Graph Theory Abstractions

Graph theory interfaces - IGraph<TNode,TEdge>, INode<TNode,TEdge> and IEdge<TNode,TEdge> - to allow for graph algorithms that do not depend on a particular graph representation.

Example implementation and usage can be found in the separate SCGraphTheory.AdjacencyList and SCGraphTheory.Search packages, respectively. Additional (test-focused) implementation examples can be found in the TestGraphs library in SCGraphTheory.Search. Notably:

• A super-simple (though rather inefficient) LINQ-powered immutable implementation. Used for tests in the search algorithm package.
• A square grid implementation using structs. Involves no up-front heap allocations other than a 2D array of node values, but performs a little worse under search because lots of data gets moved around compared to a class-based implementation (including a little boxing - see notes, below). Included in search benchmarks project because I was interested in the performance impact.
• A bare-bones adjacency matrix implementation. Doesn't actually feature in any tests - I was just curious to know what an adjacency matrix implementation of these interfaces could look like.

Notes:

• The fact that the IEdge abstraction has a "From" and a "To" doesn't make this abstraction unsuitable for undirected graphs. Graph algorithms will generally traverse edges in a particular direction, making this a useful interface, and while the AdjacencyList implementation doesn't do this (thus favouring low latency over low memory usage), there's nothing stopping an implementation (with class-valued edges) from making the IEdge implementation a struct created from the "actual" edge, depending on the current node - thus avoiding "duplicated" undirected edges on the heap.