make use of the store optional

internal/report.go

@@ -93,7 +93,10 @@ func BuildReport(ctx context.Context, reader RecordReader, writer ReportWriter,
 		store:    EphemeralStore{},
 	}, options)
 
-	buys := make(map[string]*FillerQueue)
+	buys, err := optionals.store.Load()
+	if err != nil {
+		return fmt.Errorf("loading state: %w", err)
+	}
 
 	var buysCount, sellsCount int64
 	var lastTimestamp time.Time
@@ -115,6 +118,11 @@ func BuildReport(ctx context.Context, reader RecordReader, writer ReportWriter,
 			rec, err := reader.ReadRecord(ctx)
 			if err != nil {
 				if errors.Is(err, io.EOF) {
+					err = optionals.store.Save(buys)
+					if err != nil {
+						return fmt.Errorf("saving state: %w", err)
+					}
+
 					return nil
 				}
 				return err
@@ -138,9 +146,10 @@ func BuildReport(ctx context.Context, reader RecordReader, writer ReportWriter,
 			if err != nil {
 				return fmt.Errorf("processing record: %w", err)
 			}
-
 		}
 	}
+
+	return nil
 }
 
 // processRecord either adds buys to the queue or consumes buys from the queue when processing a