# Macintosh Tiny

Back while I was preparing for /dev/world this year, I had split my attention in a few variations on a theme:

• Fixing original Macintosh hardware (the Macintosh Plus);
• Emulating Macintoshes on modern machines;
• Constructing a Raspberry Pi case that looks sort of like a scaled-down Mac.

Since I ran myself out of time at /dev/world to talk about “Macintosh Tiny”, that’s what this entry is about.

## The goal

The goal was to shove a Raspberry Pi, a small LCD panel, and maybe an amp and speaker, into a case that looks sort of like a miniature Macintosh Classic, and then run an emulator on it. Peripherals of the era, namely, floppy disk drives, hard disks, SCSI ports, serial ports, ADB ports, and so on, are not part of the project. While everything that could run on 5 volts could easily be battery powered, that was also not a consideration.

Other than the Raspberry Pi 3, I found a couple of cheap 5-inch LCDs (800x480 resolution, unnecessary resistive touch feature). The HDMI and micro USB connectors for these panels sit on one of the long edges - it’s designed to mount the Raspberry Pi directly, consuming the GPIO pins and 5V from the Pi, and get HDMI over a small nub of a circuit board which has two HDMI connectors back to back.

However, my case idea called for the Raspberry Pi to go in the bottom of the case so the USB and Ethernet ports could be exposed (analogous to where the logic board and ports are situated on a real Macintosh). This meant one thing I was worried about early on in the process was how I was going to cram a whole bulky HDMI cable and power for the screen into the front of the case, especially if the display’s natural rotation had the connectors coming out the top (Narrator: it was), because then the “head” of the Tiny would have to be made unnaturally tall to hide the cable connectors. Turns out, money solves all known problems, and later on I ordered some 15cm-long flat-ribbon HDMI cables with right-angle connectors on both ends.

I started by thinking I’d 3D print something, so began messing around in OpenSCAD. I feel like I “get” the program-like, constructive solid geometry style of OpenSCAD. However, I’ve never 3D printed anything before in my life, nor have I successfully designed the mini Mac case, so started thinking about ways to shortcut the process.

## The case: balsa wood

Someone suggested balsa wood. So I went and got some. 100% Australian plantation-grown balsa wood, and a bottle of PVC glue, from Lincraft.

Benefits of using balsa:

• It’s lightweight;
• Soft and easy to work with, using a regular blade and cutting mat;
• Not very expensive;
• Nails go in very easily;
• No need to wait for a 3D printer to print;
• Various glues stick easily, including PVC and hot glue, and
• Comes in a variety of cross-sections (flat, square, circular).

Downsides are:

• Due to how easy it is to work with, it can be easy to break;
• It’s a reasonable insulator of heat, which isn’t great for electronics;
• It burns easily, and
• I cannot saw straight to save myself.

To start making a case out of balsa, I was inspired by the Macintosh construction: the computer internals are bolted to the front case, and the back shell then slides on to cover it all up. So for the first two attempts, I started with a sheet of balsa, put the display face-down on top, and then tried to assemble a case upwards.

This approach didn’t work very well. For one thing, the 5 inch display is relatively heavy compared with the Raspberry Pi, so when the proto-Tiny was upright it was unbalanced and liked to topple onto its front again.

The third attempt at constructing a case was successful, probably because I started from the bottom instead of the front face, and used longer pieces of balsa that I cut from a square rod. This lowered the centre of gravity to avoid toppling the Tiny. It also looked more like a scaled down Macintosh at this point, because the slope of the top could be judged more easily.

At this stage I had the display locked into the balsa wood case, the Raspberry Pi mounted on the bottom, and the Pi could be powered via a USB cable coming out the back. The display was then powered via some breadboarding cables from the Pi. This mostly worked, but had some low power issues.

One problem with making a scaled down Macintosh and preserving the angles is the display: with my cheap panel, which didn’t have a great vertical viewing angle, the unit is only really usable at eye level. I fixed this after finishing the audio system by giving it some balsa wood “feet” at the front, making it look a little less like a Classic and more like a Colour Classic.

## The power supply

Once I decided to have inbuilt sound, and thus an amplifier which used more than 5 volts, the power situation became more complex - I didn’t want to run off a regular USB power source any more. The solution was to take an input of e.g. 12 volts, and feed it into a combination of a 9V regulator for the amplifiers, and a step-down converter for the Pi and display, providing an ample 3 amps (badum-tish).

## The audio

Original Macintoshes have an inbuilt speaker and line out. I didn’t much care for the line out but thought an inbuilt speaker would be a nice touch. Ultimately I ended up with stereo, mounting the speakers with glue onto some holder balsa which I then glued to the side panels. Given the emulator was outputting mono audio anyway, the second speaker is overkill, but a nice touch.

There are some major issues with the Raspberry Pi as an audio source:

• The Pi uses “PWM out” for the digital-analog conversion, which generally sounds kind of bad. Not much I can do about this issue, but it’s good enough for an emulated old computer.
• The Pi likes to detect the HDMI screen as an audio output. This behaviour can at least be tamed with the /boot/config.txt.
• The Pi cannot drive low-impedance speakers directly for very long, and even if you force it to, it sounds crap (ask me how I know). The sound chip shuts down when overloaded (good). You can’t turn the volume back up with alsamixer when this happens.
• If you use standard earbuds, the sound from a Pi is generally OK, but if you use an amplifier on the same power supply there are two huge sources of noise which overwhelm the signal: power supply noise, and ground loop noise.

I found that a 7809 regulator wasn’t enough to reject the power supply noise, and ended up making an additional C-L-C filter to remove power supply noise from the Pi and display. I wanted to remove noise down to at least 100Hz (possibly overkill), so using this old formula:

$$f = \frac{1}{2\pi\sqrt{LC}}$$

what I wanted was something like ~47µF capacitors (easy to obtain, even as tantalum caps) and 20mH of inductance. I figured the best way of producing that much inductance was to get a spool of copper wire, a toroidal ferrite core, and wind a coil myself. After wasting a few meters of wire in frustration, Hannah suggested winding it onto a pen, and then feeding the pen through the toroidal core instead of wrangling lots of loose wire. Because the ferrite and wire is kind of heavy, the centre of gravity is lowered even further depending on placement.

To get rid of the ground loop noise, I did some quick research on how ground loop isolators are implemented. The easiest seemd to be with coupling transformers, and Jaycar sold those too. Jaycar also sell pre-made isolators with 3.5mm connectors in a case, which are only a few extra dollars compared to the transformers alone.

Actually, Jaycar stocked all the electronics I ended up using for the audio system, including two cheap 1 watt amplifier kits. A single one of these and one speaker would have been fine, as would a stereo amp, but these boards are tiny. The only downside is demanding more than 5 volts, which meant the power supply was slightly more complicated.

The full-range speakers I shelled out for were pricier than a “regular” 8-ohm hobby speaker, but more compact (36mm on each edge) and pack a decent punch across the frequency spectrum.

# /dev/world 2017

10th anniversary of /dev/world!

Here’s the links from my talk, Inside Macintosh.

And here’s some source code for a game that runs on 68k Macs and emulators.

# GovHack 2017

Nice.

GovHack is wrapped up for 2017. It was finished on the weekend, but Monday and Tuesday were a waste - I have been too miserable and sick to blog about it, and only just got a burst of energy to break through the mehffort. Our entry, Death Who? (Colonial Edition), is a virtual card game based upon real lives recorded in Tasmanian historical records. I wanted to share what I’ve learned about making a super hacky multiplayer game backend from scratch in 46 hours, in case you want to try the same thing.

The past few years I have flown down to Hobart to team up with old friends. As usual, I stated up-front that I wanted no share of any prizes we might win. I’m literally in it for the fun of hanging out with my friends.

The mix was a bit different this year. Our team of 8 consisted of me, Hannah, Paris, Mars, Jon, Tim, Seb, and Shea. Our new team members Hannah, Mars, and Shea all brought their own different strengths this year; Hannah and Shea worked on a web-based component, which is unusual for our team. Previous regular Rex went and did a solo entry which you need to check out.

Similar to past years, I made a backend for the game in Go. Each time this has been a server of some kind, depending on what is needed. There are a few preliminaries that can be done while the team settles on a direction, but it’s often better to have the decisions settled on Friday night, get a good night’s sleep, and then get cracking on Saturday morning. This time, we did the latter.

### Constraints

• Serve a game based on some government-provided data.
• Work with multiple players.
• SLO: It has to work for demonstration purposes (what might be called “best effort” SLO, but I have a whole other rant about that).
• No need to worry about bandwidth, CPU, memory, disk usage, or any of that as long as it works for demo purposes.
• Players can cheat and hack the server as much as they like - demo purposes, see above.

As such, the server doesn’t have to be Google-class production-grade stuff. It just has to work for 30 minutes. Nevertheless, it’s relatively easy to do the right things in Go. I think it’s still running even now…

### Hello, GovHack

The designers of Go had web servers in mind, so adding one feels natural. I generally jump straight to a skeleton main.go in a server directory in the GitHub repo without thinking much more about it:

package main

import (
"flag"
"fmt"
"log"
"net/http"
)

var httpPort = flag.Int("http_port", 23480, "Port the webserver listens on")

func main() {
flag.Parse()

// Set up HTTP handlers
http.HandleFunc("/helloz", func(w http.ResponseWriter, r *http.Request) {
fmt.Fprint(w, "Hello, GovHack 2017!\n")
})

// Start listening on HTTP port; block.
if err := http.ListenAndServe(fmt.Sprintf(":%d", *httpPort), nil); err != nil {
log.Fatalf("Couldn't serve HTTP: %v", err)
}
}


If you look carefully through our repo history, you’ll notice that flag.Parse call was missing until Sunday. I frequently forget the basics!

The beauty of running a web server is that it lets you (1) provide a way of checking that the server is still running, and (2) provide a way of inspecting the game state at any point without logging—just leave a browser tab open on localhost:23480/something and refresh when you want to take a peek. So let’s set up a handler for that:

	http.HandleFunc("/statusz", func(w http.ResponseWriter, r *http.Request) {
// TODO: write the game state to w
})


If you’re wondering about the z at then end of /hello and /status: I really can’t help putting it there. It’s a Google-ism.

### State

It’s best to agree upon the basic structure of the state-space with the people making the client before going any further. In this case, we have a multiplayer card game with two types of cards. We whiteboarded some states:

• Lobby: the players are joining the game, the game hasn’t “started”
• When someone pushes the “start” button, transition to…
• In game: the game is underway
• Players take turns. Once one player plays, go to the next player.
• Once all players have had a turn, begin the next round.
• Once all rounds have been played, transition to…
• Game over: the winner is proclaimed.
• The game can be returned to the lobby state by pushing a button.

Start setting up some types to track the state. I like to do this in a separate package, but there’s no compelling reason to be so neat in a jam/hackathon.

package game

// Statum is some fake latin I made up
type Statum int

const (
StateLobby Statum = iota
StateInGame
StateGameOver
)

type State struct {
State     Statum          json:"state"
Players   map[int]*Player json:"players"
WhoseTurn int             json:"whose_turn"
Clock     int             json:"clock"
}

type Player struct {
// ... snip ...
Score       int     json:"score"
}


While it’s no big deal in Go to handle sending/receiving different JSON types nested inside some kind of genericised “message” type by adding MarshalJSON/UnmarshalJSON methods, to make it easier for clients I recommend avoiding doing that. In this case some parts of the state only have meaning depending on other parts of the state (e.g. WhoseTurn and Clock only mean something when State == StateInGame). To make it even easier we are also sending the entire game state to the client—we don’t care about cheating (it’s a hackathon).

Since this is a multiplayer game, be sure to smother state mutations in mutexes. sync.RWMutex is great and lets you get better performance in some cases but a sync.Mutex would be fine (it’s a hackathon). It would be possible to use the new sync.Map here, but we have to guard more than just the map from concurrent access and I like concrete types, so embedding a mutex makes the most sense.

type State struct {
//...snip...

// Non-exported fields for bookkeeping
mu     sync.RWMutex
nextID int
}

func New() *State {
return &State{
Players: make(map[int]*Players),
}
}

func (s *State) AddPlayer() int {
s.Lock()
defer s.Unlock()
id := s.nextID
s.Players[id] = new(Player)
s.nextID++
return id
}

func (s *State) RemovePlayer(id int) {
s.Lock()
defer s.Unlock()
delete(s.Players, id)
}


Player is defined in a way that its “zero value” is a sensible default. State is almost but not quite as simple, hence the New (it contains a map which we’d like to just use, but nil maps don’t work that way). It starts in the lobby state and supports arbitrarily adding and removing players. You could use a slice for storing players but then you will either have to handle nil “holes” in the slice, or fiddly logic with reslicing. Just use a map (it’s a hackathon). I might use a slice with nils next time.

### Serving a game

The next step needs you to settle on the communication between the game client and the server. We chose what seemed like the easy thing which was to send JSON messages over TCP. Typical “serious” game servers often implement a compact binary protocol over UDP. It’s straightforward to do JSON and TCP in Go, and you can see how I did it this time in server.go. The example TCP listener in the net package documentation is a nice starting point. However, some notes.

Firstly, an infinite loop will… infinitely loop, blocking its goroutine indefinitely. Since this binary is also running a web server, one of the two has to be executed in a new goroutine. (Best practice is to have the goroutine created only in the main func so it’s obvious what its lifetime is, but this is a hackathon.)

Secondly, unless the server is synchronous (in the sense that there are no messages sent from the server that aren’t in response to something from the client), you need 2 goroutines per connection: one for receiving and one for sending. One client can affect all the other clients, so this was needed. We planned for the server to just spam the clients with state objects as it pleases.

Thirdly, it is very important that goroutine leaks are avoided: they might be lightweight but they consume memory and CPU cycles, after all. Contexts are great for this, especially in larger server projects. Here they serve the purpose of keeping the sending and receiving goroutines organised. When the context is cancelled, the connection can be closed and both goroutines can end. Additionally, the context can hold some per-player state. For a while I was using the context to hold the player ID, but instead went for an explicit parameter. Instead of a cancellable context, it is pretty much equivalent to give it a “quit” channel that gets closed for cancellation, but I use contexts all the time at work and didn’t bother to think about it much (it’s a hackathon).

### Notifying all the clients

The goroutine handling outbound data (the imaginatively-named handleOutbound) is notified by a channel closing when it is time to transmit the game state, but this bears a little closer examination since there’s a great time-saving upside to this: there is no need to implement a registry of things to send notifications to, Go can handle it.

Firstly, remember that all reads on a closed unbuffered channel finish straight away and get the zero value. The outbound handler is an infinite loop around a select waiting on that channel or on the context. Here’s the part in State:

type State struct {
// ... snip ...
changedNote chan struct{}
}

func (s *State) Changed() <-chan struct{} {
s.RLock()
defer s.RUnlock()
return s.changedNote
}

func (s *State) notify() {
close(s.changedNote)
s.changedNote = make(chan struct{})
}


Every time Changed is called it returns a channel whose sole purpose in life is to be closed in the future by notify. When the state changes (and it should only be changed by methods that do the correct locking), notify is called, closing the current channel and replacing it with a new one. (Calls to notify need to be guarded by the mutex.) Anything that is interested in the state can then just call Changed, and proceed once the returned channel is closed.

However, it’s a hackathon - why not do something really cheap and use a timer to spam updates every second (or something?):

func (s *Server) handleOutbound(conn net.Conn) {
for range time.Tick(time.Second) {
s.state.Dump(conn)
}
}


This is fine, as long as steps are taken to avoid the goroutine leak (hint: it should end when the context is done, which means selecting on both the ticker and <-ctx.Done()). But it didn’t occur to me to do this at the time. Using channel-closing as a snappy notification system for arbitrarily many clients is a technique I’ve used a lot, and feels very natural in Go.

One thing that’s important for development speed is to give the client developer (Jon) a simple message to send the server that does nothing, to ensure communication works from the client side without much effort. Here’s Action:

type Act int

const (
ActNoOp      Act = iota
ActStartGame
ActPlayCard
ActReturnToLobby
)

type Action struct {
Act  Act json:"act"
Card int json:"card"
}


The zero value for Action has Act = ActNoOp, so, sending the empty JSON object {} works as a no-op message. This also helps manually testing the server: you can netcat/telnet into it and manually enter {} (or real actions as JSON).

### Unit testing

Test what it does, not how it does it.

It’s a hackathon: if you don’t have time, don’t bother with unit tests, and just test manually. However I can hardly live without at least one or two unit tests. By writing a unit test against your actual API you force yourself to understand some implications of the API design.

Go doesn’t come with a mocking framework. You don’t need one. Run and test the actual server:

func TestGame(t *testing.T) {
s := server{}
r := &response{}
if err := s.listenAndServe("localhost:0"); err != nil {
t.Fatalf("Couldn't start: %v", err)
}
defer s.Close()

// Connect player 0
if err != nil {
t.Fatalf("Couldn't connect: %v", err)
}
defer conn0.Close()

send0 := json.NewEncoder(conn0)
recv0 := json.NewDecoder(conn0)

// ... snip...

// Play a game!
for i, p := range actions {
// Send an action as one of the players.
// Check each player receives the state.
// Check the state against the desired state.
}
}


All that really has to be faked is the card deck: you don’t want a flaky test because your virtual players got dud hands. So the game state uses a deck that you give it satisfying a Deck interface, and it has two implementations: the real deck which can be Shuffled, and a fake RiggedDeck which is the same but calling Shuffle has no effect.

Adding the game logic isn’t all that interesting: with the above in place, it would be possible to make it into almost any kind of multiplayer game (with tweaks). The biggest tweak would be reducing the outbound data to only partial state updates, but that adds unnecessary complexity (it’s a hackathon).

Implementing the rules of the game is a lot like careful state bookkeeping. Having clear delineation of states helps a lot. There are explicit and implicit actions, e.g. player 1 plays card 3, versus player 2 has disconnected. It is important that concurrent actions don’t corrupt one another - use mutexes and the race detector (go test -race ...). It’s also important to think about what states could be “black holes” (often related to implicit actions). For example, if someone disconnects during a game, then the game shouldn’t wait for them to play. Or another example: if a player successfully connecting requires the game to be in the lobby state, and all the players disconnect, then the state should reset to the lobby state so people can rejoin.

Fortunately it’s a hackathon, so it’s allowed to have bugs galore, but I don’t think there are many. 😜

The game is data-driven (a bunch of historical data is churned into game cards). Data wrangling was done by Seb and Tim. We agreed early on that the data should be a JSON-formatted array of objects, which is not hard for them to encode and the server to decode. Loading the data is straightforward (hello my old friend json.NewDecoder) but the magic is in creating the cards out of them. (A few loops though.)

### Conclusion

It’s a lot of fun working with such talented people in a hackathon environment. Making a good game server involves a mix of planning, technique, experience, design, teamwork, and communication. I’m once again looking forward to GovHack next year!