feat: initial BaraDB — multimodal database engine in Nim

- LSM-Tree storage engine with WAL, bloom filter, MemTable
- BaraQL query language: lexer (80+ tokens), recursive descent parser, AST
- Vector engine: HNSW + IVF-PQ indexes, 4 distance metrics
- Graph engine: adjacency list, BFS/DFS, Dijkstra, PageRank
- Full-Text Search: inverted index, BM25 ranking, stemming, stop words
- Type system: 17 types (int/float/string/uuid/json/vector/...)
- Async TCP server
- 21 passing tests

src/barabadb/storage/wal.nim

@@ -0,0 +1,84 @@
+## WAL — Write-Ahead Log for durability
+import std/os
+import std/streams
+
+const
+  WALMagic* = 0x42415241'u32  # "BARA"
+  WALVersion* = 1'u32
+
+type
+  WalEntryKind* = enum
+    wekPut = 1
+    wekDelete = 2
+    wekCheckpoint = 3
+    wekCommit = 4
+
+  WalEntry* = object
+    kind*: WalEntryKind
+    timestamp*: uint64
+    key*: seq[byte]
+    value*: seq[byte]
+
+  WriteAheadLog* = object
+    path: string
+    stream: FileStream
+    entryCount: uint64
+    syncOnWrite: bool
+
+proc newWriteAheadLog*(dir: string, syncOnWrite: bool = true): WriteAheadLog =
+  createDir(dir)
+  let path = dir / "wal.log"
+  let stream = newFileStream(path, fmWrite)
+  if stream == nil:
+    raise newException(IOError, "Cannot open WAL: " & path)
+  stream.write(WALMagic)
+  stream.write(WALVersion)
+  stream.flush()
+  WriteAheadLog(path: path, stream: stream, entryCount: 0, syncOnWrite: syncOnWrite)
+
+proc writeEntry*(wal: var WriteAheadLog, entry: WalEntry) =
+  wal.stream.write(uint8(entry.kind))
+  wal.stream.write(entry.timestamp)
+  wal.stream.write(uint32(entry.key.len))
+  if entry.key.len > 0:
+    wal.stream.writeData(unsafeAddr entry.key[0], entry.key.len)
+  wal.stream.write(uint32(entry.value.len))
+  if entry.value.len > 0:
+    wal.stream.writeData(unsafeAddr entry.value[0], entry.value.len)
+  if wal.syncOnWrite:
+    wal.stream.flush()
+  inc wal.entryCount
+
+proc writePut*(wal: var WriteAheadLog, key, value: openArray[byte], timestamp: uint64) =
+  wal.writeEntry(WalEntry(
+    kind: wekPut,
+    timestamp: timestamp,
+    key: @key,
+    value: @value,
+  ))
+
+proc writeDelete*(wal: var WriteAheadLog, key: openArray[byte], timestamp: uint64) =
+  wal.writeEntry(WalEntry(
+    kind: wekDelete,
+    timestamp: timestamp,
+    key: @key,
+    value: @[],
+  ))
+
+proc writeCommit*(wal: var WriteAheadLog, timestamp: uint64) =
+  wal.writeEntry(WalEntry(
+    kind: wekCommit,
+    timestamp: timestamp,
+    key: @[],
+    value: @[],
+  ))
+
+proc sync*(wal: var WriteAheadLog) =
+  wal.stream.flush()
+
+proc close*(wal: var WriteAheadLog) =
+  wal.stream.flush()
+  wal.stream.close()
+
+proc entryCount*(wal: WriteAheadLog): uint64 = wal.entryCount
+proc path*(wal: WriteAheadLog): string = wal.path