Fix bug where it was possible to submit blank text in various text fields.
Caused by `String#blank?` not considering certain Unicode characters as blank. `blank?` is defined
as `match?(/\A[[:space:]]*\z/)`, where `[[:space:]]` matches ASCII spaces (space, tab, newline, etc)
and Unicode characters in the Space category ([1]). However, there are other space-like characters
not in the Space category. This includes U+200B (Zero-Width Space), and many more.
It turns out the "Default ignorable code points" [2][3] are what we're after. These are the set of 400
or so formatting and control characters that are invisible when displayed.
Note that there are other control characters that aren't invisible when rendered, instead they're
shown with a placeholder glyph. These include the ASCII C0 and C1 control codes [4], certain Unicode
control characters [5], and unassigned, reserved, and private use codepoints.
There is one outlier: the Braille pattern blank (U+2800) [6]. This character is visually blank, but is
not considered to be a space or an ignorable code point.
[1]: https://codepoints.net/search?gc[]=Z
[2]: https://codepoints.net/search?DI=1
[3]: https://www.unicode.org/review/pr-5.html
[4]: https://codepoints.net/search?gc[]=Cc
[5]: https://codepoints.net/search?gc[]=Cf
[6]: https://codepoints.net/U+2800
[7]: https://en.wikipedia.org/wiki/Whitespace_character
[8]: https://character.construction/blanks
[9]: https://invisible-characters.com
This directory contains library code used through Danbooru. This includes things like defining API clients, dealing with
sources, parsing tag searches, storing and resizing images, and so on.
Many of the files here use the Service Object pattern. Instead of putting complex code in models or controllers, it goes
here, in plain old Ruby objects (POROs). This keeps models and controllers simpler, and keeps domain logic isolated and
independent from the database and the HTTP request cycle.
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