A 36-year-old male presents with abdominal distension, pain, nausea, and vomiting for 1 week. The vomitus is copious, contains foul-smelling undigested food from the previous day, and lacks bile. Symptoms temporarily improve after vomiting. He has a history of intermittent epigastric pain for over 6 years, typically occurring on an empty stomach or at night, with seasonal autumn exacerbations. Physical examination reveals mild epigastric tenderness, visible gastric peristalsis, and a positive succussion splash. The most likely diagnosis is: