Create Str container

[brunokim]

Currently, an InsertChar atom can be a direct children of the root atom, since we assume that the whole RList corresponds to a single string. However, in the process to create a nested data structure, we may want to have multiple strings in the same tree.

To accomplish that, we'll create a container for chars, called Str. Multiple Strs may appear as children of the root empty node, and InsertChar operations are only valid within a Str.

Data structures

type Str struct {} // implements AtomValue

• Str atoms only accept InsertChar and Delete atoms as children.
• InsertChar atoms only accept InsertChar and Delete atoms as children.
• The root node must not accept InsertChar as children.

Build changes

• RList.AsString() must validate that the cursor corresponds to a Str node.
• RList.SetCursor must now be able to move in two dimensions: among the children of the root node, and over the children of a Str, as it does now.
  • Note: a good solution here should allow for deeply nested structures. We should be looking on how to specify a path within the (semantic) structure, and then translate it into a position in the array.
• Add operation to crdt/rlist_operations_test.go
• Run go test -fuzz FuzzList and fix any crashes that appear
• Write manual tests in crdt/rlist_test.go

Demo changes

• Fix cmd/demo/demo.go to create a str before starting to insert chars.
• (bonus) allow multiple strings in the user interface.

[brunokim]

The restriction that InsertChar can't be a child of root may be postponed, since it'd likely touch many places in the codebase.