wp_constrain_dimensions( int $current_width, int $current_height, int $max_width, int $max_height )

function wp_constrain_dimensions( $current_width, $current_height, $max_width = 0, $max_height = 0 ) {
	if ( !$max_width && !$max_height )
		return array( $current_width, $current_height );

	$width_ratio = $height_ratio = 1.0;
	$did_width = $did_height = false;

	if ( $max_width > 0 && $current_width > 0 && $current_width > $max_width ) {
		$width_ratio = $max_width / $current_width;
		$did_width = true;
	}

	if ( $max_height > 0 && $current_height > 0 && $current_height > $max_height ) {
		$height_ratio = $max_height / $current_height;
		$did_height = true;
	}

	// Calculate the larger/smaller ratios
	$smaller_ratio = min( $width_ratio, $height_ratio );
	$larger_ratio  = max( $width_ratio, $height_ratio );

	if ( (int) round( $current_width * $larger_ratio ) > $max_width || (int) round( $current_height * $larger_ratio ) > $max_height ) {
 		// The larger ratio is too big. It would result in an overflow.
		$ratio = $smaller_ratio;
	} else {
		// The larger ratio fits, and is likely to be a more "snug" fit.
		$ratio = $larger_ratio;
	}

	// Very small dimensions may result in 0, 1 should be the minimum.
	$w = max ( 1, (int) round( $current_width  * $ratio ) );
	$h = max ( 1, (int) round( $current_height * $ratio ) );

	// Sometimes, due to rounding, we'll end up with a result like this: 465x700 in a 177x177 box is 117x176... a pixel short
	// We also have issues with recursive calls resulting in an ever-changing result. Constraining to the result of a constraint should yield the original result.
	// Thus we look for dimensions that are one pixel shy of the max value and bump them up

	// Note: $did_width means it is possible $smaller_ratio == $width_ratio.
	if ( $did_width && $w == $max_width - 1 ) {
		$w = $max_width; // Round it up
	}

	// Note: $did_height means it is possible $smaller_ratio == $height_ratio.
	if ( $did_height && $h == $max_height - 1 ) {
		$h = $max_height; // Round it up
	}

	/**
	 * Filters dimensions to constrain down-sampled images to.
	 *
	 * @since WP-4.1.0
	 *
	 * @param array $dimensions     The image width and height.
	 * @param int 	$current_width  The current width of the image.
	 * @param int 	$current_height The current height of the image.
	 * @param int 	$max_width      The maximum width permitted.
	 * @param int 	$max_height     The maximum height permitted.
	 */
	return apply_filters( 'wp_constrain_dimensions', array( $w, $h ), $current_width, $current_height, $max_width, $max_height );
}