code · Getting Started · How-To

PICO-8 100 Doors Problem

▸ January 16, 2025 ▸ 3 min read ▸ Michael Doornbos

Introduction

I have a clear favorite in 10PRINT as my goto thing to implement whenever I get to try a new platform. Second favorite is the 100 Doors problem.

We’ve done it on many platforms, but my favorite is still the first one I did it on, the VIC-20.

PICO-8

We need to do a little more work than just poking onto the screen for this one, but it’s still easy to follow.

Slow down

One problem you’ll have is that a PICO-8 system on a modern compter can run this faster than you can see it. That’s cool if you just want the results, but we’re looking for a cool visual to follow along with.

In this version, we’re using PICO-8’s built-in game loop to naturally pace the animation.

Let me explain.

PICO-8 automatically runs the _update() function 30 times per second. Instead of using an explicit delay or sleep function, we’re leveraging this by processing just one door change per frame.

Here’s how it works:

function _update()
    -- These variables persist between frames
    if not pass then pass = 1 end
    if not door then door = pass end

    if pass <= 100 then
        -- Only handle ONE door each time _update() is called
        door_states[door] = not door_states[door]

        -- Draw just this one door
        local x = (door-1)%16*8 + 4
        local y = flr((door-1)/16)*8 + 4
        circfill(x, y, 3, door_states[door] and 12 or 7)

        -- Move to next door for the next frame
        door += pass

Each time _update() runs (30 times per second):

We only toggle and redraw ONE door
We save where we are (which pass and door) for the next frame
Then we wait until PICO-8 calls _update() again

If we wanted to slow it down even more, we could add a frame counter like this:

function _init()
    frame_count = 0
end

function _update()
    frame_count += 1
    if frame_count % 2 == 0 then  -- Only process a door every other frame
        -- rest of the code
    end
end

Make sense?

The code

function _init()
    -- Clear screen
    cls()

    -- Initialize door states table
    door_states = {}
    for i=1,100 do
        door_states[i] = false -- all doors start closed
        circfill((i-1)%16*8 + 4, flr((i-1)/16)*8 + 4, 3, 7)
    end
end

function _update()
    -- Static variable to keep track of progress
    if not pass then pass = 1 end
    if not door then door = pass end

    -- Process one door per frame
    if pass <= 100 then
        -- Toggle door state in our table
        door_states[door] = not door_states[door]

        -- Draw the circle
        local x = (door-1)%16*8 + 4
        local y = flr((door-1)/16)*8 + 4
        circfill(x, y, 3, door_states[door] and 12 or 7)

        -- Move to next door
        door += pass

        -- Move to next pass if we're done with current one
        if door > 100 then
            pass += 1
            if pass <= 100 then
                door = pass
            end
        end
    else
        -- Only print numbers once at the end
        print("open doors:", 0, 110, 7)
        local x = 0
        local y = 118
        for i=1,100 do
            if door_states[i] then
                print(i.." ", x, y, 7)
                x += 12
                if x > 120 then
                    x = 0
                    y += 6
                end
            end
        end
    end
end

function _draw()
end

How’d we do?

This is ALMOST as satisfying as the VIC-20 version ;-)