11.6: Source Code for Four-In-A-Row

This source code can be downloaded from http://invpy.com/fourinarow.py.

The image files that Four-In-A-Row uses can be downloaded from http://invpy.com/fourinarowimages.zip.

# Four-In-A-Row (a Connect Four clone)
# By Al Sweigart [e-mail]
# http://inventwithpython.com/pygame
# Released under a "Simplified BSD" license

import random, copy, sys, pygame
from pygame.locals import *

BOARDWIDTH = 7  # how many spaces wide the board is
BOARDHEIGHT = 6 # how many spaces tall the board is
assert BOARDWIDTH >= 4 and BOARDHEIGHT >= 4, 'Board must be at least 4x4.'

DIFFICULTY = 2 # how many moves to look ahead. (>2 is usually too much)

SPACESIZE = 50 # size of the tokens and individual board spaces in pixels

FPS = 30 # frames per second to update the screen
WINDOWWIDTH = 640 # width of the program's window, in pixels
WINDOWHEIGHT = 480 # height in pixels

XMARGIN = int((WINDOWWIDTH - BOARDWIDTH * SPACESIZE) / 2)
YMARGIN = int((WINDOWHEIGHT - BOARDHEIGHT * SPACESIZE) / 2)

BRIGHTBLUE = (0, 50, 255)
WHITE = (255, 255, 255)

BGCOLOR = BRIGHTBLUE
TEXTCOLOR = WHITE

RED = 'red'
BLACK = 'black'
EMPTY = None
HUMAN = 'human'
COMPUTER = 'computer'


def main():
    global FPSCLOCK, DISPLAYSURF, REDPILERECT, BLACKPILERECT, REDTOKENIMG
    global BLACKTOKENIMG, BOARDIMG, ARROWIMG, ARROWRECT, HUMANWINNERIMG
    global COMPUTERWINNERIMG, WINNERRECT, TIEWINNERIMG

    pygame.init()
    FPSCLOCK = pygame.time.Clock()
    DISPLAYSURF = pygame.display.set_mode((WINDOWWIDTH, WINDOWHEIGHT))
    pygame.display.set_caption('Four in a Row')

    REDPILERECT = pygame.Rect(int(SPACESIZE / 2), WINDOWHEIGHT - int(3 * SPACESIZE / 2), SPACESIZE, SPACESIZE)
    BLACKPILERECT = pygame.Rect(WINDOWWIDTH - int(3 * SPACESIZE / 2), WINDOWHEIGHT - int(3 * SPACESIZE / 2), SPACESIZE, SPACESIZE)
    REDTOKENIMG = pygame.image.load('4row_red.png')
    REDTOKENIMG = pygame.transform.smoothscale(REDTOKENIMG, (SPACESIZE, SPACESIZE))
    BLACKTOKENIMG = pygame.image.load('4row_black.png')
    BLACKTOKENIMG = pygame.transform.smoothscale(BLACKTOKENIMG, (SPACESIZE, SPACESIZE))
    BOARDIMG = pygame.image.load('4row_board.png')
    BOARDIMG = pygame.transform.smoothscale(BOARDIMG, (SPACESIZE, SPACESIZE))

    HUMANWINNERIMG = pygame.image.load('4row_humanwinner.png')
    COMPUTERWINNERIMG = pygame.image.load('4row_computerwinner.png')
    TIEWINNERIMG = pygame.image.load('4row_tie.png')
    WINNERRECT = HUMANWINNERIMG.get_rect()
    WINNERRECT.center = (int(WINDOWWIDTH / 2), int(WINDOWHEIGHT / 2))

    ARROWIMG = pygame.image.load('4row_arrow.png')
    ARROWRECT = ARROWIMG.get_rect()
    ARROWRECT.left = REDPILERECT.right + 10
    ARROWRECT.centery = REDPILERECT.centery

    isFirstGame = True

    while True:
        runGame(isFirstGame)
        isFirstGame = False


def runGame(isFirstGame):
    if isFirstGame:
        # Let the computer go first on the first game, so the player
        # can see how the tokens are dragged from the token piles.
        turn = COMPUTER
        showHelp = True
    else:
        # Randomly choose who goes first.
        if random.randint(0, 1) == 0:
            turn = COMPUTER
        else:
            turn = HUMAN
        showHelp = False

    # Set up a blank board data structure.
    mainBoard = getNewBoard()

    while True: # main game loop
        if turn == HUMAN:
            # Human player's turn.
            getHumanMove(mainBoard, showHelp)
            if showHelp:
                # turn off help arrow after the first move
                showHelp = False
            if isWinner(mainBoard, RED):
                winnerImg = HUMANWINNERIMG
                break
            turn = COMPUTER # switch to other player's turn
        else:
            # Computer player's turn.
            column = getComputerMove(mainBoard)
            animateComputerMoving(mainBoard, column)
            makeMove(mainBoard, BLACK, column)
            if isWinner(mainBoard, BLACK):
                winnerImg = COMPUTERWINNERIMG
                break
            turn = HUMAN # switch to other player's turn

        if isBoardFull(mainBoard):
            # A completely filled board means it's a tie.
            winnerImg = TIEWINNERIMG
            break

    while True:
        # Keep looping until player clicks the mouse or quits.
        drawBoard(mainBoard)
        DISPLAYSURF.blit(winnerImg, WINNERRECT)
        pygame.display.update()
        FPSCLOCK.tick()
        for event in pygame.event.get(): # event handling loop
            if event.type == QUIT or (event.type == KEYUP and event.key == K_ESCAPE):
                pygame.quit()
                sys.exit()
            elif event.type == MOUSEBUTTONUP:
                return


def makeMove(board, player, column):
    lowest = getLowestEmptySpace(board, column)
    if lowest != -1:
        board[column][lowest] = player


def drawBoard(board, extraToken=None):
    DISPLAYSURF.fill(BGCOLOR)

    # draw tokens
    spaceRect = pygame.Rect(0, 0, SPACESIZE, SPACESIZE)
    for x in range(BOARDWIDTH):
        for y in range(BOARDHEIGHT):
            spaceRect.topleft = (XMARGIN + (x * SPACESIZE), YMARGIN + (y * SPACESIZE))
            if board[x][y] == RED:
                DISPLAYSURF.blit(REDTOKENIMG, spaceRect)
            elif board[x][y] == BLACK:
                DISPLAYSURF.blit(BLACKTOKENIMG, spaceRect)

    # draw the extra token
    if extraToken != None:
        if extraToken['color'] == RED:
            DISPLAYSURF.blit(REDTOKENIMG, (extraToken['x'], extraToken['y'], SPACESIZE, SPACESIZE))
        elif extraToken['color'] == BLACK:
            DISPLAYSURF.blit(BLACKTOKENIMG, (extraToken['x'], extraToken['y'], SPACESIZE, SPACESIZE))

    # draw board over the tokens
    for x in range(BOARDWIDTH):
        for y in range(BOARDHEIGHT):
            spaceRect.topleft = (XMARGIN + (x * SPACESIZE), YMARGIN + (y * SPACESIZE))
            DISPLAYSURF.blit(BOARDIMG, spaceRect)

    # draw the red and black tokens off to the side
    DISPLAYSURF.blit(REDTOKENIMG, REDPILERECT) # red on the left
    DISPLAYSURF.blit(BLACKTOKENIMG, BLACKPILERECT) # black on the right


def getNewBoard():
    board = []
    for x in range(BOARDWIDTH):
        board.append([EMPTY] * BOARDHEIGHT)
    return board


def getHumanMove(board, isFirstMove):
    draggingToken = False
    tokenx, tokeny = None, None
    while True:
        for event in pygame.event.get(): # event handling loop
            if event.type == QUIT:
                pygame.quit()
                sys.exit()
            elif event.type == MOUSEBUTTONDOWN and not draggingToken and REDPILERECT.collidepoint(event.pos):
                # start of dragging on red token pile.
                draggingToken = True
                tokenx, tokeny = event.pos
            elif event.type == MOUSEMOTION and draggingToken:
                # update the position of the red token being dragged
                tokenx, tokeny = event.pos
            elif event.type == MOUSEBUTTONUP and draggingToken:
                # let go of the token being dragged
                if tokeny < YMARGIN and tokenx > XMARGIN and tokenx < WINDOWWIDTH - XMARGIN:
                    # let go at the top of the screen.
                    column = int((tokenx - XMARGIN) / SPACESIZE)
                    if isValidMove(board, column):
                        animateDroppingToken(board, column, RED)
                        board[column][getLowestEmptySpace(board, column)] = RED
                        drawBoard(board)
                        pygame.display.update()
                        return
                tokenx, tokeny = None, None
                draggingToken = False
        if tokenx != None and tokeny != None:
            drawBoard(board, {'x':tokenx - int(SPACESIZE / 2), 'y':tokeny - int(SPACESIZE / 2), 'color':RED})
        else:
            drawBoard(board)

        if isFirstMove:
            # Show the help arrow for the player's first move.
            DISPLAYSURF.blit(ARROWIMG, ARROWRECT)

        pygame.display.update()
        FPSCLOCK.tick()


def animateDroppingToken(board, column, color):
    x = XMARGIN + column * SPACESIZE
    y = YMARGIN - SPACESIZE
    dropSpeed = 1.0

    lowestEmptySpace = getLowestEmptySpace(board, column)

    while True:
        y += int(dropSpeed)
        dropSpeed += 0.5
        if int((y - YMARGIN) / SPACESIZE) >= lowestEmptySpace:
            return
        drawBoard(board, {'x':x, 'y':y, 'color':color})
        pygame.display.update()
        FPSCLOCK.tick()


def animateComputerMoving(board, column):
    x = BLACKPILERECT.left
    y = BLACKPILERECT.top
    speed = 1.0
    # moving the black tile up
    while y > (YMARGIN - SPACESIZE):
        y -= int(speed)
        speed += 0.5
        drawBoard(board, {'x':x, 'y':y, 'color':BLACK})
        pygame.display.update()
        FPSCLOCK.tick()
    # moving the black tile over
    y = YMARGIN - SPACESIZE
    speed = 1.0
    while x > (XMARGIN + column * SPACESIZE):
        x -= int(speed)
        speed += 0.5
        drawBoard(board, {'x':x, 'y':y, 'color':BLACK})
        pygame.display.update()
        FPSCLOCK.tick()
    # dropping the black tile
    animateDroppingToken(board, column, BLACK)


def getComputerMove(board):
    potentialMoves = getPotentialMoves(board, BLACK, DIFFICULTY)
    # get the best fitness from the potential moves
    bestMoveFitness = -1
    for i in range(BOARDWIDTH):
        if potentialMoves[i] > bestMoveFitness and isValidMove(board, i):
            bestMoveFitness = potentialMoves[i]
    # find all potential moves that have this best fitness
    bestMoves = []
    for i in range(len(potentialMoves)):
        if potentialMoves[i] == bestMoveFitness and isValidMove(board, i):
            bestMoves.append(i)
    return random.choice(bestMoves)


def getPotentialMoves(board, tile, lookAhead):
    if lookAhead == 0 or isBoardFull(board):
        return [0] * BOARDWIDTH

    if tile == RED:
        enemyTile = BLACK
    else:
        enemyTile = RED

    # Figure out the best move to make.
    potentialMoves = [0] * BOARDWIDTH
    for firstMove in range(BOARDWIDTH):
        dupeBoard = copy.deepcopy(board)
        if not isValidMove(dupeBoard, firstMove):
            continue
        makeMove(dupeBoard, tile, firstMove)
        if isWinner(dupeBoard, tile):
            # a winning move automatically gets a perfect fitness
            potentialMoves[firstMove] = 1
            break # don't bother calculating other moves
        else:
            # do other player's counter moves and determine best one
            if isBoardFull(dupeBoard):
                potentialMoves[firstMove] = 0
            else:
                for counterMove in range(BOARDWIDTH):
                    dupeBoard2 = copy.deepcopy(dupeBoard)
                    if not isValidMove(dupeBoard2, counterMove):
                        continue
                    makeMove(dupeBoard2, enemyTile, counterMove)
                    if isWinner(dupeBoard2, enemyTile):
                        # a losing move automatically gets the worst fitness
                        potentialMoves[firstMove] = -1
                        break
                    else:
                        # do the recursive call to getPotentialMoves()
                        results = getPotentialMoves(dupeBoard2, tile, lookAhead - 1)
                        potentialMoves[firstMove] += (sum(results) / BOARDWIDTH) / BOARDWIDTH
    return potentialMoves


def getLowestEmptySpace(board, column):
    # Return the row number of the lowest empty row in the given column.
    for y in range(BOARDHEIGHT-1, -1, -1):
        if board[column][y] == EMPTY:
            return y
    return -1


def isValidMove(board, column):
    # Returns True if there is an empty space in the given column.
    # Otherwise returns False.
    if column < 0 or column >= (BOARDWIDTH) or board[column][0] != EMPTY:
        return False
    return True


def isBoardFull(board):
    # Returns True if there are no empty spaces anywhere on the board.
    for x in range(BOARDWIDTH):
        for y in range(BOARDHEIGHT):
            if board[x][y] == EMPTY:
                return False
    return True


def isWinner(board, tile):
    # check horizontal spaces
    for x in range(BOARDWIDTH - 3):
        for y in range(BOARDHEIGHT):
            if board[x][y] == tile and board[x+1][y] == tile and board[x+2][y] == tile and board[x+3][y] == tile:
                return True
    # check vertical spaces
    for x in range(BOARDWIDTH):
        for y in range(BOARDHEIGHT - 3):
            if board[x][y] == tile and board[x][y+1] == tile and board[x][y+2] == tile and board[x][y+3] == tile:
                return True
    # check / diagonal spaces
    for x in range(BOARDWIDTH - 3):
        for y in range(3, BOARDHEIGHT):
            if board[x][y] == tile and board[x+1][y-1] == tile and board[x+2][y-2] == tile and board[x+3][y-3] == tile:
                return True
    # check \ diagonal spaces
    for x in range(BOARDWIDTH - 3):
        for y in range(BOARDHEIGHT - 3):
            if board[x][y] == tile and board[x+1][y+1] == tile and board[x+2][y+2] == tile and board[x+3][y+3] == tile:
                return True
    return False


if __name__ == '__main__':
    main()