circle.lua

print(turtle.refuel())

maxy = 1
radius = 3
intRadius = math.floor(radius)
n = 2*(intRadius+1) + 1
pixels = {}

function init()
    turtle.up()
    for i=1,intRadius+1 do
        turtle.forward()
    end
    turtle.turnLeft()
end

init()

function setpixel(x, y)
    local tx = math.floor(n/2)+x
    local ty = math.floor(n/2)+y

    pixels[ty+1][tx+1] = 'x'
end

for y=1,n do
    pixels[y] = {}
    for x=1,n do
        pixels[y][x] = '.'
    end
end

local l = math.floor(radius * math.cos(math.pi/4))

for x=0,l do
    y = math.floor(math.sqrt(radius*radius - x*x))
    setpixel(x, y)
    setpixel(-x, y)
    setpixel(x, -y)
    setpixel(-x, -y)
    setpixel(y, x)
    setpixel(-y, x)
    setpixel(y, -x)
    setpixel(-y, -x)
end

for y=1,#pixels do
    for x=1,#pixels do
        write(pixels[y][x])
    end
    print()
end

posx, posy = intRadius+2, n
vx, vy = -1, 0

function left()
    return vy*-1, vx
end

function right()
    return vy, vx*-1
end

function turnLeft()
    vx, vy = left()
    turtle.turnLeft()
end

function turnRight()
    vx, vy = right()
    turtle.turnRight()
end

function pixelLeft()
    vvx, vvy = left()
    xx, yy = posx+vvx, posy+vvy
    print('checking left: ', xx, yy)
    if (xx < 1) or (xx >= n) then
        return false
    end
    if (yy < 1) or (yy >= n) then
        return false
    end
    return pixels[yy][xx] == 'x'
end

function pixelForward()
    xx, yy = posx+vx, posy+vy
    if (xx < 1) or (xx >= n) then
        return false
    end
    if (yy < 1) or (yy >= n) then
        return false
    end
    return pixels[yy][xx] == 'x'
end

function currentSlotEmpty()
    return turtle.getItemCount(turtle.getSelectedSlot())==0
end

function reSelect()
    for i=1,16 do
        if turtle.getItemCount(i) > 0 then
            turtle.select(i)
            return
        end
    end
    -- exit the program early
    error()
end

function moveAndPlace()
    posx, posy = posx+vx, posy+vy
    turtle.forward()
    if currentSlotEmpty() then
        reSelect()
    end
    turtle.placeDown()
end

function mainLoop()
    print(posx, posy)
    if pixelLeft() then
        print('pixel left!')
        if pixelForward() then
            print('pixel forward!')
            turnRight()
        end
    else
        turnLeft()
    end
    moveAndPlace()
end

currenty = 0
repeat
    -- one circle in xz plane
    repeat
        mainLoop()
    until (posx==intRadius+2) and (posy==n)
    currenty = currenty+1
    turtle.up()
    print('one more layer finished!')
until currenty==maxy
print('done! :)')