Greeklish addresses for Greek titles

<?php

// Greek letters in the addresses (slugs) WordPress makes for new posts,
// pages, products and terms become Latin letters. Other text, other slugs and
// addresses that already exist are left as they are.
$greeklish = static function ( string $text ): string {
	return (string) preg_replace_callback(
		'/\p{Greek}+/u',
		static function ( array $m ): string {
			$word = strtr( mb_strtolower( $m[0], 'UTF-8' ), array( 'ου' => 'ou', 'ού' => 'ou' ) );
			return strtr(
				$word,
				array(
					'α' => 'a', 'ά' => 'a', 'β' => 'v', 'γ' => 'g', 'δ' => 'd', 'ε' => 'e', 'έ' => 'e',
					'ζ' => 'z', 'η' => 'i', 'ή' => 'i', 'θ' => 'th', 'ι' => 'i', 'ί' => 'i', 'ϊ' => 'i',
					'ΐ' => 'i', 'κ' => 'k', 'λ' => 'l', 'μ' => 'm', 'ν' => 'n', 'ξ' => 'x', 'ο' => 'o',
					'ό' => 'o', 'π' => 'p', 'ρ' => 'r', 'σ' => 's', 'ς' => 's', 'τ' => 't', 'υ' => 'y',
					'ύ' => 'y', 'ϋ' => 'y', 'ΰ' => 'y', 'φ' => 'f', 'χ' => 'ch', 'ψ' => 'ps', 'ω' => 'o',
					'ώ' => 'o',
				)
			);
		},
		$text
	);
};

// WordPress cuts %-encoded slugs at 200 characters (a few dozen Greek letters),
// so the title or name the slug was made from is kept to transliterate in full.
$source = '';
add_filter(
	'wp_insert_post_empty_content',
	static function ( $maybe_empty, $postarr = array() ) use ( &$source ) {
		$source = (string) ( $postarr['post_title'] ?? '' );
		return $maybe_empty;
	},
	10,
	2
);
add_filter(
	'pre_insert_term',
	static function ( $term ) use ( &$source ) {
		$source = is_string( $term ) ? $term : '';
		return $term;
	}
);

// A slug with Greek letters (WordPress stores them %-encoded) in Latin letters,
// or null when there's nothing to change.
$latin_slug = static function ( string $slug ) use ( $greeklish, &$source ): ?string {
	$decoded = urldecode( $slug );
	if ( ! preg_match( '/\p{Greek}/u', $decoded ) ) {
		return null;
	}
	if ( '' !== $source && '' !== $slug && str_starts_with( sanitize_title_with_dashes( $source, '', 'save' ), $slug ) ) {
		$decoded = $source; // The slug was made from this text: use all of it.
	}
	$latin = sanitize_title_with_dashes( $greeklish( $decoded ), '', 'save' );
	return '' === $latin ? null : $latin;
};

// Posts, pages, products…: when WordPress makes a post's address unique.
$post_slug = null;
$post_slug = static function ( $slug, $post_id = 0, $status = '', $type = '', $parent = 0 ) use ( $latin_slug, &$post_slug ) {
	if ( $post_id && '' !== (string) get_post_field( 'post_name', $post_id ) && get_post_field( 'post_name', $post_id ) === $slug ) {
		return $slug; // An address the post already has stays.
	}
	$latin = $latin_slug( (string) $slug );
	if ( null === $latin ) {
		return $slug;
	}
	remove_filter( 'wp_unique_post_slug', $post_slug, 10 );
	$slug = wp_unique_post_slug( $latin, $post_id, $status, $type, $parent );
	add_filter( 'wp_unique_post_slug', $post_slug, 10, 5 );
	return $slug;
};
add_filter( 'wp_unique_post_slug', $post_slug, 10, 5 );

// Categories, tags and other terms: when WordPress makes a new term's slug unique.
$term_slug = null;
$term_slug = static function ( $slug, $term = null ) use ( $latin_slug, &$term_slug ) {
	$latin = $latin_slug( (string) $slug );
	if ( null === $latin || ! is_object( $term ) ) {
		return $slug;
	}
	remove_filter( 'wp_unique_term_slug', $term_slug, 10 );
	$slug = wp_unique_term_slug( $latin, $term );
	add_filter( 'wp_unique_term_slug', $term_slug, 10, 2 );
	return $slug;
};
add_filter( 'wp_unique_term_slug', $term_slug, 10, 2 );