Ajuda em um jogo da velha no LuaBR

jogoDaVelha.lua

board =
{
    {1,2,3},
    {4,5,6},
    {7,8,9},
}


function play(piece)
    local pos = tonumber(io.read())
    while not isFree(board, pos) do
        print("Posicao ocupada. Tente outra:")
        pos = tonumber(io.read())
    end
    local i, j = getIndices(pos)
    board[i][j] = piece
end


function isPiece(piece)
    return piece == "O" or piece == "X"
end


function isFree(board, pos)
    local i, j = getIndices(pos)
    return not isPiece(board[i][j])
end


function getIndices(pos)
    local i = math.floor((pos-1)/3) + 1
    local j = (pos-1) % 3 + 1
    return i, j
end


function win()
    return (board[1][1] == board[1][2] and board[1][2] == board[1][3]) or
           (board[2][1] == board[2][2] and board[2][2] == board[2][3]) or
           (board[3][1] == board[3][2] and board[3][2] == board[3][3]) or
           (board[1][1] == board[2][1] and board[2][1] == board[3][1]) or
           (board[1][2] == board[2][2] and board[2][2] == board[3][2]) or
           (board[1][3] == board[2][3] and board[2][3] == board[3][3]) or
           (board[1][1] == board[2][2] and board[2][2] == board[3][3]) or
           (board[1][3] == board[2][2] and board[2][2] == board[3][1])
end


function getFreePositions(board)
    local ret = {}

    for pos = 1, 9 do
        if isFree(board, pos) then
            table.insert(ret, pos)
        end
    end

    return ret
end


function cpuPlay(piece)
    local freePlaces = getFreePositions(board)
    local index = math.random(1, #freePlaces)
    local pos = freePlaces[index]
    print(pos)
    for i = 1,3 do
        for j = 1,3 do
            if pos == board[i][j] then
                board[i][j] = piece
            end
        end
    end
end


function getGameResult(board)
    local ret = "c" -- continue
    if win() then
        ret = "w" -- Há um ganhador
    elseif #getFreePositions(board) == 0 then
        ret = "v" -- Deu velha
    end
    return ret
end


function printGameResult(result)
    local results =
    {
        w = "Ganhou",
        v = "Deu velha",
    }
    print(results[result])

end


function printBoard(board)
    for i, line in ipairs(board) do
        print(line[1] .. "|" .. line[2] .. "|" .. line[3])
    end
    print("")
end


function runGame()
    printBoard(board)

    local gameResult = "c"
    while true do
        print("Sua vez:")
        play("X")
        printBoard(board)
        gameResult = getGameResult(board)
        if gameResult ~= "c" then
            break
        end

        print("CPU:")
        cpuPlay("O")
        printBoard(board)
        gameResult = getGameResult(board)
        if gameResult ~= "c" then
            break
        end
    end

    printGameResult(gameResult)
end


runGame()