function _init()
    frame = 0

    -- Set up userdata so we can draw the screen to itself
    -- by using memcpy() to a memmapped region of memory.
    -- The userdata is now the contents of the previous frame,
    -- and it is now possible to modify the current frame.
    -- We can also call spr() with userdata
    -- to draw the previous frame to the current one

    screen = userdata("u8", 480, 270)
    memmap(0x30000, screen)
end

function set_col_table(new, current, col)
    poke(0x8000 + 64*new + current, col)
end

function _draw()
    frame += 1

    if frame == 1 then
        -- Randomise the screen for the first frame.
        for y=0,269 do
            for x=0,479 do
                if (rnd() < 0.2) pset(x, y, 7)
            end
        end

        -- Copy the current screen to the buffer, just for the first frame
        memcpy(0x30000, 0x10000, 0x20000)

    end

    --if (frame % 32 > 0) return

    cls()

    -- White cells drawn onto colour 0 will set the colour to 1, white onto 1 will be 2
    -- and so on. This counts the number of neighbouring cells very quickly
    for i=0, 7 do
        set_col_table(7, i, i+1)
    end

    -- Draw the screen 8 times in a ring, for each neighbour. The colour tables
    -- do the counting
    for y=-1,1 do
        for x=-1,1 do
            if (x!=0 or y!=0) spr(screen, x, y)
        end
    end

    -- Set up colour tables to turn the "counted" screen into the next frame
    -- Set every colour to black except for drawing black onto 3 (brought back alive)
    -- and 2 or 3 neighbours for alive (stay alive)
    for i=0, 9 do
        set_col_table(0, i, 0)
        set_col_table(7, i, 0)
    end
    set_col_table(0, 3, 7)
    set_col_table(7, 2, 7)
    set_col_table(7, 3, 7)

    -- Draw the screen to the "counted" version, with the rules set above
    spr(screen, 0, 0)

    -- Reset the draw state to make sure we have a predictable next frame
    reset()

    -- Copy the screen to the buffer for the next frame,
    -- before we pollute it with the FPS counter
    memcpy(0x30000, 0x10000, 0x20000)

    -- FPS counter
    if key("x") then
        rectfill(0, 0, 44, 8, 0)
        print("FPS: "..stat(7), 1, 1, 8)
    end

end