Previously, each code point's General Category was part of the generated
UnicodeData structure. This ultimately presented two problems, one
functional and one performance related:
* Some General Categories are applied to unassigned code points, for
example the Unassigned (Cn) category. Unassigned code points are
strictly excluded from UnicodeData.txt, so by relying on that file,
the generator is unable to handle these categories.
* Lookups for General Categories are slower when searching through the
large UnicodeData hash map. Even though lookups are O(1), the hash
function turned out to be slower than binary searching through a
category-specific table.
So, now a table is generated for each General Category. When querying a
code point for a category, a binary search is done on each code point
range in that category's table to check if code point has that category.
Further, General Categories are now parsed from the UCD file
DerivedGeneralCategory.txt. This file is a normal "prop list" file and
contains the categories for unassigned code points.
Apparently, some code points fit both categories, for example U+0345
(COMBINING GREEK YPOGEGRAMMENI). Handle this fact when determining if
a code point is a final code point in a string.
This implements unconditional special case folding, and conditional
folding for non-locale cases. Worth noting that the only conditional,
non-locale special case is for converting an uppercase sigma to
lowercase.
The Unicode standard publishes the Unicode Character Database (UCD) with
information about every code point, such as each code point's upper case
mapping. LibUnicode exists to download and parse UCD files at build time
and to provide accessors to that data.
As a start, LibUnicode includes upper- and lower-case code point
converters.
