Initial SWOOSH executor and reducer implementation

replay.go

@@ -0,0 +1,45 @@
+package swoosh
+
+import "fmt"
+
+// Replay deterministically replays WAL records on top of a snapshot and returns the resulting state.
+func Replay(wal WALStore, snapshot Snapshot) (OrchestratorState, error) {
+	base, err := cloneState(snapshot.State)
+	if err != nil {
+		return OrchestratorState{}, fmt.Errorf("clone snapshot state: %w", err)
+	}
+
+	if base.StateHash == "" {
+		hash, err := computeStateHash(base)
+		if err != nil {
+			return OrchestratorState{}, fmt.Errorf("compute snapshot hash: %w", err)
+		}
+		base.StateHash = hash
+	}
+
+	start := snapshot.LastAppliedIndex + 1
+	records, err := wal.Replay(start)
+	if err != nil {
+		return OrchestratorState{}, fmt.Errorf("replay wal: %w", err)
+	}
+
+	state := base
+	for _, record := range records {
+		if state.StateHash != record.StatePreHash {
+			return OrchestratorState{}, fmt.Errorf("wal pre-hash mismatch at index %d", record.Index)
+		}
+
+		next, err := Reduce(state, record.Transition, record.Guard)
+		if err != nil {
+			return OrchestratorState{}, fmt.Errorf("reduce wal record %d: %w", record.Index, err)
+		}
+
+		if next.StateHash != record.StatePostHash {
+			return OrchestratorState{}, fmt.Errorf("wal post-hash mismatch at index %d", record.Index)
+		}
+
+		state = next
+	}
+
+	return state, nil
+}