why did amir laugh when he faught assef and what was ironic about how he was saved

I think the fight and beating that Amir took was a cathartic experience for Amir. He had carried the guilt of not helping his friend Hassan for most of his life. The beating he takes from Assef is a release from so much of that guilt. This beating should have happened in Amir's childhood. At least now he can take it for Hassan's son.