bp_xprofile_get_member_display_name( int $user_id = null )

function bp_xprofile_get_member_display_name( $user_id = null ) {
	// some cases it calls the filter directly, therefore no user id is passed
	if ( ! $user_id ) {
		return false;
	}

	$format = bp_get_option( 'bp-display-name-format' );

	switch ( $format ) {
		case 'first_name':

			// Get First Name Field Id.
			$first_name_id = (int) bp_get_option( 'bp-xprofile-firstname-field-id' );

			$display_name = xprofile_get_field_data( $first_name_id, $user_id );

			if ( '' === $display_name ) {
				$display_name = get_user_meta( $user_id, 'first_name', true );
				if ( empty( $display_name ) ) {
					$display_name = get_user_meta( $user_id, 'nickname', true );
				}
				xprofile_set_field_data( $first_name_id, $user_id, $display_name );
			}

			// Get Nick Name Field Id.
			$nickname_id = (int) bp_get_option( 'bp-xprofile-nickname-field-id' );
			$nick_name   = xprofile_get_field_data( $nickname_id, $user_id );

			if ( '' === trim( $nick_name ) ) {
				$user = get_userdata( $user_id );
				// make sure nickname is valid
				$nickname = get_user_meta( $user_id, 'nickname', true );
				$nickname = sanitize_title( $nickname );
				$invalid  = bp_xprofile_validate_nickname_value( '', $nickname_id, $nickname, $user_id );

				// or use the user_nicename
				if ( ! $nickname || $invalid ) {
					$nickname = $user->user_nicename;
				}
				xprofile_set_field_data( $nickname_id, $user_id, $nickname );
			}

			break;
		case 'first_last_name':

			// Get First Name Field Id.
			$first_name_id = (int) bp_get_option( 'bp-xprofile-firstname-field-id' );
			// Get Last Name Field Id.
			$last_name_id      = (int) bp_get_option( 'bp-xprofile-lastname-field-id' );
			$result_first_name = xprofile_get_field_data( $first_name_id, $user_id );
			$result_last_name  = xprofile_get_field_data( $last_name_id, $user_id );

			if ( '' === $result_first_name ) {
				$result_first_name = get_user_meta( $user_id, 'first_name', true );
				if ( empty( $result_first_name ) ) {
					$result_first_name = get_user_meta( $user_id, 'nickname', true );
				}
				xprofile_set_field_data( $first_name_id, $user_id, $result_first_name );
			}

			if ( '' === $result_last_name ) {
				$result_last_name = get_user_meta( $user_id, 'last_name', true );
				xprofile_set_field_data( $last_name_id, $user_id, $result_last_name );
			}

			$display_name = implode( ' ',
				array_filter( [
					isset( $result_first_name ) ? $result_first_name : '',
					isset( $result_last_name ) ? $result_last_name : '',
				] ) );

			// Get Nick Name Field Id.
			$nickname_id = (int) bp_get_option( 'bp-xprofile-nickname-field-id' );
			$nick_name   = xprofile_get_field_data( $nickname_id, $user_id );

			if ( '' === trim( $nick_name ) ) {
				$user = get_userdata( $user_id );
				// make sure nickname is valid
				$nickname = get_user_meta( $user_id, 'nickname', true );
				$nickname = sanitize_title( $nickname );
				$invalid  = bp_xprofile_validate_nickname_value( '', $nickname_id, $nickname, $user_id );

				// or use the user_nicename
				if ( ! $nickname || $invalid ) {
					$nickname = $user->user_nicename;
				}
				xprofile_set_field_data( $nickname_id, $user_id, $nickname );
			}

			break;
		case 'nickname':

			// Get Nick Name Field Id.
			$nickname_id  = (int) bp_get_option( 'bp-xprofile-nickname-field-id' );
			$display_name = xprofile_get_field_data( $nickname_id, $user_id );

			if ( '' === trim( $display_name ) ) {
				$user = get_userdata( $user_id );
				// make sure nickname is valid
				$nickname = get_user_meta( $user_id, 'nickname', true );
				$nickname = sanitize_title( $nickname );
				$invalid  = bp_xprofile_validate_nickname_value( '', $nickname_id, $nickname, $user_id );

				// or use the user_nicename
				if ( ! $nickname || $invalid ) {
					$nickname = $user->user_nicename;
				}
				xprofile_set_field_data( $nickname_id, $user_id, $nickname );
				$display_name = $nickname;

			}
			break;
	}

	return apply_filters( 'bp_xprofile_get_member_display_name', trim( $display_name ), $user_id );
}