17 June, 2017

PRNG in goggles

I've been playing with software to run on my Adafruit Neopixel Goggles. The software gets to make pretty patterns on 32 x 24-bit RGB LEDs arranged in two rings.

One of the modes picks random colours to change each ring to. The Arduino stack supplies a random() function to do this, but it seemed to take up about 600 bytes of the limited 5kb flash program memory on the in-goggles microcontroller.

I wondered if I could make a smaller-but-less-random generator. After all, the colour pattern does not need to be cryptographically secure. I'm just trying to avoid getting the same sequence of colours every time.

Someone pointed me at Linear Feedback Shift Register PRNGs and I implemented one based around a description in Wikipedia. I chose the Galois one because pistols at dawn.

That seemed to work well for the basic generation of random numbers, but the Arduino always started up with the same seed, and so gave the same sequence of colours each time. Luckily, there are 512 bytes of EEPROM on this microcontroller and so I used two other those to generate and store a seed to be used next time.

Initially, I generated two random bytes at power-on, and stored those into the EEPROM. However, this rapidly proved to have a really short period: there were only two different patterns being displayed by the goggles in this mode!

So, next, which seems to work better, the code now initialises the PRNG from EEPROM, takes a single bit from it (and discards it) and then writes out the PRNG state into the EEPROM. That means that the start state advances by one bit every boot.

Code for the goggles is here on github: https://github.com/benclifford/goggles.

14 June, 2017

An Idris implementation of a reddit bot.

I wrote and host lsc-todaybot, a bot that moves the [TODAY] flair around on reddit.com/r/LondonSocialClub, a subreddit dedicated to social activities in London. I wrote it in Haskell, and over the last year or so have used it as a simple application for experimenting with different Haskell libraries and techniques. For example, I used it to learn about extensible effects (and gave a talk about it at the London Haskell meetup).

I've been interested in the dependently typed programming language Idris and someone claimed it was pacman-complete - that is, you can write Pacman in it. So rather than playing round with proofs and all that dependent-type gubbins, I set out to rewrite todaybot in Idris.

It's here: https://github.com/benclifford/idris-todaybot

It's not beautiful. But it works well enough to be running alongside the Haskell implementation.

I've littered the source with QUESTION/DISCUSSION blocks, but basically I had to patch up an existing JSON library, interface an HTTP library (I chose libcurl), and struggle with a whole new style of error message (possibly the hardest bit as I'm used to error messages really leading you to the right answer).

There's almost no use of dependent types, but I did find a straightforward place to make use of them, in the parameters to this libcurl function:

CURLcode curl_easy_setopt(CURL *handle, CURLoption option, parameter);

where the type of the supplied parameter depends on the particular option chosen: for example a pointer to a callback function, or a boolean verbosity level.

07 June, 2017

Fixing up my MSF radio clock

One of the first hardware things I did with a Raspberry Pi (sometime around March 2013) was interface it to a cheap 60kHz radio clock board, and write some driver software to interface it to ntpd. That was in one house, then in another, and then got dismantled, put in a box, and smashed up a bit. Based on the

I spent a few hours yesterday fixing it up: resoldering the antenna onto the circuit board, getting rid of the dodgy soldering and putting it on a prototype board, putting a capacitor across the power supply because I heard rumour that might make it receive better (it doesn't seem to), and implementing parity checking in the decoder.

It's still terribly awkward to position: I have it velcroed up on top of a curtain rail to try to get it high and away from all my other electronics and it is still very sensitive to antenna positioning; and it is still estimated by ntpd to be less accurate than getting time off the internet; and even with parity checking it is still fairly common for it to decode a time that is wrong.

But it has a nice flashing red LED.

Software: https://github.com/benclifford/msf

05 March, 2017

toad.com open mail server

So there was some controversy decades ago the past about John Gilmore's public open SMTP relay server. I wondered if it still existed.

It does!

benc@dogger:~$ telnet new.toad.com 25
Connected to new.toad.com.
Escape character is '^]'.
220 new.toad.com ESMTP Sendmail 8.12.9/8.12.9; Sun, 5 Mar 2017 08:12:44 -0800
EHLO dogger.cqx.ltd.uk
250-new.toad.com Hello dynamic-91.hawaga.org.uk [] (may be forged), pleased to meet you
250-SIZE 89000000
250 HELP
MAIL FROM:benc@hawaga.org.uk
250 2.1.0 benc@hawaga.org.uk... Sender ok
RCPT TO:benc@hawaga.org.uk
250 2.1.5 benc@hawaga.org.uk... Recipient ok
354 Enter mail, end with "." on a line by itself
Subject: test 1

250 2.0.0 v25GCiXw019546 Message accepted for delivery
221 2.0.0 new.toad.com closing connection

Received: from new.toad.com (new.toad.com [])
    by smtp-in.biscay.cqx.ltd.uk (8.14.4/8.14.4/Debian-2ubuntu2.1) with ESMTP id v25F9dpo009917
    for <benc@hawaga.org.uk>; Sun, 5 Mar 2017 15:09:40 GMT
Received: from dogger.cqx.ltd.uk (dynamic-91.hawaga.org.uk [] (may be forged))
    by new.toad.com (8.12.9/8.12.9) with ESMTP id v25F96Xw016104
    for benc@hawaga.org.uk; Sun, 5 Mar 2017 07:09:28 -0800
Date: Sun, 5 Mar 2017 07:09:06 -0800
From: benc@hawaga.org.uk
Message-Id: <201703051509.v25F96Xw016104@new.toad.com>
Subject: test 1

02 February, 2017

Why a US dollar and a Euro are roughly the same value.

A US dollar and a Euro are roughly the same value: 1 USD is about 1 EUR, very roughly (rather than, say, 1 USD = 1000 ITL = 10^25 ZWD = 60 RUR).

There is history behind that which I've not seen presented in one place. Here is my vague understanding of it.

For a few decades before the early 1970s, the Bretton Woods system fixed USD and gold (at $35 = 1oz) and many other currencies were pegged to gold/USD.

This ended in 1971, when the USD would no longer freely converted into gold by the US government, and major world currencies became free floating.

Around that time, although at little bit before, the IMF Special Drawing Right came into existence. This was defined, at the start, to equal 1 USD, which it did until the end of Bretton Woods, but it wa. The only time I've really encountered this in real life was in small print on the back of aeroplane tickets, where compensation amounts were denominated in SDRs.

The SDR was composed of a basket of different world currencies (at time of writing, USD, EUR, CNY, JPY, GBP in defined ratios) so as the USD-value of those component currencies change, so does the USD-value of 1 SDR, rather than being fixed to the initial 1 SDR = 1 USD ratio. Over time, though, the exchange rate has stayed very roughly 1:1

In Europe, they invented the European Unit of Account which was a basket of specifically European currencies. This was scaled so that at the start, 1 unit of account was equal to 1 SDR (and so inherited the property of being roughly 1 USD). Being composed of a different based from the SDR, it varied in value with respect to both the SDR and the USD.

Next came the ECU (which was invented just before I was born), another basket of European currencies which replaced the European unit of account at 1:1

Finally, along came the Euro, an actual currency with paper notes and metal coins. This replaced the ECU again at 1:1, but again was a different basket: While the ECU basket had included GBP (British pounds), DKK (Danish crowns) and GRD (Greek Drachmas), the Euro did not include those.

So the Euro has the US dollar as its great-great-grandfather through a series of basket currencies, each branched from the previous at a 1:1 ratio.

Of course, all that 1:1 substition could have gone very differently: the Zimbabwe dollar was equivalent to a Rhodesian Dollar which was equivalent to half a Rhodesian Pound (as happened with most pound decimalisations) which replaced the Rhodesia and Nyasaland pound which replaced the Southern Rhodesian Pound, equivalent to a pound sterling - but the GBP:ZWD exchange rate at the very end was something like 1:10^25.

20 December, 2016

Lua Fibre-optic Christmas Tree

In 1998, I was given a fibre optic christmas tree, lit by two coloured light bulbs powered by 2 x AA batteries. In the intervening years, the electrics broke. Finally today I got round to doing something about it.

The tree now has a bunch of neopixels where the bulbs used to be, and an ESP8266 microcontroller providing the flashing (as well as a Wifi/telnet accessible Lua command prompt).

Unfortunately, I can't see much of a different between the illumination provided by the seven pixels - I was hoping that each one would light the fibres up very differently, but that hasn't turned out to be the case. No big deal though, and maybe more optical isolation between the different LEDs and the base of the fibre bundle would help.

Interestingly, the fibres manage to project an image of the layout of the LEDs on the board (see link above) onto the wall behind the tree! and all the fibres emit a very slight blue light because there's a blue LED on the controller board.

25 November, 2016

smuggling things in a dirty bottom

The Haskell unit type, (), has just one value, also written (), right?

smuggle :: Typeable t => t -> ()
discover :: Typeable t => () -> Maybe t

x :: ()
x = smuggle "hello world"

discover x :: Just String
Just "hello world"

These allow you to inject an arbitrary (Typeable) Haskell value into unit and retrieve it later. Just don't try to inspect the resulting () value.

Rather than Haskell 98, you'll need unsafePerformIO and extensible exceptions, put together in a way that lets you hide arbitrary stuff in a thunk, and force evaluation at just the right time.

smuggle :: Typeable t => t -> ()
smuggle v = unsafePerformIO $ throw (toDyn v)

discover :: Typeable t => () -> Maybe t
discover v = either (fromDynamic) (const Nothing)
  $ unsafePerformIO
  $ try
  $ case v of () -> return ()

I could write more. But it's Friday night and I want to drink my wine.

Edit: This is now available on hackage as acme-smuggler.

24 November, 2016


When you're in a cafe

64 bytes from icmp_seq=3 ttl=59 time=26.8 ms
64 bytes from icmp_seq=4 ttl=59 time=23.9 ms
64 bytes from icmp_seq=5 ttl=59 time=26.0 ms

and then their internet dies

From icmp_seq=28 Destination Host Unreachable
From icmp_seq=29 Destination Host Unreachable
From icmp_seq=30 Destination Host Unreachable

and then there is a power cut

From icmp_seq=51 Destination Host Unreachable
From icmp_seq=52 Destination Host Unreachable
From icmp_seq=53 Destination Host Unreachable

but that reset the router

64 bytes from icmp_seq=1786 ttl=59 time=20.7 ms
64 bytes from icmp_seq=1787 ttl=59 time=24.0 ms
64 bytes from icmp_seq=1788 ttl=59 time=21.5 ms

and all is good in the world again.

10 November, 2016

a different technical writing form

My blogger draft box is full of half written posts that I haven't finished - sometimes because I'm lazy; sometimes because the draft is a pile of notes rather than something that could form a coherent post. I've tried embracing that, by dumping a lot of stuff into github at benclifford/text/wiki. This contains a bunch of rambling notes on miscellaneous topics that I have been updating as I think of stuff.

01 August, 2016

spherical camera

This week I've been playing with a Theta S spherical camera. I'm pretty impressed at what it does, given the price. Here's a gallery of images that you can scroll around.