Global current_post counter, WP_Query agnostic

wp-global-post-count.php

<?php
/**
 * Create a globally accessible counter for all queries
 * Even custom new WP_Query! Include inside functions.php in your theme.
 * This allows you to have a post counter available in your sub-templates
 * accessible even if you use get_template_part();
 *
 * Example
 *
 * global $current_loop_post_index;
 * while(have_posts()): the_post();
 *     echo $current_loop_post_index; // Will output 0
 *     the_title();
 * 
 *     $custom_query = new WP_Query(array('post_type' => 'portfolio'));
 *     while($custom_query->have_posts()): $custom_query->the_post();
 *         echo $current_loop_post_index; // Will output 0, 1, 2, 3
 *         the_title();
 *     endwhile;
 * 
 *     echo $current_loop_post_index; // Will output 0 again
 * endwhile;
 */

/**
 * Initialize your variables
 */
add_action('init', function(){
    global $current_loop_post_index;
    $current_loop_post_index = -1;
});

/**
 * At loop start, always make sure the counter is -1
 * This is because WP_Query calls "next_post" for each post,
 * even for the first one, which increments by 1
 * (meaning the first post is going to be 0 as expected)
 */
add_action('loop_start', function($q){
    global $current_loop_post_index;
    $current_loop_post_index = -1;
}, 100, 1);

/**
 * At each iteration of a loop, this hook is called
 * We store the current instance's counter in our global variable
 */
add_action('the_post', function($p, $q){
    global $current_loop_post_index;
    $current_loop_post_index = $q->current_post;
}, 100, 2);

/**
 * At each end of the query, we clean up by setting the counter to
 * the global query's counter. This allows the custom $current_loop_post_index variable
 * to be set correctly in the main page, post or query, even after
 * having executed a custom WP_Query.
 */
add_action( 'loop_end', function($q){
    global $wp_query, $current_loop_post_index;
    $current_loop_post_index = $wp_query->current_post;
}, 100, 1);