Obviously, “hello עולם” backwards is “םלוע olleh”. The first encoded character appears rightmost in the right word, while the last encoded displays rightmost of the leftmost word, i.e. If we trivially reverse this string, we get “olleh םלוע” as it starts rendering from the right margin. When the latin script is first, the string starts from the left margin, with the first encoded character to the left. A mix such as “hello עולם” will render “hello” LTR and “עולם” RTL (the “ם” is encoded last, but displays leftmost in that word). Reversal issues occur naturally in bidirectional text. Obviously, it also has to handle explicit directional embedding, U+202A and U+202B, which are similar but not identical to directional overrides. ![]() Your string reverser doesn’t actually reverse strings. With trivial reversion, it becomes “hello world” followed by a RLO immediately cancelled by a LRO. In this direction, everything from the second character onward is shown right-to-left as “hello world”. It happens to be encoded with left-to-right and right-to-left overrides as “U+202D U+202E dlrow olleh”. What’s “hello world” backwards? It’s “hello world” if your implementation is to be believed. ![]() If you try to reverse it while keeping the macron after the “e”, you end up with “nae_m” (“na em“) rather than the original, correct “na_em” (“n aem”). To put a ‘double macron below’ under the characters “ea” in “mean”, you’d encode “me_an” which renders as “m ean”. Double composing characters go between the characters they compose. Just reverse it.īy the way, if you try to fix this by ensuring that composing characters stay behind their preceding character, you’ll introduce a regression. Please don’t shuffle diacritical marks in the input string. If the string is trivially reversed, it becomes “adaloc a~nip” which will render as “adaloc ãnip”. In this way, you can encode “pin~a colada”, and it will render as “piña colada”. While there is a separate character for “ñ”, n with tilde, it can also be written as two characters: regular “n” (U+006E) plus composing tilde (U+0303), which I’ll write as a regular tilde for illustration. Trashing characters in the string is not a property of correct string reversers. Java’s dePointAt(int)), reversing them produces an invalid character. If two chars form a single code point (see e.g. Characters in so-called supplementary planes will not fit in a 16-bit char, and will be encoded as a surrogate pair – two chars next to each other. At least that’s a simple fix, right? Surrogate pairsĮnvironment based around 16-bit character types, like Java and C#’s char and some C/++ compilers’ wchar_t, had an awkward time when Unicode 2.0 came along, which expanded the number of characters from 65536 to 1114112. It’s obviously a bug if the BOM ends up at the end of the string when it’s reversed. BOM use is optional, and, if used, should appear at the start of the text stream.” It is encoded at U+FEFF byte order mark (BOM). Wikipedia says that “The byte order mark (BOM) is a Unicode character used to signal the endianness (byte order) of a text file or stream. The following are cases that a string reversal algorithm could reasonably be expected to handle, but which your initial, naive implementation most likely fails: Byte order marks Reversing a string is much harder than one would think. No, you will in fact need several hours and hundreds of lines of code. Well… Java, C#, Python, Haskell and all other modern languages have native Unicode string types, so at most you’ll just need another minute to verify that it does indeed work, right? What if I say that this is 2013 and your software can’t just fail because a user inputs non-ASCII data? ![]() If I ask you to prove your hacker worth by implementing it in your favorite language, how long would it take you and how many tries will you need to get it right?įive minutes with one or two tries? 30 seconds and nail it on the first try? ![]() Oh, string reversal! The bread and butter of Programming 101 exams.
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