Dan Gohman 107879735b Add new normalization algorithms using Standardized Variants ...

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

2020-12-06 08:18:20 -08:00

..

unicode.py

Add new normalization algorithms using Standardized Variants

2020-12-06 08:18:20 -08:00