Today is the 110th birthday of Martin Gardner (1914–2010)! Every year around this date, various groups hold “Celebration of Mind” festivals. You can find some examples on the Gathering 4 Gardner site, although I see at the moment it’s a little sparse and 404’ish.

A few good blog posts and things (by other people) that I’ve seen lately:

The other day I got an intriguing question from Sándor Dargó. In “const all the things?” (2022-01-23) I had written:

Returning “by const value” is always wrong. Full stop.

Sándor writes: “I was wondering — given that it’s really the case, even the Core Guidelines says so, and it seems to be easy to identify — do you know why we don’t have compiler warnings for such return types?”

The other week I was contacted by a fellow digital antiquarian re an erratum in the “Adventure Family Tree” (managed by Nathanael Culver and hosted by Mike Arnautov). That page had listed under “No known download”:

BROO_XXX: “Enhanced” version of WOOD0350, with “added locations, text added, puzzles reworked,” by Nigel Brooks. […] This version was later renamed “A Serf’s Tale, A Retelling of the Original Adventure”.

In fact, wrote our correspondent, A Serf’s Tale is very much alive and well; the ZX Spectrum tape image survives, and is playable in any Spectrum emulator, such as in JSSpeccy on spectrumcomputing.co.uk. Ctrl+F “speccy” on that page, or (modulo link-rot) click here, to play A Serf’s Tale online.

So I spent a few days trying to beat A Serf’s Tale — and eventually succeeded, with the help of several vintage walkthroughs and hint files. Spoilers galore below the break.

In Milton’s Paradise Lost, book II (1667), Belial counsels against an assault on Heaven:

What if the breath that kindl’d those grim fires
Awak’d should blow them into sevenfold rage
And plunge us in the flames? or from above
Should intermitted vengeance arm again
His red right hand to plague us?

This last image is lifted from Horace’s Odes I.2:

Iam satis terris nivis atque dirae
grandinis misit Pater et rubente
dextera sacras iaculatus arces
terruit urbem […]

Yesterday John Graham-Cumming posted about “Steve Ballmer’s incorrect binary search interview question,” and in the Hacker News discussion, among the predictable “interview culture is flawed!!” complaints, there was some discussion of whether binary search was even the right way to solve Ballmer’s puzzle. Here’s a stab at an answer.

“Moses was learned in all the wisdom of the Egyptians, and was mighty in words and in deeds.” Flavius Josephus, in Antiquities of the Jews II.10 (circa A.D. 94), recounts one of those mighty deeds: While on the march against the Ethiopians

I’m blogging this mainly so that I’ll have a quick cut-and-pasteable starting point the next time I need to use boost::interprocess::offset_ptr as a motivating example for something. Many thanks to StackOverflow user “sehe” for the great SO answer from which I distilled this code.

But also see the end of this post for a bonus about allocator-aware inplace_vector.

Via Andy Miller (2007), an amusing metaphor for Linux memory overcommit. Originally posted by Andries Brouwer to the linux-kernel mailing list, 2004-09-24, in the thread titled “oom_pardon, aka don’t kill my xlock”:

The other day I learned a new place where adding or removing noexcept can change the performance of your program: GNU libstdc++’s hash-based associative containers change the struct layout of their nodes depending on the noexceptness of your hash function. This is laid out fairly clearly in the docs; it’s simply bizarre enough that I’d never thought to look for such a thing in the docs!

A quotation from the preface to Donald Knuth’s The TeXBook (Amazon, archive.org):

[A] noteworthy characteristic of this manual is that it doesn’t always tell the truth. When certain concepts of TeX are introduced informally, general rules will be stated; afterwards you will find that the rules aren’t strictly true. […] The author feels that this technique of deliberate lying will actually make it easier for you to learn the ideas. Once you understand a simple but false rule, it will not be hard to supplement that rule with its exceptions.

On the occasion of the confirmation of BB(5) (a problem involving the enumeration of Turing machines), Thomas Baruchel asks the following simpler question about finite state machines:

Given an alphabet of size \(L\), how many distinct languages over that alphabet are recognizable by a complete DFA with \(n\) states?

This question comes up a lot on the cpplang Slack. Suppose I have a class named my::Book, and I want to put it into a std::unordered_set. Then I need to write a std::hash specialization for it. So I write:

Via Enrico Mauro, an interesting variation on the priority_tag trick. Compare the following example to the HasSomeKindOfSwap example in “priority_tag for ad-hoc tag dispatch” (2021-07-09).

template<class T, int N = 2, class = void>
struct HasSomeKindOfValueType
  : HasSomeKindOfValueType<T, N-1> {};

template<class T>
struct HasSomeKindOfValueType<T, 2, std::void_t<typename T::value_type>>
  : std::true_type {};

P2927 “Inspecting exception_ptr,” proposes a facility (formerly known as std::try_cast) with this signature:

This past week I was at the WG21 meeting in St Louis, Missouri. My lovely wife and I spent a fair bit of time exploring the city in vacation mode: City Museum (fantastic); the penguin house at the St Louis Zoo (fantastic); cocktails, carpaccio, and hot chicken at Blood & Sand (fantastic). But in between, there were some movements of papers.

One of the papers on the docket for this week’s WG21 meeting in St Louis is P3116 “Policy for explicit” (Zach Laine, 2024). The idea of “policy,” in this context, is that LEWG wants to have something like a “style guide” for proposal-authors. If a proposal comes in with noexcept in the wrong places, or explicit, or [[nodiscard]], we want to be able to quickly tell the author how it ought to be, without a lot of the same discussion happening on every paper. Like a house style guide in newspaper-editing: if our newspaper uses the Oxford comma, and you bring in an article without it, then we can just point to the style guide, make the fix, and move on [see?], without a lot of repeated discussion of the pros and cons of the comma except insofar as you can argue that it belongs in this particular article for a really good reason.

This past weekend I finished reading Gretchen McCulloch’s entertaining 2019 book on “the new rules of language,” Because Internet. Top takeaways:

On Friday, June 28th, there will be a discussion at the C++ Committee meeting about the state of “trivial relocation” in C++. (The meeting is physically in St Louis, but also streamed remotely via Zoom and open to visiting experts per WG21’s Meetings and Participation Policy.) If you’re an expert (read: a library maintainer or developer) with opinions or experience on “trivial relocation” — especially if you have implemented it in your own codebase — I encourage you to read the participation policy and try (virtually) to show up for the discussion!

Over on Puzzling StackExchange, Brandan Williams poses the following interesting question:

Find a strictly decreasing sequence of integers \(r_0, r_1, r_2,\ldots, r_n\) such that you can place kissing circles of radii \(r_1, r_2,\ldots, r_n\) around a central circle of radius \(r_0\). That is, circle \(r_0\) will be tangent to all \(n\) other circles, and circle \(r_i\) will be tangent to circle \(r_{i+1}\) for all \(1\leq i<n\), and circle \(r_n\) will be tangent to circle \(r_1\).