1	import Base
2	import ./Keys.bend as Keys
3	import ./MemTable.bend as MemTable
4	import ./Sstable.bend as Sstable
5	import ./SortedRun.bend as SortedRun
6	
7	# Merge iterator (Task 6): concat sources (memtable newest-first, then
8	# SSTables newest-level-first) and canonicalize with balanced stable merge;
9	# first-inserted wins ties = newest wins). The range+tombstone filter is
10	# pump+fuel+leaf: String.cmp hands both strings back beside the verdict,
11	# so the key survives the keep/drop decision with no clone primitive.
12	# Order of defs is load-bearing (Bend 2: only backward references).
13	
14	def scan_put2(pair: (String & String) & Cmp, +val: String, acc: List<&2, MemTable.Entry>) -> List<&2, MemTable.Entry>:
15	  match pair:
16	    case ((k3, _), c):
17	      match c:
18	        case LT{}:
19	          Con{MemTable.Entry{k3, Some{val}}, acc}
20	        case EQ{}:
21	          acc
22	        case GT{}:
23	          acc
24	
25	def scan_put(pair: (String & String) & Cmp, +hi: String, +val: String, acc: List<&2, MemTable.Entry>) -> List<&2, MemTable.Entry>:
26	  match pair:
27	    case ((k2, _), c):
28	      match c:
29	        case LT{}:
30	          acc
31	        case EQ{}:
32	          scan_put2(String.cmp(k2, hi), val, acc)
33	        case GT{}:
34	          scan_put2(String.cmp(k2, hi), val, acc)
35	
36	def scan_go(fuel: Nat, +lo: String, +hi: String, acc: List<&2, MemTable.Entry>, xs: List<&2, MemTable.Entry>) -> List<&2, MemTable.Entry>:
37	  match fuel:
38	    case 0n:
39	      List.reverse(&2, MemTable.Entry, acc)
40	    case 1n+f:
41	      match xs:
42	        case Nil{}:
43	          List.reverse(&2, MemTable.Entry, acc)
44	        case Con{MemTable.Entry{key, val}, t}:
45	          match val:
46	            case None{}:
47	              scan_go(f, lo, hi, acc, t)
48	            case Some{v}:
49	              scan_go(f, lo, hi, scan_put(String.cmp(key, lo), hi, v, acc), t)
50	
51	# Newest-first concat: memtable entries, then each level's tables in order.
52	def flatten(mem: List<&2, MemTable.Entry>, levels: List<&2, List<&2, MemTable.Entry>>) -> List<&2, MemTable.Entry>:
53	  match levels:
54	    case Nil{}:
55	      mem
56	    case Con{h, t}:
57	      List.append(&2, MemTable.Entry, mem, List.concat(&2, MemTable.Entry, Con{h, t}))
58	
59	# Full merge: sorted, unique, newest version wins each key.
60	def merge(mem: List<&2, MemTable.Entry>, levels: List<&2, List<&2, MemTable.Entry>>) -> List<&2, MemTable.Entry>:
61	  SortedRun.sort_newest(flatten(mem, levels))
62	
63	# Range scan over a merged run: lo <= key < hi, tombstones dropped.
64	def scan(+merged: List<&2, MemTable.Entry>, lo: String, hi: String) -> List<&2, MemTable.Entry>:
65	  scan_go(List.length(&2, MemTable.Entry, merged), lo, hi, Nil{}, merged)
66	
67	# --- Closed-vector laws live in laws/MergeIter.bend (spec laws 4, 5, 10) ---