A 40-year-old woman suddenly developed severe headache, occipital pain, and vomiting for 8 hours, without fever. No history of hypertension. Physical examination: clear consciousness, mild hypertension. Right pupil dilated with absent pupillary light reflex, right upper eyelid ptosis, and inability to move the eye upward, downward, and inward. Nuchal rigidity, Kernig's sign (+). Brain CT shows hyperdense signals in the interhemispheric fissure, lateral sulcus, and basal cistern. The most likely diagnosis is: