Monthly Archives: February 2016

OpenTafl v0.1.6.1b, and progress generally

I’ve released a new version of OpenTafl. Among several other changes, there are two I would like to highlight.

First off, the groundwork for OpenTafl Notation-style engines is complete: there is code in OpenTafl which can produce and read in OTN rules records, code which can produce and read in OTN position records, and code which can produce OTN move records. This is a major milestone: producing and reading position and rules records is tricky, as is producing move records. (Move records should be fairly easy to read in, compared to the others; they encode a much smaller amount of critical data.) Note that the notation spec has changed somewhat, based on lessons I’ve learned over the course of my weekend of implementation. If you’ve done any work based on it yourself, you’ll want to go over the spec again. Hopefully, it won’t change much going forward, although the specification for OpenTafl-compatible engines is likely to change as I work on implementing that in the next month or two. You can see some of the work in version each position should provide its OTN position record, and games against the AI will report the AI’s thinking using OTN move records.

Second, OpenTafl’s memory usage has once again been dramatically reduced, to the point where it should be comfortably playable to the maximum depth at which it’s feasibly playable (currently depth 6-7 for brandub, depending on speed, and depth 4-5 for other games, depending on size and complexity) without running into memory issues, on any reasonable computer. I intend to go into a little more depth on how I accomplished this, but for the moment, you’ll just have to take my word for it.

You can find the full change logs at the OpenTafl site.

Old-School Service Rifles: Mauser Kar 98k

While I’m primarily an AR guy, and think that’s the best overall choice right now for the vast majority of rifle-y things that a guy might do, I’m also fond of old service rifles. They tell stories. Today we’re going to look at my oldest, a Mauser Karabiner 98k.

The Karabiner 98k, or Kar 98k, was a development of the Gewehr 98, by way of the Karabiner 98b. The Kar 98b wasn’t really a carbine at all, just a G 98 with better sights. It was still a long-barreled rifle. But after World War I, the Germans finally got to figuring that maybe they should standardize on one, shortish carbine for everyone who needed a rifle, rather than worry about infantry rifles and cavalry carbines. So, in 1934, they made what was to be the last in the long line of Mauser 98 designs, the Kar 98 kurz.1 In addition to the obviously shorter length, it also has a turned-down bolt handle, which makes mounting optics easier.

The Kar 98k has that wonderful, controlled-feed action that Mauser is famous for, and that so many have copied. It holds five rounds of 7.97x57mm ammunition, and proved to be a reliable and accurate weapon. It was the standard service rifle for the Wehrmacht Heer during the Second World War, and also saw use by the Soviets and many smaller powers after the war. It was also widely copied.

Let’s look at mine. It was made in 1938 in Suhl by J.P. Sauer und Sohn. Due to the time period of manufacture being before production had ramped up, Sauer was using some older parts. For this reason, the receiver bears both Weimar Waffenamnt proof marks and Third Reich Waffenamnt proof marks, which is kinda cool. Based on the age, we can conclude that this rifle saw plenty of service. Several parts are marked by an electropen with a different serial number than what is stamped on the gun. From this, we can conclude that this rifle was on the Eastern Front, was captured by the Soviets, and spent time reissued and in their arsenals. It has an X marking on the receiver that indicates it was eventually mustered out of Red Army service, and it eventually made its way to America and then to me.

Conditionwise, the rifle is in solid, but not excellent condition. The soviet arsenals have mixed up a few of the smaller parts, and they do not have matching serials with the rest of the gun. I’m happier that way, because it means the price is lower. The wood and finish show some wear, but are generally in good condition, and the bore doesn’t show too much wear either. There isn’t any pitting, and the grooves aren’t too worn out. When I got it, I was missing a few incidentals, which I decided to pick up. I got a surplus, beat-up looking sword bayonet of the appropriate late-thirties era, with oversized 9.75 inch blade, a cleaning rod, and a new-production sling.

For all its age, my Mauser shoots really well. The action is smooth, and the trigger is pretty good for a service rifle. It’s more or less two stage, and is somewhat heavy, but not gritty or creepy. The sights are ok. If you take your time and line them up right, the rifle is very accurate. They’re a simple notch and wedge-shaped post though, so these aren’t altogether fast or precise. Hardly my choice, but I didn’t design this. As is fitting and proper, the sight has range markings out to a hopelessly-optimistic two kilometers. I haven’t tried to hit anything at this range.

Recoil isn’t terrible. It’s certainly not a .22, but it’s not abusive the way a Mosin is. When I’ve brought it out for friends, I’ve gotten neither complaints nor habitual flinches, which is a good endorsement. The bolt isn’t as fast as a Lee Enfield, but it has never given me trouble.

My Mauser is a really nifty piece of history. It’s nearly eighty years old, but it still looks and shoots great. It’s a real treat to have and to run some rounds through.

If only it could talk.

1.) Short. Because it’s actually a carbine-length carbine as opposed to a longer, rifle-length carbine.

Underhanded C Code’s 2015 winner: why I love computer science

If you’re a software person, read the post. Linus Åkesson, this year’s winner, is brilliant, and I want you to appreciate just how much.

If you aren’t, stick around, because you should appreciate Mr. Åkesson’s brilliance, too. I’m going to explain this in layman’s terms. The International Underhanded C Code Competition, or IOUCCC, is an annual competition where software engineers and computer scientists from all the world over compete to write the most innocent-looking yet fiendishly guilty C programs possible. This year’s challenge is as follows:

There are two countries, which have signed a nuclear disarmament treaty. Each wants to carry out inspections to ensure that the other side is decommissioning real bombs, and not fakes, but neither side wants to expose the actual gamma ray spectra produced by their real bombs—that’s sensitive information on their nuclear programs. So, they have need of a tool which takes an observed spectrum and compares it to a reference spectrum, returning either yes (if the observed spectrum matches the reference spectrum, and is therefore a bomb), or no (if that is not so). Here’s the underhanded bit: one country wants to cheat. They want to keep more bombs than the treaty permits, but they have to fool the inspectors—that is, they have to figure out a way to show inspectors fake decommissioned bombs, which nevertheless yield a ‘yes’ when inspected with the tool. They can’t just fake the reading, because the reading has two parts: first, the placement of the peaks in the spectrum and their relative sizes, which only come from the specific mixes of fissile materials used in bombs; and second, the overall radioactive energy level of the bomb, which only comes from having a large amount of radiation coming from the bomb. It’s easy to replicate the peaks with a small amount of fissile material, and easy to replicate the total energy with an x-ray source, but impossible to combine them to look like a bomb without being a bomb.

We need to establish some background information about how spectra appear to digital tools. The way you collect a spectrum is to point a detector at a radioactive source of some sort. Energetic particles strike the detector face. An event is recorded, and deposited into a ‘bin’ based on the energy of the event. The collection of bins composes the spectrum. So, a spectrum, to a computer, looks like this: energies between 0 and 100, 20 events; energies between 101 and 200, 40 events; energies between 201 and 300, 153 events; and so on. In C, the easiest way to represent this is an array—that is, a region of computer memory which holds a list of numbers all in a row. Since, to a computer, numbers are a fixed number of bits (binary digits), the numbers don’t need to have any space between them; number N+1 is a fixed length from the start of number N.

We also need to talk briefly about floating point numbers. Floating point numbers are how computers represent decimals (generally speaking). Since there are an infinite number of real numbers between any two real numbers, a computer obviously can’t represent them precisely. Instead, it uses scientific notation, akin to this example: 1.2 \times 10^2, or 120. (In binary, obviously, but that’s not important yet.) We call the 1.2 part the mantissa, and the 2 the exponent. (For a computer, the base of the exponent is always two, because of binary.) Moreover, they are arranged like this: sign bit, exponent bits (highest place to lowest place), mantissa bits (highest place to lowest place), and the mantissa is always assumed to be one, unless the exponent is zero. (Instead of writing 1.2, we just write .2, and remember that we usually put a one before the decimal point.)

Finally, we need to discuss how one might want to compare two spectra. First, we’ll introduce a quantity called the dot product: if you have two lists of numbers of the same length, and go number-by-number, multiplying the first by the first, the second by the second, the third by the third, and so on, then add all of those products, the number you get is called the dot product. If you take the dot product of a list of numbers with itself, then take the square root of the dot product, you get the magnitude. A list of numbers, each entry in which has been divided by the list’s magnitude, is said to be normalized. Finally, if you take the dot product of two normalized lists of numbers, you get a number between 0 and 1. If you interpret the lists of numbers as multidimensional vectors (remember your geometry?) the number you just got is the cosine of the angle between them. For our purposes, it’s a number called the spectral contrast angle. If it’s 1, the two vectors are completely identical. If it’s -1, they’re completely different. So, when we get our two spectra as lists of numbers, we want to calculate the spectral contrast angle. If it’s sufficiently similar, then the spectra match.

So, Mr. Åkesson’s program takes two lists of numbers, representing spectra. It runs some preprocessing steps: smoothing the data, taking its first-order derivative to remove any constant-slope noise1, and smoothing the derivative to obtain a spectral fingerprint: a representation which contains the locations and relative heights of the peaks. Before going any further, it checks to see whether the total energy in the spectrum is above a threshold—a decommissioned bomb will still have a small signature, because of the remains of the fissile materials inside—then hands off the data to another source code file, which handles normalizing the spectra and doing the dot product stuff.

The underhandedness comes in that handoff. The data is defined as a list of numbers of type float_t. float_t is, in turn, defined in a header file (a file which holds definitions) as a double, a 64-bit (i.e. double-precision) floating point number, of which one bit is the sign of the number, 11 bits are the exponent, and the remaining bits are the mantissa. It is important to note a few things about doubles and Mr. Åkesson’s code: first, data in a double-precision number is left-justified. In cases where a given number can be expressed with sufficient precision, the bits on the right-hand side of the double’s position in memory are set to 0, unused. Second, the values in this particular program are generally fairly small—no more than a few thousand. Third, the preprocessing has the effect of making the numbers smaller: a derivative, as a rate of change, will always be smaller than the total value, except when it changes instantly. So, after the preprocessing, you have a list of double-precision floating point numbers, whose values range from about 0 to 100. Representing these numbers is simple, and doesn’t require using very much of the 64-bit length of the number.

The list is passed to the second file as the argument to a function, still a list of numbers of type float_t. The thing is, in this file, float_t means something else. This file does not include the definitions header from the first file, because it doesn’t need it. Instead, it includes a standard math library header file, and the math library header file defines float_t as a 32-bit (i.e. single-precision) floating point number, of which one bit is a sign bit, eight bits are the exponent, and the remaining bits are the mantissa. So, instead of a list of 64-bit numbers of length n, you have a list of 32-bit numbers of length 2n—but you’ve told the code you have a list of size n, so the code only looks at n numbers. That is, the code only looks at the first half of the spectrum. This lets you evade the total energy threshold without having to have a bunch of radioactive material in the bomb: place an x-ray source in your fake bomb. The total energy goes up so that it passes the threshold, and it won’t be considered in the peak-matching code, because it’s on the right side (the second half, the high-energy side) of the spectrum.

You still have to get the peak matching right, but the underhandedness has a second, subtler component which will help. It has to do with what happens when you convert a 64-bit floating point number into a 32-bit floating point number. Remember that, when a floating-point number doesn’t need its full length to express a number, all of the significant bits are on the left side of the number. Therefore, representing numbers between 1 and 100, especially whole numbers (which the preprocessing helps to ensure) doesn’t take anywhere near 64 bits. It doesn’t even take 32 bits. So, when you turn an array of 64-bit numbers into an array of 32-bit numbers, you split each 64-bit number in half. The second half is all zero bits, and ends up being zero. Since this happens to both the reference and the observed spectra, every other number in both arrays ends up being zero, and zero is the same as zero, so we don’t need to look at them any more.

The first half is the interesting half. The sign bit works exactly the same. The exponent, though, gets shortened from 11 bits to 8 bits, and the three bits that we leave off are the highest bits. Removing the low three bits moves the high bit over three places, which is the same as dividing by eight2. The three bits which were the end of the exponent become the beginning of the mantissa. This is fine: it preserves the relative sizes of numbers. (The least significant digits of the exponent are more significant than the most significant bits of the mantissa; you’ll just have to trust me here, or do some examples in base 10 yourself, following the rules I gave for floating point numbers.)

So what we’ve done is dramatically reduced the sizes of each energy level bin in the spectrum. Furthermore, we’ve also compressed their dynamic range: the difference between the lowest peaks and the highest peaks is much smaller, and so they resemble each other much more closely. The recipe for a fake bomb, then, is to take a small amount of fissile material and a powerful x-ray source, and put them in the casing. The x-ray source takes care of the total energy level, and the dynamic range compression from the 64-bit to 32-bit switch takes care of the low-energy peaks.

Congratulations! You just convinced the other country that you’re about to decommission a real bomb, when in actuality, it’s a fake.

Congratulations also to Linus Åkesson, who wrote such a clever piece of deception into a mere 66 lines, and did so with such panache I could not resist writing on it.

1. This appears in real-world spectra, because there’s naturally a lot more low-energy noise than high-energy noise. In support of this assertion, I offer the evidence that you are not suffering from radiation sickness right now. (Probably.)
2. 1000 is 8, in binary. (From right to left, you have the 1s-place, the 2s-place, the 4s-place, and the 8s-place.) Remove three zeros, and you have a 1 in the 1s place: 1.

Resurrected Weapons: RUM-125 Sea Lance

Submarines have been a serious threat to shipping since the Great War. Recently, the Russians are putting subs to sea like they did in the Cold War, ready to menace the shipping lanes once more. And submarines are more deadly then ever, with modern torpedoes like the Mk. 48 ADCAP having a range of upwards of twenty seven nautical miles. By detonating under the keel, they can split many ships in half. And, unlike antiship missiles, there aren’t many good ways to deal with torpedoes. You’re basically limited to a few decoy systems. So what’s a surface ship to do? Why, attack the sub, of course. This usually involves helicopters that can drop sonobuoys and dip sonars. They can also drop torpedoes if they find a sub.

What if the surface ship needs to engage a submarine directly? Suppose the helicopter isn’t nearby, or is out of torpedoes, or the surface ship detected the sub with her own sensors? Modern lightweight (read: anti-submarine) torpedoes have a range of anywhere from about five to about twelve nautical miles, depending on what speed setting they’re using. That’s a bit less than half of what the submarine’s torpedoes can do, giving him the shot long before you have it. What other options do we have for engaging?

We could use a rocket to get the torpedo closer before we drop it. If you have Mark 41 VLS cells, you could use the RUM-139 VL-ASROC, which puts a Mk. 46 torpedo about fifteen nautical miles from the launching ship. There are versions available with the more recent Mk. 54 lightweight torpedo, which has a much better seeker. Depending on speed settings, this gives us very nearly the range that the opposing sub has with his torpedo. Detente.

For those of you who’ve forgotten your high school French, or you uncultured swine who never had any, detente is a French word that means “you both get to die”. Yay. Personally, I’d rather not die, and would love to have the range for the first shot given a good sonobuoy contact and no torpedo-equipped helicopters nearby. For this, we come to another casualty of dwindling budgets in the ’90s, the RUM-125B Sea Lance.1

The Sea Lance has a bigger motor and a better inertial navigation system. It still fits in a regular Mk. 41 VLS cell. The RUM-125B was originally specced around the Mark 50 lightweight torpedo, but an enterprising designer could fit most any NATO lightweight torpedo in, since they’re all about the same size. The RUM-125B had a range of thirty five nautical miles, so if you see him first, you can shoot him first, helicopters or no. With a powerful weapon like this, it makes the surface ship a more active participant in the search for subs, rather than just a mothership to provide fuel.

But wait, there’s more. You may be wondering why the designation started with B. It didn’t. B is just the normal, conventional-warhead
model. Throw a torpedo, have it engage. When you really, really want range, when Ivan’s sub just absolutely, positively has got
to die, and when you want to really piss off greenpeace, there’s the RUM-125A. This missile variant can lob a 200 kiloton nuclear depth bomb out to a range of one hundred nautical miles. So you’re probably going to be safe from that blast. Maybe. It’s not very accurate, but then, it doesn’t have to be. This is the mother of all depth charges. Guaranteed to crush hulls, kill marine life, and cause an international incident, or your money back!

That’s not all. There were variants (designated UUM-125A and UUM-125B) that could be launched from submarines. These would get launched from the torpedo tubes in a buoyant capsule that would float to the surface and then launch the missile. It’s a great way to give attack subs a long range punch if they’re aware of a sub threat. Or just want to nuke the whales.

So go ahead, Captain Viktor Tupolev. Push your pissant Alfa-class boat as hard as you want. You’ll only die overheated.

Now, if only Sea Lance would work on those pesky land whales on Twitter.

Verdict: Approved by the Borgundy War Department Procurement Board

1.) Yes, this is a lower designation number. Trust me, it’s more advanced. Or don’t. More for me.
2.) This post is all in nautical miles, because we’re talking about things at sea. If you’re a communist, and prefer metric units, multiply all range figures above by 1.85.