Hangul Syllables and several other ranges are defined in UnicodeData.txt
as just their first and last values:
```
AC00;<Hangul Syllable, First>;Lo;0;L;;;;;N;;;;;
D7A3;<Hangul Syllable, Last>;Lo;0;L;;;;;N;;;;;
```
Teach the unicode.py script how to recognize these, so that it correctly
classifies them as assigned ranges, for the `is_public_assigned`
predicate.
Add an `is_public_assigned` predicate, which tests whether a given
`char` is assigned (`General_Category` != `Unassigned`) in the currently
supported version of Unicode, and not Private-Use (`General_Category`
!= `Private_Use`).
This comes up in some use cases sensitive to the stability of NFC over
Unicode version changes. An unassigned codepoint could become assigned in
the future, and new normalizations could apply to it.
For further details, see
- <https://unicode.org/reports/tr15/#Versioning>
Switch to a dedicated `svar()` iterator function, which just does
standardized variation sequences, rather than framing this functionality
as an open-ended "extended" version of the standard normalization
algorithms. This makes for a more factored API, gives users more control
over exactly what transformations are done, and has less impact on users
that don't need this new functionality.
The standard normalization algorithm decomposes CJK compatibility ideographs
into nominally equivalent codepoints, but which traditionally look different,
and is one of the main reasons normalization is considered destructive in
practice.
[Unicode 6.3] introduced a solution for this, by providing
[standardized variation sequences] for these codepoints. For example, while
U+2F8A6 "CJK COMPATIBILITY-IDEOGRAPH-2F8A6" canonically decomposes to U+6148
with a different appearance, in Unicode 6.3 and later the standardized variation
sequences in the StandardizedVariants.txt file include the following:
> 6148 FE00; CJK COMPATIBILITY IDEOGRAPH-2F8A6;
which says that "CJK COMPATIBILITY IDEOGRAPH-2F8A6" corresponds to
U+6148 U+FE00, where U+FE00 is "VARIATION SELECTOR-1".
U+6148 and U+FE00 are both normalized codepoints, so we can transform text
containing U+2F8A6 into normal form without losing information about the
distinct appearance. At this time, many popular implementations ignore these
variation selectors, however this technique at least preserves the information
in a standardized way, so implementations could use it if they chose.
This PR adds "ext" versions of the `nfd`, `nfc`, `nfkd`, and `nkfd`
iterators, which perform the standard algorithms extended with this technique.
They don't match the standard decompositions, and don't guarantee stability,
but they do produce appropriately normalized output.
I used the generic term "ext" to reflect that other extensions could
theoretically be added in the future. The standard decomposition tables are
limited by their stability requirements, but these "ext" versions could be
free to adopt new useful rules.
I'm not an expert in any of these topics, so please correct me if I'm mistaken
in any of this. Also, I'm open to ideas about how to best present this
functionality in the API.
[Unicode 6.3]: https://www.unicode.org/versions/Unicode6.3.0/#Summary
[standardized variation sequences]: http://unicode.org/faq/vs.html
Once the decompose iterator sees a starter, it should immediately start
returning characters from the preceeding sequence. If the input happens
to be stream-safe, it should never get more than MAX_NONSTARTERS + plus
boundary values ahead of its inner iterator.
Having the Some() constructor in each match arm body puts extras stress
on rustc's item_bodies_checking pass. We can work around that by moving
the Some() constructor around the match, and directly returning None
from the default arm, which is the only one that generates a None value.
On my box, this almost cuts the time spent in item_bodies_checking in
half, going from about 8.7s to about 4.6s, and reduces the complete
compile time from about 13s to about 9s, so about a third less.
There are no changes in performance in `cargo bench`.
Note that doing the same for the composition_table() function does not
yield any compile time wins, but would cause a performance regression.
cc #29
Jacob Kiesel