(note: this is not CS homework)
I've tried implementing the minimax game-tree searching algorithm in Coffeescript, and continue to get errors with my algorithm. There appear to be 2 main issues: 1) the algorithm itself doesn't seem to return the proper value during alpha-beta pruning (obviously the bigger issue), and 2) my gameboard object, represented by an array of 9 integers, seems to be attached to the DOM, making duplication of the board and passing it as a parameter to the recursive calls of the minimax search function problematic.
There are 3 classes: board, bot (where the minimax algorithm lives), and game. Be aware that a new Game is initiated upon load (debugging alerts popup accordingly) and that a board is mocked with preconfigured plays to ease debugging.
You'll note that I've tried three separate attempts at a minimax solution (been banging my head against this for weeks in my spare time), the latter two are now commented out. In my final solution I've been following this pseudocode.
index.html
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<head>
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<title>Tic Tac Toe | JMS</title>
<meta name="description" content="Tic-tac-toe using the minimax algorithm">
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<body>
<h1 id="output">empty</h1>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"></script>
<script>window.jQuery || document.write('<script src="js/vendor/jquery-1.9.0.min.js"><\/script>')</script>
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<script src="js/rules.js"></script>
<script src="js/board.js"></script>
<script src="js/game.js"></script>
<script src="js/bot.js"></script>
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board.coffee
class Board
constructor: ->
@spaces = [0,1,2,3,4,5,6,7,8]
reset: ->
@spaces = [0,1,2,3,4,5,6,7,8]
setSpace: (index, side) ->
console.log "board.setSpace: played at index #{index} with side #{side}"
@spaces[index] = side
$('#output').text(@spaces)
setSpaces: (array) ->
@spaces = array
getSpace: (index) ->
@spaces[index]
getSpaces: ->
@spaces
bot.coffee
class Bot
constructor: (side) ->
console.log "created a new bot!"
@name = "Gandalf"
@infinity = 99
@side = side
calculateMove: (board) ->
console.log "Bot.calculateMove with #{board.getSpaces()}"
debugger
isBoardEmpty = (board) -> # works
console.log "Bot.calculateMove: board is #{board.getSpaces()}"
boardSpaces = board.getSpaces()
for space in boardSpaces
console.log "Bot.calculateMove: checking if space #{space} is empty"
if typeof space is "string"
console.log "Bot.calculateMove: board isn't empty"
return false
return true
return 4 if isBoardEmpty(board)
boardCopy = jQuery.extend({}, board) # this copies the board
# but still refers to the same spaces
# in the copy. Necessary?
console.log "about to call Bot.move"
move = @search(boardCopy, @side, 0, -@infinity, +@infinity)
return move
search: (board, side, depth, alpha, beta)->
# needs to return the index of the move
debugger
##### TRY 1 #####
#################
value = @nodeValue(board, side)
if value isnt 0
if value > 0 then return value - depth else return value + depth
otherside = if side is 'X' then 'O' else 'X'
moves = @generateMoves board
boardSpaces = board.getSpaces()
boardCopy = new Board()
boardCopy.setSpaces(boardSpaces)
if side is 'O'
for move in moves
boardCopy.setSpace(move, side)
score = @search(boardCopy, otherside, depth + 1, beta, alpha)
alpha = score if score > alpha
@undoMove(boardCopy, move)
return alpha if alpha >= beta
if side is 'X'
for move in moves
boardCopy.setSpace(move, side)
score = @search(boardCopy, otherside, depth + 1, beta, alpha)
beta = score if score < beta
@undoMove(boardCopy, move)
return beta if alpha >= beta
##### TRY 2 #####
#################
# value = @nodeValue(board, side)
# console.log "Bot.search: depth is #{depth}"
# console.log "Bot.search: value is #{value}"
# if value isnt 0
# if value > 0 then return value - depth else return value + depth
# moves = @generateMoves board
# return value if moves.length is 0
# otherside = if side is 'X' then 'O' else 'X'
# for move in moves
# console.log "Bot.search: #{move} in moves"
# boardSpaces = board.getSpaces()
# boardCopy = new Board()
# boardCopy.setSpaces(boardSpaces) # This could be rolled into a optional argument
# # on the board constructor
# @makeMove(boardCopy, move, side)
# potentialAlpha = -@search(board, otherside, depth + 1, -beta, -alpha)
# @undoMove(boardCopy, move) # THINK ITS SOMETHING WITH THE BOARD BEING USED
# break if beta <= alpha
# if potentialAlpha > alpha
# alpha = potentialAlpha
# if depth is 0
# bestMove = move
# if depth isnt 0 then return alpha else return bestMove
##### TRY 3 #####
#################
# value = @nodeValue(board, side)
# console.log "Bot.search: depth is #{depth}"
# console.log "Bot.search: value is #{value}"
# if value isnt 0
# if value > 0 then return value - depth else return value + depth
# moves = @generateMoves board
# return value if moves.length is 0 # ?
# otherside = if side is 'X' then 'O' else 'X'
# boardSpaces = board.getSpaces()
# boardCopy = new Board()
# boardCopy.setSpaces(boardSpaces) # This could be rolled into a optional argument
# # on the board constructor
# if side is 'O'
# for move in moves
# boardCopy.setSpace(move, side)
# score = @search(boardCopy, otherside, depth + 1, beta, alpha)
# alpha = score if score > alpha
# return alpha if alpha >= beta
# break if beta > alpha
# else
# for move in moves
# boardCopy.setSpace(move, side)
# score = @search(boardCopy, otherside, depth + 1, beta, alpha)
# beta = score if score < beta
# return beta if alpha >= beta
# break if alph > beta
nodeValue: (board, side) ->
console.log "Bot.nodeValue: board is #{board.getSpaces()} and side is #{side}"
gameResult = checkGameOver board
console.log "Bot.nodeValue: gameResult is #{gameResult}"
if gameResult is false or gameResult is 'tie'
console.log "returning 0 for nodeValue"
return 0
else if gameResult is side
console.log "returning #{@infinity} for nodeValue"
return @infinity
else
console.log "returning #{-@infinity} for nodeValue"
return -@infinity
generateMoves: (board) ->
console.log "Bot.generateMoves: board is #{board.getSpaces()}"
moves = []
boardSpaces = board.getSpaces()
for space in boardSpaces
if typeof space is "number"
moves.push(space)
console.log "Bot.generateMoves: moves are #{moves}"
return moves
makeMove: (board, move, side) ->
console.log "makeMove: board before makeMove with move #{move} is #{board.getSpaces()}"
board.setSpace(move, side)
# board[move] = side
console.log "makeMove: board after makeMove is #{board.getSpaces()}"
return board #####
undoMove: (board, move) ->
console.log board.getSpaces()
boardSpaces = board.getSpaces()
board.setSpace(move, move)
console.log board.getSpaces()
return board
# minimax = (player, board) ->
# minimax (player, board) ->
# winner if gameOver(currentPosition)
rules.coffee
checkGameOver = (board) ->
opportunities = 8
result = false
check = (space) ->
board.getSpace(space)
winningCombinations = [[0,1,2],[3,4,5],[6,7,8],[0,3,6],
[1,4,7],[2,5,8],[0,4,8],[2,4,6]]
for combo in winningCombinations
firstPlay = combo[0]
secondPlay = combo[1]
thirdPlay = combo[2]
if typeof check(firstPlay) is "string" and
typeof check(secondPlay) is "string" and
typeof check(thirdPlay) is "string"
opportunities -= 1
console.log "checkGameOver: opportunities decreased to #{opportunities}"
if check(firstPlay) is check(secondPlay) is check(thirdPlay)
alert "Winner is #{board.getSpace(firstPlay)}"
return result = board.getSpace(firstPlay)
if opportunities is 0
alert "tie"
return result = "tie"
return result
game.coffee
class Game
constructor: ->
@board = new Board()
$('#output').text(@board.getSpaces())
new: ->
@board.reset()
@result = false
@side = "X"
@bot = new Bot "O"
@moves = 0
firstTurn: ->
# Production
# space = prompt "What space are you playing?"
# @makeMove space
# Testing
@board.setSpaces ['X',1,2,3,'O',5,6,7,8] # Testing – Bot should output 1 for calMove
@makeMove 2
makeMove: (space) =>
@board.setSpace(space, @side)
@moves += 1
@concludeTurn()
listenForMove: ->
space = prompt "What space are you playing?"
@makeMove space
concludeTurn: ->
@result = checkGameOver @board
console.log "Result is #{@result}"
if @result is 'X' or @result is 'O' or @result is 'tie'
alert "Game.concludeTurn: game is over, heading into gameOver"
return @gameOver @result
@changeTurn()
changeTurn: ->
@side = if @side is 'X' then 'O' else 'X'
console.log "in change turn, side is now #{@side}"
@listenForMove() if @side is 'X'
console.log "Game.changeTurn: bot (#{@bot}) is about to calc move"
@makeMove(@bot.calculateMove @board) if @side is 'O'
# placement = @bot.calculateMove @board if @side is 'O'
# console.log placement
# @board.setSpace(placement, 'O') # give a secondary argument to makeMove and remove
# @concludeTurn()
gameOver: (winner) ->
console.log "Game.concludeTurn: winner is #{@result}"
return
# answer = prompt "Winner is #{winner}! Would you like to play again?"
# @playAgain answer
# playAgain: (answer) ->
# alert "Suite yourself" if answer is false
# @new() if answer is true
# check if game is over
# check moves
# if even then human
# else
# bot
#
# increment moves?
# reset grid
# turn = "X"
# move = 0
g = new Game()
g.new()
g.firstTurn()
Very grateful for any help.
