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//usr/share/mysql-8.4/charsets/README

This directory holds configuration files that enable MySQL to work with
different character sets.  It contains:

charset_name.xml
    Each charset_name.xml file contains information for a simple character
    set.  The information in the file describes character types,
    lower- and upper-case equivalencies and sorting orders for the
    character values in the set.

Index.xml
    The Index.xml file lists all of the available charset configurations,
    including collations.

Each collation must have a unique number.  The number is stored
    IN THE DATABASE TABLE FILES and must not be changed.

The max-id attribute of the <charsets> element must be set to
    the largest collation number.

Compiled in or configuration file?
    When should a character set be compiled in to MySQL's string library
    (libmystrings), and when should it be placed in a charset_name.xml
    configuration file?

If the character set requires the strcoll functions or is a
    multi-byte character set, it MUST be compiled in to the string
    library.  If it does not require these functions, it should be
    placed in a charset_name.xml configuration file.

If the character set uses any one of the strcoll functions, it
    must define all of them.  Likewise, if the set uses one of the
    multi-byte functions, it must define them all.  See the manual for
    more information on how to add a complex character set to MySQL.

Syntax of configuration files
    The syntax is very simple.  Words in <map> array elements are
    separated by arbitrary amounts of whitespace. Each word must be a
    number in hexadecimal format.  The ctype array has 257 words; the
    other arrays (lower, upper, etc.) take up 256 words each after that.