4)
board: net.brianvaughan.scala.chess.BoardState = 
bRo|bKn|bBi|bQu|bKi|bBi|bKn|bRo|
bPa|   |bPa|   |bPa|bPa|bPa|bPa|
   |bPa|   |   |   |   |   |   |
   |   |   |bPa|   |   |   |   |
   |   |wBi|   |wPa|   |   |   |
   |   |   |   |   |   |   |   |
wPa|wPa|wPa|wPa|   |wPa|wPa|wPa|
wRo|wKn|wBi|wQu|wKi|   |wKn|wRo|

scala> board = board.movePiece(2,3,3,4)
board: net.brianvaughan.scala.chess.BoardState = 
bRo|bKn|bBi|bQu|bKi|bBi|bKn|bRo|
bPa|   |bPa|   |bPa|bPa|bPa|bPa|
   |bPa|   |   |   |   |   |   |
   |   |   |wBi|   |   |   |   |
   |   |   |   |wPa|   |   |   |
   |   |   |   |   |   |   |   |
wPa|wPa|wPa|wPa|   |wPa|wPa|wPa|
wRo|wKn|wBi|wQu|wKi|   |wKn|wRo|

scala> board = board.movePiece(5,6,5,4)
board: net.brianvaughan.scala.chess.BoardState = 
bRo|bKn|bBi|bQu|bKi|bBi|bKn|bRo|
bPa|   |bPa|   |bPa|   |bPa|bPa|
   |bPa|   |   |   |   |   |   |
   |   |   |wBi|   |bPa|   |   |
   |   |   |   |wPa|   |   |   |
   |   |   |   |   |   |   |   |
wPa|wPa|wPa|wPa|   |wPa|wPa|wPa|
wRo|wKn|wBi|wQu|wKi|   |wKn|wRo|

scala> board = board.movePiece(4,3,5,4)
board: net.brianvaughan.scala.chess.BoardState = 
bRo|bKn|bBi|bQu|bKi|bBi|bKn|bRo|
bPa|   |bPa|   |bPa|   |bPa|bPa|
   |bPa|   |   |   |   |   |   |
   |   |   |wBi|   |wPa|   |   |
   |   |   |   |   |   |   |   |
   |   |   |   |   |   |   |   |
wPa|wPa|wPa|wPa|   |wPa|wPa|wPa|
wRo|wKn|wBi|wQu|wKi|   |wKn|wRo|