Why can't you reverse a string with a flag emoji?

Case is totally wild, it only applies to like 5% of the symbols in ASCII, but in the process it means they each need two codepoints and you're expected to carry around tech for switching back and forth between cases.

And then there are several distinct types of white space, each gets a codepoint, some of them try to mess with your text's "position" which may not make any sense in the context where you wanted to use it. What does it mean to have a "horizontal tab" between two parts of the text I wanted to draw on this mug? I found a document which says it is the same as "eight spaces" which seems wrong because surely if you wanted eight spaces you'd just write eight spaces.

And after all that ASCII doesn't have working quotation marks, it doesn't understand how to spell a bunch of common English words like naïve or café, pretty disappointing.

What we need now is a standardized, sane subset of Unicode that implementations can support while rejecting the insane scope creep that got added on top of that. I guess the BMP is a good start, even though it already contains superfluous crap like "dingbats" and boxes.

Do you see the US flag after "copy and paste this emoji" on https://emojipedia.org/flag-united-states/?

Of course, various programming languages have primitives and concepts which they may label "string". But you still need to specify that context, drawing in the additional specification those languages provide. Plus, traditionally and in practice, such concepts often serve the function of importing or exporting unstructured data. So even in the context of a specific programming language, the label "string" is often used to elide details necessary to understanding the content and semantics of some particular chunk of data.

All you gain by having Unicode code point strings is the illusion of Unicode support until you test anything that uses combining characters or variant selectors. In essence, languages opting for such strings are making the same mistake at Windows/Java/etc. did when adopting UTF-16.

When you’re handling any kind of C pointer you need to know how big the buffer is around that pointer where pointer-arithmetic accesses make sense - but for a string, you also want to know ‘how much of the buffer is full of meaningful character data?’ - or else you’re stuck with fixed width text fields like some kind of a COBOL caveman.

But because C was designed by clever people for clever people they figured the standard string functions can just be handed a char pointer without any buffer bounds info because you can be trusted to always make sure that the pointer you give them is below a \0 within a single contiguous char buffer.

And that worked out great.

For instance, if you started with an image of the Washington Monument with the Lincoln Memorial in the background, the "reverse" would be an image of the Washington Monument with assorted Smithsonian museum buildings behind it. Or whatever you theorize would be on the east of the Washington Monument if no reference is available.

Is an absolute value signed as positive?

Are you referring to a grouping not covered by the definition of grapheme clusters (which I am only passingly familiar with)? If so, then I don't think it's any more non-meaningful to reverse it than to reverse an English string. The result is gibberish to humans either way - it sounds more like you're saying that there is no universally "meaningful to humans" way to reverse some text in potentially any language, which is true regardless of what encoding or written language you're using. I was thinking of it more from the programmer side - i.e. that Unicode provides ways to reverse strings that are more "meaningful" (as opposed to arbitrary) than e.g. just reversing code points.

Of course, if an interviewer is really asking you how to do this, they're probably either 1) working in bioinformatics, in which case there are exactly four ASCII characters they really care about and the problem is well-defined, or 2) it's implementing something like rev | cut -d '-' -f1 | rev to get rid of the last field and it doesn't matter how you implement "rev" just so long as it works exactly the same in reverse and you can always recover the original string.

Not if the ASCII string employed the backspace control character to accomplish what is today done with Unicode combining characters.

Or, in fact, if it employed any other kind of control sequence.

By your logic any software that has a single bug would be useless, and if that were the case this entire profession wouldn't exist.

Sometimes that's unacceptable, because you really do care about 100% of cases. When it isn't, you get really cool "impossible" tools out of it :)

Even in mathematics if you add additional constraints you can solve subclasses of problems.

I think the example shows quite clearly the problem really comes down to dividing the string to logical parts (atoms, grapheme clusters, however you want to call it).

I actually should admit, for all my protesting above that you never need to do this, I did once actually implement something that "required", as part of the process, reversing a string. It should be apparent once I share what it was why I put scare-quotes around "required" though.

We wanted to test and demonstrate the localization and unicode-readiness capabilities of our software, and to verify that every UI string was actually coming from the resource file for the selected locale, and handled in a unicode-safe way.

So I implemented a program that took in the en-GB resource file, and outputted an en-AU one that contained all the original strings, just flipped upside down. This being, of course, the canonical way to localize a product for Australia.

And to turn a string upside down, you need to reverse the order of the characters, before mapping them to their unicode upside-down equivalent.

Unfortunately, the Unicode consortium do not make available a comprehensive database of which glyphs are 180º reversals of other glyphs, so my solution ended up not having comprehensive coverage of all unicode codepoints, but since my source data was en-US text that wasn't that important; what was more important was that some of the resource strings used a 'safe subset' of HTML so I needed to not turn <strong> into <ƃuoɹʇs>.

More than anything, it was probably that experience that gave me a true appreciation for what nonsense it is to try to break a string into characters and manipulate them.

(Also, while I do love the ingenuity of string reversal for suffix-based indexing, reversing a domain name for efficient prefix-based lookup can of course also be done by breaking the name up into subcomponents (thus not requiring you to care about character composition at all between dots), reversing the sequence of those parts and reassembling the string from the components in reverse order - which has the added benefit of preserving human readability of the domain name, and a natural sort order...)

To expand, if the most-significant-bit is a 0, it's an ASCII codepoint. If the top two are '10', it's a continuation byte, and if they're '11', it's the start of a multibyte codepoint (the other most-significant-bits specify how long it is to facilitate easy codepoint counting).

So a naive codepoint reversal algorithm would start at the end, and move backwards until it sees either an ASCII codepoint or the start of a multibyte one. Upon reaching it, copy those 1-4 bytes to the start of a new buffer. Continue until you reach the start.

[0]: https://en.wikipedia.org/wiki/UTF-8#Encoding

But before their introduction, you could decide if there's a grapheme cluster break between codepoints just by looking at the two codepoints in question. Now, you may need to parse a whole sequence of codepoints to see how flags pair up.

I don't think reversing a string is a meaningful operation though; there's no reason to think of a string as a "list of characters" when it's also a "list of words" and several other things. Swift provides more than one kind of iteration for that reason.

For example, Wales was essentially just straight up conquered, and so for long periods Wales did not have any distinct legal identity from England. You'll see that today there's a bunch of laws which are for England and Wales but notably not Scotland, including criminal laws. In living memory Wales got some measure of independent control over its own affairs, via an elected "Assembly" but what powers are "devolved" to this assembly are in effect the gift of the Parliament, in Westminster, which is sovereign. Whether taking away those powers would go well is a good question.

On the other hand, Northern Ireland is what's left of English/ British dominion over the entire island of Ireland, most of which today is the Republic of Ireland, a sovereign entity with its own everything. It's only existed for about a century, and is a result of the agreed "partition" when the Irish rebelled because most of the Irish wanted independence but those in the North not so much. Feel free to read about euphemistically named "Troubles". In the modern era, Northern Ireland, like Wales, gets a devolved government in Stormont. Unlike Wales, the Northern Ireland government is a total mess, and e.g. they have abortion (like the rest of the UK, and like the rest of Ireland) only because Stormont was so broken that Westminster imposed abortion legalisation on them since they weren't actually governing. If you think the US Congress is dysfunctional, check out Stormont...

Finally Scotland was for a very long time an independent but closely related sovereign nation. It agreed to join this United Kingdom about three hundred years ago in the Acts of Union after about a century with the same Monarch ruling both countries. However, it too got a devolved government, a Parliament, probably the most powerful of the three, in Holyrood, Edingburgh in the 20th century and it has a relatively powerful pro-independence politics, the Scottish National Party is the dominant power in Scottish politics, although how many of its voters actually support independence per se is tricky to judge.

Brexit changed all this again, because as part of the EU a bunch of the powers you could reasonably localise, and so were "devolved" to Wales, Scotland and Northern Ireland, had been controlled by EU law. So Westminster could say they were devolved, knowing that the constituent entities couldn't actually do much with this supposed power. Having left the EU, those powers were among the thing Brexiteers seemed to have imagined now lay at Westminster, but of course the devolved countries said no, these are our powers, we get to decide e.g. how agricultural subsidies are distributed to suit our farmers.

That's even more fun in Northern Ireland, because they share a border with the Republic, an EU member, and so they're not allowed to have certain rules that would obviously result in a physical border with guards and so on. Their Unionists (the people who are why it isn't just part of the Republic of Ireland because they want to be in the United Kingdom) feel like they were sold out by Westminster politicians, while the Republicans (those who'd rather be part of the Republic) see this as potentially a further argument in favour of that. All of which isn't helping at all to keep the peace between these rivals, that peace being the whole reason we don't want to put up a border...