While I was setting up the wiki, I realized some problems with non-English letters, such as ö. Therefore I had to find out more details about Unicode and encoding (a task that does not happen frequently if you program in languages such as Fortran, where you normally have other class of problems to handle). I found this very interesting page on JoelOnSoftware, and another FAQ for Unix/Linux.
Basically, if I understood correctly, everything can be summed up to the following:
- Unicode is a standard that define how to handle all the conceivable symbols (roman letters, numbers, Japanese and Chinese ideograms, arabic letters etc...). Actually, there is both ISO 10646 and Unicode, and they are not the same: Unicode does a lot more. So Unicode is a superset of ISO 10646, but for our discussion we will talk about Unicode even if talking about ISO 10646 would suffice just because is easier to type.
- Unicode assigns to every symbol a given number, the code point. For example, the letter A is assigned to code point 0x41, and 食 (taberu), which is the japanese character for the verb "to eat", has code point 0x98DF. The code point set can be eventually expanded to include any symbol, even those from invented languages (although unofficially, Tolkien's Tengwar and Klingon are included).
- The code point specifies an abstract symbol, not a way to draw that symbol. How this symbol is drawn is a task involving the font, which is used to produce a concrete glyph representation for each code point it covers.
- To write code points into files or in memory, you have to use an encoding, basically a technique to represent the code point value in practice. There is an insane amount of encodings, but they can be classified in two rough categories: fixed length encodings and variable length encodings.
- In fixed length encodings, every symbol require a fixed number of bytes, say 4. This is the approach UTF-32 uses: every code point requires four bytes to be written, always. This solution increases the space occupation four-fold even if you are only using plain English characters. Moreover, it is not backward compatible, so you cannot read a pure English text encoded in UTF-32 with cat. The advantage of this solution is that you know how much you have to skip to jump to the next or previous character, four bytes, and you know very easily how many characters you have: the occupied size divided by four.
- In variable length encodings, whose major representative is UTF-8, the number of bytes needed to encode a code point depends on the code point itself. For code points in the interval 0x0-0x7F, UTF-8 uses just one byte. Code points between 0x80–0x7FF will need two bytes, code points between 0x800–0xD7FF will require three bytes and so on. With this approach, simple English text will continue to be small and backward compatible, because each code point will be represented with a single byte, and this code point is mapped as in ASCII. Another important advantage is that the space occupation is of course more efficient. The disadvantage is that it is more difficult to go forward or backward one symbol: in C, buf++ will not work, because you could instead jump in the middle of a two-byte encoded character. With a fixed length encoding you know how many bytes you have to skip. With UTF-8 you have to parse the code point representation, and then determine how many bytes to move. Normally, however, proper API is provided to take care of these issues once and for all.
The problems I had with the ö letter arised from a wrong encoding WikkaWiki was assuming: it specified the document as iso-8859-1 encoded. ISO-8859-1 (also known as Latin-1) is basically a single-byte encoding like ASCII, where values from 0x80 to 0xFF map to most European symbols. However, the data I wrote was UTF-8 encoded, and this introduced a mismatch between the data and the decoding performed by the browser (in ISO-8859-1, as specified in the HTML). Since ö has code point U+00F6 (binary 11110110), this resulted in a two-byte UTF-8 encoding of the type 110yyyyy 10zzzzzz: 11000011 10110110, or 0xC3 0xB6. Each of these two bytes was interpreted as a single-byte ISO-8859-1 value, leading to a weird Ã¶ (capital A tilde and pilcrow end paragraph) instead of an ö (o with umlaut).
To solve, I basically changed the default meta tag content in WikkaWiki, from
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
and now it works correctly.