WooCommerce get product default variation id (most efficient way)

wc-variation.php

<?php

/**
 * Get default product variation
 *
 * note: this method check for exact match.
 *
 * @param mixed  $product_id WC_Product|WP_Post|int|bool $product Product instance, post instance, numeric or false to use global $post.
 * @param string $return Optional. The format to return the results in. Can be 'id' to return id of variation or 'object' for the product variation object. Default 'id'.
 *
 * @return int|WC_Product_Variation return 0 if not found.
 */
function wc_get_default_variation( $product_id = false, $return = 'id' ) {
	// do not use wc_get_product() to bypass some limits
	$product = WC()->product_factory->get_product( $product_id );

	if ( empty( $product ) || ! $product instanceof WC_Product_Variable ) {
		return 0;
	}

	if ( $product->has_child() ) {
		$attributes = $product->get_default_attributes();

		if ( ! empty( $attributes ) ) {
			$check_count      = true;
			$attributes_count = count( $attributes );

			// get in-stock (if enabled in wc options) and visible variations
			$variations = $product->get_available_variations( 'objects' );

			foreach ( $variations as $variation ) {
				$variation_attributes = $variation->get_attributes();

				// check count for first time
				// if not match, it means that user do not set default value for some variation attrs
				if ( $check_count && $attributes_count !== count( $variation_attributes ) ) {
					break;
				}

				// no need to check count anymore
				$check_count = false;

				// remove 'any' attrs (empty values)
				$variation_attributes = array_filter( $variation_attributes );

				// add 'any' attrs with default value
				$variation_attributes = wp_parse_args( $variation_attributes, $attributes );

				// check is default
				if ( $variation_attributes == $attributes ) {
					if ( $return === 'id' ) {
						return $variation->get_id();
					}

					return $variation;
				}
			}
		}
	}

	return 0;
}