The game Connect-4 is played on a board that has seven columns. There are six spaces on each column. The board is initially empty. Two players take turns dropping one piece (black or grey in the diagram below, but "X" or O" in our game) in a column. The piece falls into the lowest unoccupied row. A player cannot move in a column if the top-most row of that column already has a piece in it. The first player to get four of their counters in a line (horizontally, vertically, or diagonally) is the winner.
You have been provided with two files for this problem: minimax.lisp, which contains lisp code for the minimax algorithm, and connect-4.lisp, which contains a working Lisp implementation of the Connect-4 game.
As the Connect-4 implementation currently stands, you should have absolutely no problem beating the program. Try it:
[1]> (load 'minimax)
;; Loading file C:\CSE2AIF\minimax.lisp ...
;; Loaded file C:\CSE2AIF\minimax.lisp
T
[3]> (load connect-4)
;; Loading file C:\CSE2AIF\connect-4.lisp ...
;; Loaded file C:\CSE2AIF\connect-4.lisp
T
[3]> (play)
The program plays poorly because it has a poor-quality heuristic. The function static, which evaluates a position from the point of view of a particular player, is currently defined as follows: (defun static (board player) 0)
This means that each board configuration receives exactly the same evaluation; i.e., 0. Your task for this question is to develop and implement a good heuristic for the Connect-4 game. The only LISP code that you are required to write is a LISP function static, which accepts a board and player as input arguments, and returns an evaluation of the board state from the point of view of player. You can, of course, write as many helper functions as you like.
To assist you, the code you have been supplied with contains a parameter *all-c4-lines* which is a list of all of the 69 possible ways to win in Connect-4. Each element of this list is a list of length four, such as
((3 5) (4 5) (5 5) (6 5))
Each element of this is the column and row. For example, the above list indicates that there is a line of length four that includes a piece at the 4th column of the 6th row, the 5th column of the 6th row, the 6th column of the 6th row, and the 7th column of the 6th row.
Note that rows and column indexing starts at 0.
The Lisp built-in function aref can be used to find what piece, if any, is at a specified location. For example, to find what piece, if any, occupies column col and row row of the current board state, evaluate the following code
(aref board col row)
This will return either nil, x or o.