A HashMap managed graph model with the concept of ownership.
The flow_arena package consists of trait Flow and struct FlowArena, along with its node representation trait Node and struct FlowNode.
FlowArena grants you the ability to:
1. represent a tree / graph data model in a memory-safe way
2. use the concept of "ownership" to switch between the tree form and the graph form, along with:
1. the ability to move / delete nodes recursively in a controlled way - if node A is owned by node B, then B's removal will trigger A's removal
2. the ability to visit and traverse the data model in two different ways, namely the tree-ish way and the graph-ish way
In fact, it all started with Rust's ownership rules...
When it comes to relation driven data models, it's easy to make Rustaceans headache - just try and write a safe doublely-linked list. The concept of Arena, therefore, is raised to implement a simple and memory-safe representation of such models. [^1]
Consider a tree model. Basically it's a group of nodes where each node can point to some other nodes. How do we denote this relation? Well, we give every node a unique mark which claims its reference. We used to use pointers to cleverly mark this uniqueness with the help of memory addresses; but due to Rust's ownership rule, this approach is verbose to implement. So, why not mark this uniqueness by ourselves, with indices or ids?
The idea of the Arena is simple: use a Vec<Node> or a HashMap<Id, Node<Id>> to contain all the nodes, where each node contains a collection of indices / ids, e.g. Vec<Id>, as well as its own data. We can mark a node with its index / id, thus we can designate a root node; to traverse, just recursively get a node and traverse its "node marker collection".
FlowArena represents a directed graph with the idea of Arena. More than that, it's also able to represent the "flow model".
We call such a data model a "flow model", which clearly indicates the functionality of FlowArena.
struct FlowNode<Id> and the struct FlowArena<Id, Entity> to represent a flow modeltrait Node<Id> and trait Flow - see Implementation for reference// Todo: Introduce a series of Flow traits.
The FlowArena is serving as the underlying data model of flow.er, a notebook, mindmap, todo-list and agenda app.