Extending Looped Music for Fun, Relaxation and Productivity

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Some work (like programming) takes a lot of concentration, and I use noise-cancelling headphones to help me work productively in silence. But for other work (like doing business paperwork), I prefer to have quiet music in the background to help me stay focussed. Quiet background music is good for meditation or dozing, too. If you can’t fall asleep or completely clear your mind, zoning out to some music is the next best thing.

The best music for that is simple and repetitive — something nice enough to listen too, but not distracting, and okay to tune out of when needed. Computer game music is like that, by design, so there’s plenty of good background music out there. The harder problem is finding samples that play for more than a few minutes.

So I made loopx, a tool that takes a sample of music that loops a few times, and repeats the loop to make a long piece of music.

When you’re listening to the same music loop for a long time, even slight distortion becomes distracting. Making quality extended music audio out of real-world samples (and doing it fast enough) takes a bit of maths and computer science. About ten years ago I was doing digital signal processing (DSP) programming for industrial metering equipment, so this side project got me digging up some old theory again.

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Woothee (HTTP User Agent Parser)

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I’ve written a D implementation of the Project Woothee multi-language HTTP user agent parser. Here are some notes about what it’s useful for, and a few things special about the D implementation.

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Pi from High School Maths

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Warning: I don’t think the stuff in this post has any direct practical application by itself (unless you’re a nuclear war survivor and need to reconstruct maths from scratch or something). Sometimes I like to go back to basics, though. Here’s a look at π \pi and areas of curved shapes without any calculus or transcendental functions.

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Tricks with Pseudorandom Number Generators

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Pseudorandom number generators (PRNGs) are often treated like a compromise: their output isn’t as good as real random number generators, but they’re cheap and easy to use on computer hardware. But a special feature of PRNGs is that they’re reproducible sources of random-looking data:

import std.random;
import std.stdio;

void main()
{
    // Seed a PRNG and generate 10 pseudo-random numbers
    auto rng = Random(42);
    foreach (_; 0..10) write(uniform(0, 10, rng), ' ');
    writeln();
    // Reset the PRNG, and the same sequence is generated again
    rng = Random(42);
    foreach (_; 0..10) write(uniform(0, 10, rng), ' ');
    writeln();

    // Output:
    // 2 7 6 4 6 5 0 4 0 3
    // 2 7 6 4 6 5 0 4 0 3
}

This simple fact enables a few neat tricks.

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Glico (Weighted Rock Paper Scissors)

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This still isn’t the blog post I said I was going to write about now, but I figured some game theory would make a good post at the moment, especially when a lot of people I know are working at home with kids who need entertaining. Here’s some stuff about a traditional Japanese kids’ game called Glico, a form of weighted Rock Paper Scissors (RPS).

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Some Useful Probability Facts for Systems Programming

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Probability problems come up a lot in systems programming, and I’m using that term loosely to mean everything from operating systems programming and networking, to building large online services, to creating virtual worlds like in games. Here’s a bunch of rough-and-ready probability rules of thumb that are deeply related and have many practical applications when designing systems.

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Analysing D Code with KLEE

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KLEE is a symbolic execution engine that can rigorously verify or find bugs in software. It’s designed for C and C++, but it’s just an interpreter for LLVM bitcode combined with theorem prover backends, so it can work with bitcode generated by ldc2. One catch is that it needs a compatible bitcode port of the D runtime to run normal D code. I’m still interested in getting KLEE to work with normal D code, but for now I’ve done some experiments with -betterC D.

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Why Sorting is O(N log N)

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Any decent algorithms textbook will explain how fast sorting algorithms like quicksort and heapsort are, but it doesn’t take crazy maths to prove that they’re as asymptotically fast as you can possibly get.

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Counting Sudoku Solution Grids using Monte Carlo

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How many ways can you fill a 9x9 grid, obeying all the rules of the sudoku puzzle? The answer is too big to just calculate directly on a computer, so an exact answer takes careful analysis. But if an absolutely exact answer isn’t required, we can get a good statistical approximation using a Monte Carlo algorithm. As a bonus, the algorithm doesn’t need any application-specific analysis and works on many other problems, too. It’s a handy “stupid things that work” approach to solving problems.

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Compression, Complexity and Software System Design

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Information theory gives us a way to understand communication systems, by giving us a way to understand what happens to information as it’s transmitted or re-encoded.

We can also study what happens to complexity in a software system as components depend on or interface each other. Just like we can make rigorous arguments about how much information can be compressed, we can make arguments about how much complexity can be simplified, and use this to make better choices when designing software systems.

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