CustomUppercaseCollation.php

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<?php
class CustomUppercaseCollation extends NumericUppercaseCollation {
 
    private $alphabet;
 
    private $puaSubset;
 
    public function __construct( array $alphabet, Language $lang ) {
        // It'd be trivial to extend this past 64, you'd just
        // need a bit of bit-fiddling. Doesn't seem necessary right
        // now.
        if ( count( $alphabet ) < 1 || count( $alphabet ) >= 64 ) {
            throw new UnexpectedValueException( "Alphabet must be < 64 items" );
        }
        $this->alphabet = $alphabet;
 
        $this->puaSubset = [];
        $len = count( $alphabet );
        for ( $i = 0; $i < $len; $i++ ) {
            $this->puaSubset[] = "\xF3\xB3\x80" . chr( $i + 128 );
        }
        parent::__construct( $lang );
    }
 
    private function convertToPua( $string ) {
        return str_replace( $this->alphabet, $this->puaSubset, $string );
    }
 
    public function getSortKey( $string ) {
        return $this->convertToPua( parent::getSortKey( $string ) );
    }
 
    public function getFirstLetter( $string ) {
        // In case a title has a PUA code in it, make it sort
        // under the header for the character it would replace
        // to avoid inconsistent behaviour. This class mostly
        // assumes that people will not use PUA codes.
        return parent::getFirstLetter(
            str_replace( $this->puaSubset, $this->alphabet, $string )
        );
    }
}