Data Structure with Ordering and Offsets

Design a data structure that maintains an ordered sequence of items, each with an associated length, and supports two operations:

1. add(item_name, length) – appends the item to the end of the sequence (or updates the length if the item already exists, replacing its old position and collapsing the gap).
2. offset(item_name) – returns the sum of the lengths of all items that appear before the named item in the current order (i.e., the byte-offset at which this item’s data would start in a flat serialization). If the item is not present, return -1.

The structure must efficiently handle these operations as the sequence grows, and queries should return the correct offset even after any number of interleaved additions and updates.