4 ms·

One of the requirements is: 4) The first byte should indicate the number of bytes to follow in a multibyte sequence. This is actually a pretty smart requirem

by fefe23 7y ago

One of the requirements is: 4) The first byte should indicate the number of bytes to follow in a multibyte sequence. This is actually a pretty smart requirement for efficiency. This way the parser can know if the input bytes it has constitute a UTF-8 sequence by only looking at the first byte. That saves a lot of unnecessary processing. Unfortunately the wording here breaks that advantage: https://en.cppreference.com/w/cpp/string/multibyte/mbrtoc16 https://en.cppreference.com/w/cpp/string/multibyte/mbrtoc16 if the next n bytes constitute an incomplete, but so far valid, multibyte character. Nothing is written to The "so far valid" forces implementations to still look at the other bytes, which undoes the optimization in UTF-8. I suspect somebody was asleep at the wheel there. Regarding your question about the wasted bits: I don't think it matters much. It certainly does not for English text like we exchange here, where the important case is the 0-0x7f case which UTF-8 handles optimally. Your maximum compactness variant means if you start in the middle of a sequence, you can't tell you are in the middle and not at the start. UTF-8 is doing this well in my opinion. You can seek anywhere in a file and then move forwards or backwards till the beginning of a sequence without fear of misparsing the middle of a correct sequence as a different sequence.

kstenerud 7y ago
Yes, the ultra-compact encoding will not be self-sequencing. But the bit-6-continuation variant yields more bits per byte, which would give better compression in many languages. Regarding efficiency, you still have to read every character, with the difference being one check every 1, 2, 3, or 4 characters in a more complex algorithm vs a check on every character in a simpler algorithm (I haven't checked to see which beats the other in performance, but they look pretty similar). This feels a lot like mixing transport layer metadata into the data format, potentially giving a small processing performance benefit at the cost of huge data wastage when certain languages are encoded.
- marcosdumay 7y ago
  > huge data wastage when certain languages are encoded Is there a language that consistently uses codepoints with more than 2 bytes? It bothers me a bit that UTF-8 is not an infinitely extendable encoding. But that also isn't an important objection, because it is finite, but huge.
  - kstenerud 7y ago
    Yeah, it's fine that the encoding is not infinitely extendable. But if it didn't encode the length into the first byte, you'd have 1 extra bit for 2-byte sequences, 2 extra bits for 3-byte sequences, etc. That means that you can double the number of possible glyphs per 2-byte sequence (for an extra 2048 glyphs in the 2-byte range). For 3 bytes, it's 2 bits for an extra almost 200,000 glyphs before having to jump to 4 bytes. This would be a huge boon to East Asian languages like Chinese and Japanese.
  - XaspR8d 7y ago
    > Is there a language that consistently uses codepoints with more than 2 bytes? There are definitely (small) communities using scripts that lie entirely in the SMP. For example, Mru, Adlam, Takri, Pracalit, Miao, Wancho, etc. Most of these are either historic scripts that have mostly been supplanted by unified ones (esp. Devanagari) but retain usage in some areas, or languages that did not have a pre-colonial writing system that are attempting to reclaim cultural identity with a new script. But yes, I don't think there are major communities that consistently do so. My anecdata from a few Mandarin- and Japanese-speaking friends is that SIP characters rarely occur. Really if anything, emoji obsessives, mathematicians using bold/fraktur characters, and historical linguists/anthropologists would have the biggest savings. https://en.wikipedia.org/wiki/Mru_language#Alphabet https://en.wikipedia.org/wiki/Mru_language#Alphabet https://en.wikipedia.org/wiki/Adlam_script https://en.wikipedia.org/wiki/Adlam_script https://en.wikipedia.org/wiki/Takri_script https://en.wikipedia.org/wiki/Takri_script https://en.wikipedia.org/wiki/Pracalit_script https://en.wikipedia.org/wiki/Pracalit_script https://en.wikipedia.org/wiki/Pollard_script https://en.wikipedia.org/wiki/Pollard_script
  - boomlinde 7y ago
    > Is there a language that consistently uses codepoints with more than 2 bytes? Kanji, hiragana, katakana etc. At least 3 bytes with some 4-byte sequences. This compares unfavorably to for example Shift-JIS.

by fefe23 7y ago

kstenerud 7y ago

Yes, the ultra-compact encoding will not be self-sequencing. But the bit-6-continuation variant yields more bits per byte, which would give better compression in many languages. Regarding efficiency, you still have to read every character, with the difference being one check every 1, 2, 3, or 4 characters in a more complex algorithm vs a check on every character in a simpler algorithm (I haven't checked to see which beats the other in performance, but they look pretty similar). This feels a lot like mixing transport layer metadata into the data format, potentially giving a small processing performance benefit at the cost of huge data wastage when certain languages are encoded.

marcosdumay 7y ago
> huge data wastage when certain languages are encoded Is there a language that consistently uses codepoints with more than 2 bytes? It bothers me a bit that UTF-8 is not an infinitely extendable encoding. But that also isn't an important objection, because it is finite, but huge.
- kstenerud 7y ago
  Yeah, it's fine that the encoding is not infinitely extendable. But if it didn't encode the length into the first byte, you'd have 1 extra bit for 2-byte sequences, 2 extra bits for 3-byte sequences, etc. That means that you can double the number of possible glyphs per 2-byte sequence (for an extra 2048 glyphs in the 2-byte range). For 3 bytes, it's 2 bits for an extra almost 200,000 glyphs before having to jump to 4 bytes. This would be a huge boon to East Asian languages like Chinese and Japanese.
- XaspR8d 7y ago
  > Is there a language that consistently uses codepoints with more than 2 bytes? There are definitely (small) communities using scripts that lie entirely in the SMP. For example, Mru, Adlam, Takri, Pracalit, Miao, Wancho, etc. Most of these are either historic scripts that have mostly been supplanted by unified ones (esp. Devanagari) but retain usage in some areas, or languages that did not have a pre-colonial writing system that are attempting to reclaim cultural identity with a new script. But yes, I don't think there are major communities that consistently do so. My anecdata from a few Mandarin- and Japanese-speaking friends is that SIP characters rarely occur. Really if anything, emoji obsessives, mathematicians using bold/fraktur characters, and historical linguists/anthropologists would have the biggest savings. https://en.wikipedia.org/wiki/Mru_language#Alphabet https://en.wikipedia.org/wiki/Mru_language#Alphabet https://en.wikipedia.org/wiki/Adlam_script https://en.wikipedia.org/wiki/Adlam_script https://en.wikipedia.org/wiki/Takri_script https://en.wikipedia.org/wiki/Takri_script https://en.wikipedia.org/wiki/Pracalit_script https://en.wikipedia.org/wiki/Pracalit_script https://en.wikipedia.org/wiki/Pollard_script https://en.wikipedia.org/wiki/Pollard_script
- boomlinde 7y ago
  > Is there a language that consistently uses codepoints with more than 2 bytes? Kanji, hiragana, katakana etc. At least 3 bytes with some 4-byte sequences. This compares unfavorably to for example Shift-JIS.