Inpsyde_Autoloader.php

<?php
spl_autoload_register( function ( $class_name ) {

    $camelcase_to_snakecase = function ( $string ) {
		return preg_replace( '/([a-z])([A-Z])/', '$1_$2', $string );
	};

	$split  = explode( '\\\\', $class_name );
	// remove "Inpsyde" namespace
	$vendor = array_shift( $split );

	if ( ! strlen( $vendor ) )
		$vendor = array_shift( $split );
	
	// only load classes prefixed inside "Inpsyde" namespace
	if ( $vendor != "Inpsyde" )
		return false;

	// remove plugin namespace as it is not represented as dir
	array_shift( $split );
	
	// class name without namespace
	$class_name = array_pop( $split );
	// CamelCase to snake_case
	$class_name = $camelcase_to_snakecase( $class_name );

	// the rest of the namespace, if any
	$namespaces = $split;

	// library directory
	$base_dir = dirname( dirname( __FILE__ ) );
	$libs = array(
		$base_dir . '/inc/',
		$base_dir . '/lib/'
	);

	$class_file_name = "class-$class_name.php";
	
	// register all possible paths for the class
	$possibilities = array();
	foreach ( $libs as $lib ) {
		if ( count( $namespaces ) >= 1 ) {
			$possibilities[] = $lib . strtolower( implode( '/', array_map( $camelcase_to_snakecase, $namespaces ) ) . '/' . $class_file_name );
		} else {
			$possibilities[] = $lib . strtolower( $class_file_name );
		}
	}

	// search for the class
	foreach ( $possibilities as $file ) {
		if ( file_exists( $file ) ) {
			require_once( $file );
			return true;
		}
	}
	
	return false;
} );