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A WordPress-centric search engine for devs and theme authors

function get_template_hierarchy( $slug, $is_custom = false, $template_prefix = '' ) {
	if ( 'index' === $slug ) {
		/** This filter is documented in wp-includes/template.php */
		return apply_filters( 'index_template_hierarchy', array( 'index' ) );
	}
	if ( $is_custom ) {
		/** This filter is documented in wp-includes/template.php */
		return apply_filters( 'page_template_hierarchy', array( 'page', 'singular', 'index' ) );
	}
	if ( 'front-page' === $slug ) {
		/** This filter is documented in wp-includes/template.php */
		return apply_filters( 'frontpage_template_hierarchy', array( 'front-page', 'home', 'index' ) );
	}

	$matches = array();

	$template_hierarchy = array( $slug );
	// Most default templates don't have `$template_prefix` assigned.
	if ( ! empty( $template_prefix ) ) {
		list( $type ) = explode( '-', $template_prefix );
		// We need these checks because we always add the `$slug` above.
		if ( ! in_array( $template_prefix, array( $slug, $type ), true ) ) {
			$template_hierarchy[] = $template_prefix;
		}
		if ( $slug !== $type ) {
			$template_hierarchy[] = $type;
		}
	} elseif ( preg_match( '/^(author|category|archive|tag|page)-.+$/', $slug, $matches ) ) {
		$template_hierarchy[] = $matches[1];
	} elseif ( preg_match( '/^(taxonomy|single)-(.+)$/', $slug, $matches ) ) {
		$type           = $matches[1];
		$slug_remaining = $matches[2];

		$items = 'single' === $type ? get_post_types() : get_taxonomies();
		foreach ( $items as $item ) {
			if ( ! str_starts_with( $slug_remaining, $item ) ) {
					continue;
			}

			// If $slug_remaining is equal to $post_type or $taxonomy we have
			// the single-$post_type template or the taxonomy-$taxonomy template.
			if ( $slug_remaining === $item ) {
				$template_hierarchy[] = $type;
				break;
			}

			// If $slug_remaining is single-$post_type-$slug template.
			if ( strlen( $slug_remaining ) > strlen( $item ) + 1 ) {
				$template_hierarchy[] = "$type-$item";
				$template_hierarchy[] = $type;
				break;
			}
		}
	}
	// Handle `archive` template.
	if (
		str_starts_with( $slug, 'author' ) ||
		str_starts_with( $slug, 'taxonomy' ) ||
		str_starts_with( $slug, 'category' ) ||
		str_starts_with( $slug, 'tag' ) ||
		'date' === $slug
	) {
		$template_hierarchy[] = 'archive';
	}
	// Handle `single` template.
	if ( 'attachment' === $slug ) {
		$template_hierarchy[] = 'single';
	}
	// Handle `singular` template.
	if (
		str_starts_with( $slug, 'single' ) ||
		str_starts_with( $slug, 'page' ) ||
		'attachment' === $slug
	) {
		$template_hierarchy[] = 'singular';
	}
	$template_hierarchy[] = 'index';

	$template_type = '';
	if ( ! empty( $template_prefix ) ) {
		list( $template_type ) = explode( '-', $template_prefix );
	} else {
		list( $template_type ) = explode( '-', $slug );
	}
	$valid_template_types = array( '404', 'archive', 'attachment', 'author', 'category', 'date', 'embed', 'frontpage', 'home', 'index', 'page', 'paged', 'privacypolicy', 'search', 'single', 'singular', 'tag', 'taxonomy' );
	if ( in_array( $template_type, $valid_template_types, true ) ) {
		/** This filter is documented in wp-includes/template.php */
		return apply_filters( "{$template_type}_template_hierarchy", $template_hierarchy );
	}
	return $template_hierarchy;
}