local function checkFuel( )
    write("Checking fuel level...")
    if turtle.getFuelLevel() < 20 then
        print("need refuel.")
        for i=1,16 do
            turtle.select(i)
            if turtle.refuel(turtle.getItemCount(i)) then
                write("Fueled from slot ")
                print(i)
            end

            if turtle.getFuelLevel() >= 500 then
                return
            end

        end
    else
        print("fuel status is ok.")
    end
end

local function goToGround( )
    write("goToGround...")
    while turtle.detectDown() == false do
        turtle.down()
    end
    print("ok")
end

local function makeCorridoor( length )
    goToGround()
    write("makeCorridoor")

    for i=0,length do
        write(".");
        turtle.dig()
        turtle.down()
        turtle.dig()
        turtle.up()
        turtle.forward()
    end
    print("done")
end

local function makeCorridoorReturn( length )
    makeCorridoor(length)

    for i=0,length do
        turtle.back()
    end
end

local function makeMatrix( rows, cols )
    local coord, size
    coord = {}
    size  = {}

    coord.x = 0
    coord.y = 0

    size.x  = 2 * rows + 1
    size.y  = 2 * cols + 1

    print("Cols...")
    for i=1,cols,1 do
        checkFuel()
        write("i=")
        print(i)
        makeCorridoorReturn( size.x )

        turtle.turnRight()
        makeCorridoor(3)
        turtle.turnLeft()
    end

    turtle.turnRight()
    turtle.turnRight()

    for i=0,size.y,1 do
        turtle.forward()
    end

    turtle.turnRight()

    print("Rows...")
    for i=1,rows,1 do
        checkFuel()
        write("i=")
        print(i)
        makeCorridoorReturn( size.y )

        turtle.turnLeft()
        makeCorridoor(3)
        turtle.turnRight()
    end
end

makeMatrix( 6, 6 )