-// scanFile gives each line of the day file at path from offset on to add.
-func scanFile(ctx context.Context, path string, day uint32, offset uint64, add func([]byte, position) error) (err error) {
+// scanFile gives each line of the day file at path from offset on to add. In
+// the last day file, it cuts a part of a line at the end.
+func scanFile(ctx context.Context, path string, day uint32, offset uint64, last bool, add func([]byte, position) error) (err error) {