Dijkstra - support undirected graphs

[assafsun]

Dijkstra - support undirected graphs

issue #42

[assafsun]

The changes are based on the work of PR after rebase, review and running the tests https://github.com/dagrejs/graphlib/pull/92

[BudgieInWA]

Currently inEdges and outEdges are undefined for undirected graphs with the documentation recommending nodeEdges instead. Instead of having outEdges behave exactly like nodeEdges, I think it would be useful for outEdges(v) to guarantee that it returns edges oriented away from v, by reversing those oriented towards v (and similarly for inEdges). outEdges behaving exactly like nodeEdges is a convenience at best and I argue that it is confusing and likely to lead to incorrect usage.

With the existing API, an algorithm that wishes to traverse both directed and undirected graphs in the normal way (respecting the direction of edges in directed graphs but not in undirected graphs) might do something like this:

function visit(v) {
  let edges;
  // Let's not do any more work than we need to:
  if (g.isDirected()) {
    edges = g.outEdges(v);
  } else {
    function orientAwayFromV(e) { ... }
    edges = g.nodeEdges(v).map(orientEdgeAwayFromV);
  }
  // For the rest of this function edges are oriented in the direction
  // that we are traversing them so it doesn't matter if the graph is
  // directed or not.

  for (const e of edges) {
    if (shouldTraverse(e)) {
       console.log("traversing edge from", e.v, "to", e.w);
       visit(e.w);
    }
  }
}

The new API implemented in this PR cannot help us here if we want to avoid unnecessarily checking edge orientation. Also, if the documentation said that outEdges works on directed graphs, const edges = g.outEdges(v); would look correct, but some edges would be backwards.

I propose implementing outEdges something like this:

Graph.prototype.outEdges = function(v, w) {
  return this.isDirected()
    ? this.getOutEdges(v, w)
    : this.getOutEdges(v, w).concat(reverseEdges(this.getInEdges(v, w));
};

That way, I could write my example like this supporting directed and undirected graphs without having to manually check edge direction:

function visit(v) {
  const edges = g.outEdges(v);
  
  for (const e of edges) {
    if (shouldTraverse(e)) {
       console.log("traversing edge from", e.v, "to", e.w);
       visit(e.w);
    }
  }
}