Unicode is a standard for coding multingual text. ISO has standardized a portion of Unicode as ISO646
Unicode is complex. Whereas ASCII defines 127 characters, Unicode defines 1,114,112 code points, and characters are composed of one or more code points. Unicode provides code charts, but doesn't stop there. The following things are also specified by Unicode:
RS: Until version 3.0, 16 bits (\u0000-\uFFFD: the "Basic Multilingual Plane", BMP) were sufficient for any Unicode. From 3.1, we must expect longer codes - up to 31 bits long, as specified in ISO 10646. Why 31 bits? Because that is the maximum that can be expressed in UTF-8: 6 bytes, omitting the taboo values \xFE and \xFF.
1111110a 10aaaaaa 10bbbbbb 10bbcccc 10ccccdd 10dddddd
, where small letters stand for "payload" bits of bytes a..d, highestmost has only 7 bits
comp.lang.tcl 2008-04:
Newsgroups: comp.lang.tcl From: [email protected] Date: Sat, 26 Apr 2008 11:55:45 -0700 (PDT) Local: Sat, Apr 26 2008 2:55 pm Subject: unicode - get character representation from \uxxx notation Hello, to show my problem see the following example: > set tcl_patchLevel 8.5.3b1 > set str "n\u00E4mlich" nämlich > set c 0xE4 > set str "n\\uformat %04.4X $chmlich" n\u00E4mlich How do I get the \u00E4 in the character representation let's say iso8859-1 ? > encoding convertto iso8859-1 $str Newsgroups: comp.lang.tcl From: [email protected] Date: Sat, 26 Apr 2008 14:21:27 -0700 (PDT) Local: Sat, Apr 26 2008 5:21 pm Subject: Re: unicode - get character representation from \uxxx notation To convert the hex number expressed as a string 0x00e4 to a Unicode character, use: format "%c" 0x00e4 You can then use encoding convertto to convert this to another encoding, e.g.: encoding convertto iso8859-1 format "%c" 0x00e4
LV 2008-07-08:
I've a request from a developer concerning whether Tcl is capable of handling characters larger than the Unicode BMP. His application was using tdom and it encountered the 𝒜 character, which is a script-A, unicode value 0x1D49C, which tdom reports it can't handle because it is limited to UTF-8 chars up to 3 bytes in length.
What do Tcl programmers do to properly process the longer characters?
Note this is in an enterprise setting. Finding a solution is critical in the publishing (web or print) arena.
RS 2008-07-09: Unicode out of BMP (> U+FFFF) requires a deeper rework of Tcl and Tk: we'd need 32 bit chars and/or surrogate pairs. UTF-8 at least can deal with 31-bit Unicodes by principle.
LV During July, 2008, there was some discussion in the TCT mailing list [L1 ] (let's see how long that URL lasts...) about ways that the Tcl code itself could evolve to handle things better. But for right now, users have to face either dealing with their wide unicode via a different programming language in some way (whether converting wide characters to some other similar character, using some sort of macro representation, etc.)
AMG, 2015: It's been seven years since the above discussion. What progress has been made?
tcl.h contains the comment:
Fast random access to characters is quite important, e.g. for regular expressions, so I don't see how standard UTF-16 meets Tcl's needs unless augmented by some kind of indexing mechanism. Maybe the thought is reduced performance is acceptable for strings outside the BMP due to their assumed rarity, though I hope for logarithmic rather than linear, perhaps with some caching to further optimize the common situation of the sought-for character indexes being near each other.
But this is kind of a worst-of-both-worlds sort of deal. If you're going to have to pay for variable-width representation, might as well go with UTF-8 rather than -16.
AMG: I invented a (hopefully) fast indexing scheme for UTF-8 strings, though it could certainly be adapted for UTF-16.
Instead of the current linear time UTF-16 conversion step, make an array storing the byte index of every nth character other than the first. During lookup, divide the sought-for character index by n, then subtract one to get the array slot which stores the byte index for the start of the segment. (No need to store the start of the first segment; it's always zero!) Then scan forward one character at a time, covering at most n-1 characters.
For best performance, let n be a compile-time constant power of 2. This allows all division and modulo operations to be implemented in terms of shifts and masks.
The most obvious optimization is to discard the indexing array if the byte count and the character count turn out to be equal. This means the string is ASCII, so no UTF-8 magic is required.
For compression, instead of byte_index, have the array store (byte_index-char_index), i.e. the number of UTF-8 continuation bytes preceding the segment. Upon lookup, add (char_index-(char_index%n)). This can reduce memory usage by letting strings with fewer continuation bytes use unsigned 8- or 16-bit array slots. This subtraction also makes the next optimization simpler.
Take the difference between the current and next array slot values (subtract n if not doing the above compression) to get the number of UTF-8 continuation bytes in the segment. During the scan, decrement this value as each is encountered. When the remaining count is zero, it's known that the remaining characters are one byte each, so finish the scan and jump straight to the character. If a segment is all ASCII to begin with, this optimization kicks in immediately.
Another potential optimization is to scan backwards from the start of the next segment (or end of string) if the character index modulo n is greater than some threshold. Probably should put the threshold at 3n/4 since backward UTF-8 scans have to be done one byte at a time, whereas forward scans are one whole character at a time. This can be combined with the above optimization.
Yet another optimization is to remember the result of the previous index lookup and scan relative to it if within n characters forward or whatever threshold backwards.
The maximum number of array slots is (byte_count/n)-1, so allocation can be done right away if the byte count is known but not the character count. Though if the string is merely NUL-terminated and not (byte)length-counted, then it's necessary to either make two passes through the string or to allocate an initial guess then geometrically grow the allocation if the guess is short.
Using the backwards scan optimization, the above could have instead started at b=f[i]=4 and c=n-(c&m)=2, then scanned backwards. Decrement b at each byte. At each non-continuation byte (i.e. (e[b]&0xc0)!=0x80), decrement c. (Be careful near the end of the string.)
If the continuation byte counting optimization is used, it's known that the segment contains only two continuation bytes because f[i]-f[i-1]=4-2=2. When characters 17 (á) and 20 (þ) are found, it's also known that no continuation bytes remain, so after þ, simply add the remaining c to b to get the final b.
I haven't thought as much about updating the fast seek index when modifying the string. I don't see why appending to the string would invalidate the already-computed index; simply add to the end. But inserting/replacing/deleting a substring would probably take linear time due to recomputing the index from the start of the edit. I don't really mind that though because this operation already takes linear time due to memmove().
AMG: How are combining characters handled? They seem to be treated as individual characters, and they're only combined in the display. Trouble with this is that the cursor can go between combining characters, along with similar problems like cutting a string in the middle of what's called a grapheme cluster.
I wish for a way to treat a character along with all its combining characters (a grapheme cluster) as a logical unit. For instance, [string length] would return the number of grapheme clusters, not the number of code points. New commands would have to be defined to pick apart a grapheme cluster into constituent code points. I imagine most programs will want grapheme clusters to be atomic.