Reading time and automatic table of contents

<?php
/**
 * Reading time and table of contents.
 *
 * @package Acme_Theme
 */

defined( 'ABSPATH' ) || exit;

function acme_reading_time( int $post_id = 0 ): int {
	$content = get_post_field( 'post_content', $post_id ?: get_the_ID() );
	$words   = str_word_count( wp_strip_all_tags( strip_shortcodes( (string) $content ) ) );
	return max( 1, (int) ceil( $words / 220 ) );
}

function acme_add_toc( string $content ): string {
	if ( ! is_singular( 'post' ) || ! in_the_loop() || ! is_main_query() ) {
		return $content;
	}

	$items = array();
	$used  = array();

	$content = preg_replace_callback(
		'/<h([23])([^>]*)>(.*?)<\/h\1>/is',
		static function ( array $m ) use ( &$items, &$used ): string {
			$text = wp_strip_all_tags( $m[3] );
			$id   = sanitize_title( $text ) ?: 'section';
			$base = $id;
			for ( $n = 2; isset( $used[ $id ] ); $n++ ) {
				$id = $base . '-' . $n;
			}
			$used[ $id ] = true;
			$items[]     = array( 'id' => $id, 'text' => $text, 'level' => (int) $m[1] );
			return sprintf( '<h%1$d%2$s id="%3$s">%4$s</h%1$d>', $m[1], $m[2], esc_attr( $id ), $m[3] );
		},
		$content
	);

	if ( count( $items ) < 3 ) {
		return $content;
	}

	$list = '';
	foreach ( $items as $item ) {
		$list .= sprintf(
			'<li class="acme-toc__l%1$d"><a href="#%2$s">%3$s</a></li>',
			$item['level'],
			esc_attr( $item['id'] ),
			esc_html( $item['text'] )
		);
	}

	$toc = sprintf(
		'<nav class="acme-toc" aria-label="%1$s"><p class="acme-toc__title">%2$s</p><ol>%3$s</ol></nav>',
		esc_attr__( 'Table of contents', 'acme-theme' ),
		/* translators: %d: minutes */
		esc_html( sprintf( _n( '%d min read', '%d min read', acme_reading_time(), 'acme-theme' ), acme_reading_time() ) ),
		$list
	);

	return $toc . $content;
}
add_filter( 'the_content', 'acme_add_toc' );