count_users( string $strategy = 'time', int|null $site_id = null )

function count_users( $strategy = 'time', $site_id = null ) {
	global $wpdb;

	// Initialize
	if ( ! $site_id ) {
		$site_id = get_current_blog_id();
	}
	$blog_prefix = $wpdb->get_blog_prefix( $site_id );
	$result = array();

	if ( 'time' == $strategy ) {
		if ( is_multisite() && $site_id != get_current_blog_id() ) {
			switch_to_blog( $site_id );
			$avail_roles = wp_roles()->get_names();
			restore_current_blog();
		} else {
			$avail_roles = wp_roles()->get_names();
		}

		// Build a CPU-intensive query that will return concise information.
		$select_count = array();
		foreach ( $avail_roles as $this_role => $name ) {
			$select_count[] = $wpdb->prepare( "COUNT(NULLIF(`meta_value` LIKE %s, false))", '%' . $wpdb->esc_like( '"' . $this_role . '"' ) . '%');
		}
		$select_count[] = "COUNT(NULLIF(`meta_value` = 'a:0:{}', false))";
		$select_count = implode(', ', $select_count);

		// Add the meta_value index to the selection list, then run the query.
		$row = $wpdb->get_row( "
			SELECT {$select_count}, COUNT(*)
			FROM {$wpdb->usermeta}
			INNER JOIN {$wpdb->users} ON user_id = ID
			WHERE meta_key = '{$blog_prefix}capabilities'
		", ARRAY_N );

		// Run the previous loop again to associate results with role names.
		$col = 0;
		$role_counts = array();
		foreach ( $avail_roles as $this_role => $name ) {
			$count = (int) $row[$col++];
			if ($count > 0) {
				$role_counts[$this_role] = $count;
			}
		}

		$role_counts['none'] = (int) $row[$col++];

		// Get the meta_value index from the end of the result set.
		$total_users = (int) $row[$col];

		$result['total_users'] = $total_users;
		$result['avail_roles'] =& $role_counts;
	} else {
		$avail_roles = array(
			'none' => 0,
		);

		$users_of_blog = $wpdb->get_col( "
			SELECT meta_value
			FROM {$wpdb->usermeta}
			INNER JOIN {$wpdb->users} ON user_id = ID
			WHERE meta_key = '{$blog_prefix}capabilities'
		" );

		foreach ( $users_of_blog as $caps_meta ) {
			$b_roles = maybe_unserialize($caps_meta);
			if ( ! is_array( $b_roles ) )
				continue;
			if ( empty( $b_roles ) ) {
				$avail_roles['none']++;
			}
			foreach ( $b_roles as $b_role => $val ) {
				if ( isset($avail_roles[$b_role]) ) {
					$avail_roles[$b_role]++;
				} else {
					$avail_roles[$b_role] = 1;
				}
			}
		}

		$result['total_users'] = count( $users_of_blog );
		$result['avail_roles'] =& $avail_roles;
	}

	return $result;
}