143Build a collaborative rich-text editor with remote cursors and offline synchronization

The core of a collaborative editor is a data structure that lets several people edit the same text at the same time, even offline, and always end up with the same document. Implement a text CRDT (a Replicated Growable Array, RGA):

createReplica(siteId) returns:

• insert(index, text): insert at a visible index; returns the operations to broadcast (one per character).
• delete(index, length): delete visible characters; returns the operations to broadcast.
• apply(ops): apply remote operations. Operations may arrive in any order, more than once, and later (after being offline). Applying must be idempotent, and an operation whose dependency has not arrived yet must wait until it does.
• getText(): the current visible text.
• anchorAt(index) and indexOf(anchor): a remote cursor's position that stays attached to the same place in the text as others edit before it.

Convergence: any replicas that have applied the same set of operations, in any order, have the same getText().

(Rich-text formatting is built on top of this as marks over character ranges; the follow-up questions discuss it.)