1	import Base
2	
3	# Describes which source heads advance and whether a visible record is emitted.
4	type Decision is Data:
5	  Finished{}
6	  EmitBase{}
7	  EmitDelta{}
8	  EmitBoth{}
9	  SkipDelta{}
10	  SkipBoth{}
11	
12	# Emits a delta value or advances past a delta tombstone without a base head.
13	def delta.without.base(live: Bool) -> Decision:
14	  match live:
15	    case True{}: EmitDelta{}
16	    case False{}: SkipDelta{}
17	
18	# Emits an SST head when no newer delta head is available.
19	def base.without.delta() -> Decision:
20	  EmitBase{}
21	
22	# Applies ordering and tombstone precedence when both sources have a head.
23	def both.live(live: Bool, order: Cmp) -> Decision:
24	  match live:
25	    case True{}:
26	      match order:
27	        case LT{}: EmitDelta{}
28	        case EQ{}: EmitBoth{}
29	        case GT{}: EmitBase{}
30	    case False{}:
31	      match order:
32	        case LT{}: SkipDelta{}
33	        case EQ{}: SkipBoth{}
34	        case GT{}: EmitBase{}
35	
36	# Exposes the two-head comparison transition for the top-level selector.
37	def both(live: Bool, order: Cmp) -> Decision:
38	  both.live(live, order)
39	
40	# Selects the next merge transition for sorted SST and delta heads.
41	def choose(
42	  base_exists: Bool,
43	  delta_exists: Bool,
44	  delta_live: Bool,
45	  order: Cmp
46	) -> Decision:
47	  match base_exists:
48	    case True{}:
49	      match delta_exists:
50	        case True{}: both(delta_live, order)
51	        case False{}: base.without.delta()
52	    case False{}:
53	      match delta_exists:
54	        case True{}: delta.without.base(delta_live)
55	        case False{}: Finished{}