PowerTab for PowerShell

PowerShell is excellent.

The default command line environment was obviously written by a bunch of monkeys (who on Earth wants to block select rectangles of text rather than lines of text?).

PowerTab is a module that goes some way towards fixing the woeful keep-tabbing-until-you-hit-the-completion-you-want tab-completion behaviour.

Dates and Times and Timezones and .NET

Yesterday I learned something about parsing dates and times.  I was trying to get a .NET application to process date/time strings of the form "2010-08-04T16:30:00 CEST" using DateTime.TryParse.  This approach didn't work because .NET doesn't grok timezone strings like "CEST" (Central European Summer Time).

My first thought was that since CEST is defined as UTC+2 (i.e., Universal Coordinated Time plus two hours), I could rewrite these date/time strings as "2010-08-04T16:30:00+02:00", which DateTime.TryParse can handle (note that the timezone delta on the end must be subtracted from the preceding date/time to obtain UTC).  My attempts to tackle this were stymied by the fact that timezone names are not unique.  For example, CST can mean either Central Summer Time or Central Standard Time, and that's just in Australia!

So, to conclude, for the love of all that is holy, don't use timezone names in your date/time strings.  Use attached UTC deltas instead.

[For those who are wondering, as I did, the difference between UTC and GMT is never allowed to exceed 0.9s; UTC is atomic clock time with some leap seconds added here and there; GMT is not.]

DNS Problems with Blizzard's StarCraft II Patch Installer

After much anticipation, my copy of StarCraft II arrived in the post yesterday.  I installed it.  I ran it.  It immediately started downloading a patch.  Ten megabytes later, I lost all internet connectivity.  Again, again, and a-bloody-gain.  Now, it turns out I had recently upgraded to Windows 7, a thing of beauty, which automatically ran a network diagnostic tool and reported that there was a problem with the DNS service.  Hmm, thinks I, maybe iiNet (who are my otherwise rather good ISP) have cocked up their DNS server.  After a bit of resettery, I discover that Google have a publically available DNS server (8.8.8.8).  I add this as the backup DNS server and, lo and behold, glorious sustained connectivity.

Having shared this with you, I am now about to abandon society and join the StarCraftiverse for some time.  Adieu.

Changing Jobs!

After nine excellent years at the University of Melbourne, I'm leaving to take up a new position as a senior software engineer at an IT firm in the heart of Melbourne.  I'll miss working with everyone at the University, but I'm also very excited about the new challenge.  The chaps here had a whip-round and bought me a 1997 Speyside single malt.  FANTASTIC!  This bottle is what I believe is cask strength, at nearly 60% alchohol, and will have to be approached with caution.

Farewell drinks tonight!

Searle on Deconstructionism

This is John Searle's priceless take on deconstructionism. One footnote is particularly amusing:

I said that deconstruction had found little appeal among professional philosophers. But there are some notable exceptions, much prized by deconstructionists. They tend to be ambiguous allies. One of these characterized Derrida as "the sort of philosopher who gives bullshit a bad name." We cannot, of course, exclude the possibility that this may be an expression of praise in the deconstructionist vocabulary.

A marvellous crytpic crossword clue

Clue: hijklmno (5 letters)

Answer: water (highlight with your mouse to see the solution).

C# Namespaces

Do not use the same name for a namespace and a class.

It only leads to madness and divorce.

I've just spent a merry half hour finding this out.

WotD: tropology and tropometer

From Jarrold's Dictionary of Difficult Words:

tropology, n. figurative style of writing; interpretation of Bible stressing figurative nature of language.

tropometer, n. instr. measuring rotation.

WotD: ambry and ambsace

From Jarrold's Dictionary of Difficult Words:

ambry n. niche; cupboard, espec. containing sacred vessels in chancel.

ambsace n. double ace; lowest score; bad luck.

False positives

Consider an individual X who is tested for a disease.
Let D mean "X has the disease" and -D mean "X does not have the disease".
Let T mean "X has tested positive for the disease".
Let P(A) mean "the probability that A is true".
Let P(A|B) mean "the probability that A is true given that B is true".

We are given P(D), P(T|D), and P(T|-D).
(i.e.: the probability that X has the disease; the probability of getting a positive test if X really does have the disease; and the probability of getting a false positive test if X actually doesn't have the disease, respectively).
Note that P(-D) = 1 - P(D).

What is P(D|T)?
(i.e.: what is the probability that X has the disease given that X tested positive for the disease?)

Let u = P(D).P(T|D) be the probability that X tests positive and has the disease.
Let v = P(-D).P(T|-D) be the probability that X tests positive and yet doesn't have the disease.

These are the only ways X can get a positive test result. It follows that

Solution: P(D|T) = u/(u + v)
(i.e.: the probability that X has the disease given X tested positive is the fraction of positive results for X that are actually correct).

Practical application.

Since disease is rare, P(-D) will be much larger than P(D). Therefore if P(T|-D) (the chance of a false positive) is not very small, then v = P(-D).P(T|-D) will be large, hence P(D|T) will be relatively low (i.e., a positive test will probably not mean that X actually has the disease).

Bottom line: make sure your tests have a low false-positive rate.

Covariance and Contravariance

On occasion I've mixed these terms up. Taking X : T to mean "X has/is compatible with type T", then:

X : T is covariant if X can also take on a subtype of T;

X : T is contravariant if X can also take on a supertype of T.

Consider the expression Ys = map(F, Xs) where
Xs : list(Tx),
Ys : list(Ty), and
F : U -> V.

Clearly F must accept values of type Tx (or some supertype thereof), therefore U must be of type Tx (or some supertype thereof), hence F is contravariant in its argument.

On the other hand, F must return values of type Ty (or some subtype thereof), therefore V must be of type Ty (or some subtype thereof), hence F is covariant in its return type.

Ugly Betty - Nooooooooo!

Ugly Betty has aired it's final episode. We grow sad...

UB was the finest, smartest comedy I've seen on television and the cast and scriptwriters managed to maintain the level from start to end throughout all four of its seasons. Good for them for finishing it well rather than running the show into the ground.

UB was the comedy that the West Wing writers could have written. It was just that good.

My First StarCraft On-line Win!

Kum bay yah, I have finally won a game. This is more of an achievement than you think given how bad I am at StarCraft. (I play as 'theoctopi' on ICCup if anyone's interested.)

Extended C type generator tools

I've extended my set of C tools for generating common data types. Now there are generators for list types, 'maybe' types, and expanding array types. Now with one command line invocation you can generate hundreds of lines of bug-free (!) boilerplate C code.

Tool for generating algebraic types in C

I've spent a few evenings crafting a tool to automatically generate C code implementing algebraic data types (or discriminated union types) from short specification files. For example, the following specification

@ type a_b_tree
@ constructor a
@ constructor b
@ constructor branch
l : a_b_tree *
r : a_b_tree *
@{
    if (l == NULL || r == NULL) {
        fprintf(stderr, "a_b_tree: branch cannot have NULL arguments.\n");
 exit(1);
    }
}@
@ end

expands to 144 lines of C (counting the .h and .c files).

The transformation supports

• generation of constructor, deconstructor, and freeing functions;
  
• optimised handling of constructors with no arguments (memory is not allocated for separate instances of these);
  
• optimised handling of types with only a single constructor (there is no need in this case to distinguish between the type and its constructor);
  
• user specifiable malloc and free functions;
  
• user specifiable header and footer code for the generated .h and .c files.
  

You can download this tool here.

The anomaly method and UHI

This is rather interesting. It seems the anomaly method of only considering digressions from the mean of a temperature series (i.e., the series' anomalies) does account for the urban heat island effect (i.e., it effectively removes the unwanted UHI component from the signal).

Grammar

This is witty and helpful.

I particularly like

• Don't string too many prepositional phrases together unless you are walking through the valley of the shadow of death.
  

Cute "proof" of Pythagoras' theorem

[For reasons I have yet to fathom, the ASCIIMathML plugin doesn't want to work for this page...]

I'm reading The Mathematical Mechanic by Mark Levi, a tour of various mathematical results that can be obtained much more directly (albeit without proof) by appealing to our mechanical intuition.

Here's a beautiful proof from the book of Pythagoras' theorem, `a^2 + b^2 = c^2` where `a` and `b` are the opposite and adjacent sides of a right angled triangle and `c` is the hypotenuse.

Consider an ice skater of mass `m` on a perfectly smooth ice rink. The skater starts in the South West corner of the rink and pushes off against the South wall; the skater is now moving North with velocity `a` and hence has kinetic energy `ma^2/2`. Next the skater pushes off against the West wall and adds an Easterly component `b` to their velocity. The energy acquired from this second push is `mb^2/2`. Now, the skater's overall velocity is `c` and overall kinetic energy is `mc^2/2`. But this must be the sum of the energy acquired from each push, hence `ma^2/2 + mb^2/2 = mc^2/2`. Cancel the `m/2` terms and you have Pythagoras' theorem!

A roguelike I can enjoy

Dungeon Crawl is a roguelike that I can actually enjoy over a coffee break (i.e., I don't have to dedicate years of my life to it). And you can play it over ssh.

Latin phrases

• "Nil carborundum illigitimi" - don't let the bastards get you down.;
• "Nullius in verba" - take no-one's word for it (motto of the Royal Society, apparently suspended for climate related issues);

and my personal favourite:

• "Sic biscuitus disintegrat" - that's the way the cookie crumbles.