Source

File: wp-includes/class-wp-query.php

public function generate_postdata( $post ) {

		if ( ! ( $post instanceof WP_Post ) ) {
			$post = get_post( $post );
		}

		if ( ! $post ) {
			return false;
		}

		$id = (int) $post->ID;

		$authordata = get_userdata( $post->post_author );

		$currentday   = mysql2date( 'd.m.y', $post->post_date, false );
		$currentmonth = mysql2date( 'm', $post->post_date, false );
		$numpages     = 1;
		$multipage    = 0;
		$page         = $this->get( 'page' );
		if ( ! $page ) {
			$page = 1;
		}

		/*
		 * Force full post content when viewing the permalink for the $post,
		 * or when on an RSS feed. Otherwise respect the 'more' tag.
		 */
		if ( get_queried_object_id() === $post->ID && ( $this->is_page() || $this->is_single() ) ) {
			$more = 1;
		} elseif ( $this->is_feed() ) {
			$more = 1;
		} else {
			$more = 0;
		}

		$content = $post->post_content;
		if ( false !== strpos( $content, '<!--nextpage-->' ) ) {
			$content = str_replace( "\n<!--nextpage-->\n", '<!--nextpage-->', $content );
			$content = str_replace( "\n<!--nextpage-->", '<!--nextpage-->', $content );
			$content = str_replace( "<!--nextpage-->\n", '<!--nextpage-->', $content );

			// Remove the nextpage block delimiters, to avoid invalid block structures in the split content.
			$content = str_replace( '<!-- wp:nextpage -->', '', $content );
			$content = str_replace( '<!-- /wp:nextpage -->', '', $content );

			// Ignore nextpage at the beginning of the content.
			if ( 0 === strpos( $content, '<!--nextpage-->' ) ) {
				$content = substr( $content, 15 );
			}

			$pages = explode( '<!--nextpage-->', $content );
		} else {
			$pages = array( $post->post_content );
		}

		/**
		 * Filters the "pages" derived from splitting the post content.
		 *
		 * "Pages" are determined by splitting the post content based on the presence
		 * of `<!-- nextpage -->` tags.
		 *
		 * @since 4.4.0
		 *
		 * @param string[] $pages Array of "pages" from the post content split by `<!-- nextpage -->` tags.
		 * @param WP_Post  $post  Current post object.
		 */
		$pages = apply_filters( 'content_pagination', $pages, $post );

		$numpages = count( $pages );

		if ( $numpages > 1 ) {
			if ( $page > 1 ) {
				$more = 1;
			}
			$multipage = 1;
		} else {
			$multipage = 0;
		}

		$elements = compact( 'id', 'authordata', 'currentday', 'currentmonth', 'page', 'pages', 'multipage', 'more', 'numpages' );

		return $elements;
	}