function getResult(board)
    local tie = true
    for _, node in ipairs(board) do
        if node == " " then
            tie = false
            break
        end
    end
    if tie then
        return 3
    end
    return (
        (board[1] == board[2] and board[1] == board[3] and board[1] ~= " ") and (board[1] == "X" and 1 or 2) or
        (board[4] == board[5] and board[3] == board[6] and board[4] ~= " ") and (board[4] == "X" and 1 or 2) or
        (board[7] == board[8] and board[7] == board[9] and board[7] ~= " ") and (board[7] == "X" and 1 or 2) or
        (board[1] == board[4] and board[1] == board[7] and board[1] ~= " ") and (board[1] == "X" and 1 or 2) or
        (board[2] == board[5] and board[2] == board[8] and board[2] ~= " ") and (board[2] == "X" and 1 or 2) or
        (board[3] == board[6] and board[3] == board[9] and board[3] ~= " ") and (board[3] == "X" and 1 or 2) or
        (board[1] == board[5] and board[1] == board[9] and board[1] ~= " ") and (board[1] == "X" and 1 or 2) or
        (board[3] == board[5] and board[3] == board[7] and board[3] ~= " ") and (board[3] == "X" and 1 or 2) or
        0
    )
end

function drawBoard(board)
    print (
        board[1] .. " | " .. board[2] .. " | " .. board[3] .. "\n" ..
        board[4] .. " | " .. board[5] .. " | " .. board[6] .. "\n" ..
        board[7] .. " | " .. board[8] .. " | " .. board[9]
    )
end

function removeElement(array, element)
    for index, value in ipairs(array) do
        if value == element then
            table.remove(array, index)
            return
        end
    end
end

function makeEnemyMove(array)
    local options = {}
    for index, value in ipairs(array) do
        if value == " " then
            table.insert(options, index)
        end
    end
    if #options > 0 then
        array[options[math.random(#options)]] = "O"
    end
    return array
end

while true do
    local grid = {" ", " ", " ", " ", " ", " ", " ", " ", " "}
    local result;
    repeat
        drawBoard(grid)
        local choice = tonumber(io.read())
        if grid[choice] == " " then
            grid[choice] = "X"
        end
        grid = makeEnemyMove(grid)
        result = getResult(grid)
    until result > 0
    drawBoard(grid)
    print (
        result == 1 and "You win!" or
        result == 2 and "You lose!" or
        "It's a tie!"
    )
    io.write("Press enter to play again.\n")
    io.read()
end