Pixel Art

function refuel()
    while turtle.getFuelLevel() < 10 do
        print("Waiting for gas...")
        sleep(5)
        turtle.select(10)
        turtle.refuel(64)
    end
end

function placeBlock( slot )
    while turtle.getItemCount( slot ) < 1 do
        print("Waiting for blocks...")
        sleep(5)
    end
    turtle.select(slot)
    trutle.place()
end

local f = fs.open("test.txt","r")
backwards = false
while 1 do
    local line = f.readLine()
    if not line then
        break
    else
        if backwards then
            start = #line
            ending = 1
            step = -1
        else
            start = 1
            ending = #line
            step = 1
        end
        for i=start, ending, step do
            local c = string.sub(line,i,i)
            if c == "0" then
                turtle.back()
            elseif c == "1" then
                placeBlock(1)
                refuel()
                turtle.back()
            elseif c == "2" then
                placeBlock(2)
                refuel()
                turtle.back()
            elseif c == "3" then
                placeBlock(3)
                refuel()
                turtle.back()
            end
        end
        refuel()
        turtle.up()
        turtle.turnLeft()
        turtle.turnLeft()
        turtle.back()
        turtle.back()
        turtle.back()
        backwards = not backwards
    end
end