Trainee pilot on a solo flight reported a NMAC with a helicopter while in the pattern at a non towered airport.

Narrative

I was in pattern at ZZZ; turning downwind. Aircraft Y made a call that they were 3 miles out to cross midfield at 1800. I was watching for them so I made a call that I was entering downwind. I was trying to watch for them because I knew they would be crossing near my location. Seconds later I see them at my wing tip as they start to make a call 'crossing midfield...' they're at my altitude and about 500ft away 2'oclock. They continue their radio call so I can't call my position; so I pulled power and descend below them. Aircraft Y finished their radio call as I'm diving below them. The call was something along the lines of ...1800 ft crossing above the aircraft on final' I wasn't on final; but I was the only one in pattern. I'm sure they thought it was final because of my rapid descend. I leveled out and returned to pattern altitude after reaching about 1500ft. Company A regularly does this; but never this close to me. I had actually just spoken to someone in the FBO about this stuff happening with Company A yesterday. I am 100% that had I not executed an emergency descend while in downwind leg; midfield; I would have been struck by Aircraft Y. They need to either be aware of traffic or cross outside of traffic altitude (and still be aware of traffic). Company A often cruises through pattern and somewhat disrupts the flow; but we generally try to take into consideration their purpose. This time however; they had full disregard for my aircraft established in pattern and chose to fly directly toward me; forcing me out of pattern.