Pro tip: Start your passwords with /!

Posted by Michał ‘mina86’ Nazarewicz on 11th of April 2021

Anyone who uses a screen locker surely can recall a situation where they approached their computer and started typing their password to unlock it even though it was never locked. Even if the machine is configured to lock automatically after a period of inactivity, there may be situations when power saving blanks the monitor even before the automatic locking happens.

If one’s lucky, they realise their mistake in time before hitting Return in a chat window. It’s not uncommon however that one ends with the password blasted into ether over IRC or Google Docs; lazy people might ignore the secret getting saved in their shell history file but even that should facilitate, often annoying, password change.

What if I told you there’s a way to avoid those problems? A one simple trick which will eliminated at least some forms of possible leaks. Simply prefix all your passwords with /! (slash followed by an exclamation mark).

Fun fact: ∞ is even

Posted by Michał ‘mina86’ Nazarewicz on 1st of April 2021

Some people find it surprising that zero is an even number. Turns out it’s such a controversial point that Wikipedia’s article on the subject has nearly 5000 words and 75 citations. That’s ten times as long as an entry on toast sandwich which is clearly a more important topic. On the other hand perhaps the confusion is to be expected considering that for centuries zero has been judged an odd digit (if a digit at all). Regardless, zero being even is not news and this is not what this post is about.

Rather, I wish to share another bit of knowledge. As it turns out, infinity is even. Who would have thought! As the working group for the C programming language (SC22 WC14) explains in C99 rationale, ‘all large positive floating-point values are even integers’.

There you go. Next time your Maths teacher asks what’s -42 to infinite power go ahead and exclaim with conviction that it’s plus infinity! The teacher will fail you of course (and rightfully so) unless you’re so lucky that they turn out to secretly be an expert on IEEE 754 standard.

Dark theme with media queries, CSS and JavaScript

Posted by Michał ‘mina86’ Nazarewicz on 28th of March 2021

Split view of Tower Bridge during the day and at night.
(photo by Franck Matellini)

No, your eyes are not deceiving you. This website has gone through a redesign and in the process gained a dark mode. Thanks to media queries, the darkness should commence automatically according to reader’s system preferences (as reported by the browsers). You can also customise this website in settings panel in top right (or bottom right).

What are media queries? And how to use them to adjust website’s appearance based on user preferences? I’m glad you’ve asked, because I’m about to describe the CSS and JavaScript magic that enables this feature.

Media queries overview

body { font-family: sans-serif; }
@media print {
	body { font-family: serif; }
}

Media queries grew from the @media rule present since the inception of CSS. At first it provided a way to use different styles depending on a device used to view the page. Most commonly used media types where screen and print as seen in the example on the right. Over time the concept evolved into general media queries which allow checking other aspects of the user agent such as display size or browser settings. A simple stylesheet respecting reader’s preferences might be as simple as:

body {
	/* Black-on-white by default */
	background: #fff;
	color: #000;
}
@media (prefers-color-scheme: dark) {
	/* White-on-black if user prefers dark colour scheme */
	body {
		background: #000;
		color: #fff;
	}
}

That’s enough to get us started but not all browsers support that feature or provide a way for the user to specify desired mode. For example, without a desktop environment Chrome will report light theme preference and Firefox users need to go deep into the bowels of about:config to change ui.systemUsesDarkTheme flag if they are fond of darkness. To accommodate such situations, it’s desirable to provide a JavaScript toggle which defaults to option specified in system settings.

Fortunately, media can be queried through JavaScript and herein I’ll describe how it’s done and how to marry theme switching with browser preferences detection. TL;DR version is to grab a demonstration HTML file which includes a fully working CSS and JavaScript code that can be used to switch themes on a website.

sRGB↔L*a*b*↔LChab conversions

Posted by Michał ‘mina86’ Nazarewicz on 21st of March 2021

After writing about conversion between sRGB and XYZ colour spaces I’ve been asked about a related process: moving between sRGB and CIELAB (perhaps better known as L*a*b*). As this may be of interest to others, I’ve decided to go ahead and make an article out of it. I’ll also touch on CIELChab which is a closely related colour representation.

Panther Chameleon
Picture of a chameleon with its decomposition into L*, a* and b* channels. Photo by Dr Pratt Datta.

The L*a*b* colour space was intended to be perceptually uniform. While it’s not truly uniform it’s nonetheless useful and widely used in the industry. For example, it’s the basis of the ΔE*00 colour difference metric. LChab aim to make L*a*b* easier to interpret by replacing a* and b* axes with more intuitive chroma and hue parameters.

Importantly, the conversion between sRGB and L*a*b* goes through XYZ colour space. As such, the full process has multiple steps with a round trip conversion being: sRGB​→​XYZ​→​L*a*b*​→​XYZ​→​sRGB. Because of that structure I will describe each of the steps separately.

Will the real ARG_MAX please stand up? Part 1

Posted by Michał ‘mina86’ Nazarewicz on 14th of March 2021

arg max is a set of values from function’s domain at which said function reaches its maxima. That’s certainly an arg max but spelled without an underscore thus not the one we are searching for. No, this article is regarding the ARG_MAX that limits the length of arguments to an executable.

Or in other words, why you are getting:

bash: command: Argument list too long

HTML: No, you don’t need to escape that

Posted by Michał ‘mina86’ Nazarewicz on 7th of March 2021

This website being my personal project allows me to experiment and do things I’d never do in professional settings. Most notably, I’m rather found of trying everything I can to reduce the size of the page. This goes beyond mere minification and eventually lead me to wonder if all those characters I’ve been escaping in HTML code really need such treatment.

Libraries offering HTML support will typically provide a function to indiscriminately replace all ampersands, quote characters, less-than and greater-then signs with their corresponding HTML-safe representation. This allows the result to be used in any context in the document and is a good choice for user-input validation. It’s a different matter when it comes to squeezing every last byte. Herein I will explore just which characters and when exactly really need to be escaped in an HTML document.

Regular expressions are broken

Posted by Michał ‘mina86’ Nazarewicz on 28th of February 2021

Quick! What does re.search('foo|foobar', 'foobarbaz').group() produce? Or for those not fluent in Python, how about /foo|foobar/.exec('foobarbaz')? Or to put it into words, what part of string foobarbaz will a foo|foobar regular expression match?

foo foobar

Perhaps it’s just me, but I expected the result to be foobar. That is, for the regular expression to match the longest leftmost substring. Alas, that’s not what is happening. Instead, Python’s and JavaScript’s regex engine will only match foo prefix.

Knowing that, what does re.search('foobar|foo', 'foobarbaz').group() produce (notice the subexpressions in the alternation are swapped). This can be reasoned in two ways: either order of branches in the alternation doesn’t matter — in which case the result should be the same as before, i.e. foo — or it does matter — and now the result will be foobar.

A computer scientist might lean towards the first option but a software engineer will know it’s the second.

Reading stdin with Emacs Client

Posted by Michał ‘mina86’ Nazarewicz on 21st of February 2021

One feature Emacs doesn’t have out of the box is reading data from standard input. Trying to open - (e.g. echo stdin | emacs -) results in Emacs complaining about unknown option (if it ends up starting in graphical mode) or that ‘standard input is not a tty’ (when starting in terminal).

With sufficiently advanced shell one potential solution is the --insert flag paired with command substitution: echo stdin | emacs --insert <(cat). Sadly, it’s not a panacea. It messes up initial buffer (and thus may break setups with custom initial-buffer-choice) and doesn’t address the issue of standard input not being a tty when running Emacs in terminal.

For me the biggest problem though is that it isn’t available when using emacsclient. Fortunately, as previously mentioned the Emacs Server protocol allows for far more than just instructions to open a file. Indeed, my solution to the problem revolves around the use of --eval option:

#!/usr/bin/perl

use strict;
use warnings;

my @args = @ARGV;
if (!@args) {
	my $data;
	$data = join '', <STDIN>;
	$data =~ s/\\/\\\\/g;
	$data =~ s/"/\\"/g;
	$data = <<ELISP;
(let ((buf (generate-new-buffer "*stdin*")))
  (switch-to-buffer buf)
  (insert "$data")
  (goto-char (point-min))
  (x-focus-frame nil)
  (buffer-name buf))
ELISP
	@args = ('-e', $data);
}

exec 'emacsclient', @args;
die "emacsclient: $!\n";

People allergic to Perl may find this Python version more palatable:

Emacs remote file editing over SSHFS

Posted by Michał ‘mina86’ Nazarewicz on 14th of February 2021

Previous article described how to use emacsclient inside of an SSH session. While the solution mentioned there relied on TRAMP, I’ve confessed that it isn’t what I’m actually using. From my experience, TRAMP doesn’t cache as much information as it could and as a result some operations are needlessly slow. For example, delay of find-file prompt completion is noticeable when working over connections with latency in the range of tens of milliseconds or more. Because for a long while I’d been working on a workstation ‘in the cloud’ in a data centre in another country, I’ve built my setup based on SSHFS instead.

It is important to note that TRAMP has myriad of features which won’t be available with this alternative approach. Most notably, it transparently routes shell commands executed from Emacs through SSH which often results in much faster execution than trying to do the same thing over SSHFS. grep command in particular will avoid copying entire files over network when done through TRAMP.

Depending on one’s workflow, either TRAMP-based or SSHFS-based solution may be preferred. If you are happy with TRAMP’s performance or rely on some of its feature, there’s no reason to switch. Otherwise, you might want to try an alternative approach described below.

Greyscale, you might be doing it wrong

Posted by Michał ‘mina86’ Nazarewicz on 7th of February 2021

While working on ansi_colours crate I’ve learned about colour spaces more than I’ve ever thought I would. One of the things were intricacies of greyscale. Or rather not greyscale itself but conversion from sRGB. ‘How hard can it be?’ one might ask following it up with a helpful suggestion to, ‘just sum all components and divide by three!’

Taking an arithmetic mean of red, green and blue coordinates is an often mentioned method. Inaccuracy of the method is usually acknowledged and justified by its simplicity and speed. That’s a fair trade-off except that equally simple and fast algorithms which are noticeably more accurate exist. One such method it is built on an observation that green contributes the most to the perceived brightens of a colour. The formula is (r + 2g + b) / 4 and it increases accuracy of the calculation by taking green channel twice and speed by changing division operation into a bit shift. But that’s not all. Even if speed is an important factor, there are better algorithms.

TL;DR

fn grey_from_rgb_avg_32bit(r: u8, g: u8, b: u8) -> u8 {
    let y = 3567664 * r as u32 + 11998547 * g as u32 + 1211005 * b as u32;
    ((y + (1 << 23)) >> 24) as u8
}

The above implements the best algorithm for converting sRGB into greyscale if speed and simplicity is main concern. It does not involve gamma thus forgoes most complicated and time-consuming arithmetic. It’s much more precise and as fast as arithmetic mean.

That’s hardly the end of the story though as slightly more complicated methods can increase accuracy much more.