[Ordering] add ability to define order constraints within rank

[izhan]

Allows for node order constraints like:

dagre.layout(g, {constraints: [{left: "node-B", right: "node-A"}]})

Addresses: #112, #201, #251, among others
Related PRs: #263, #252

Usage

interface Constraint {
    left: string;
    right: string;
}

interface Options {
    constraints?: Constraint[];
}

function layout(g: graphlib.Graph, opts: Options) {...}

Example

Without constraints:

With constraints:

const g = new dagre.graphlib.Graph({}).setGraph({}).setDefaultEdgeLabel(function () {
    return {}
})
g.setNode("root")
g.setNode("node-A")
g.setNode("node-B")
g.setEdge("root", "node-A")
g.setEdge("root", "node-B")
dagre.layout(g, {constraints: [{left: "node-B", right: "node-A"}]})

Details

I'm following a suggestion made by @cpettitt in dagrejs/dagre-d3#64 (comment)

To specify the constraints, I'm following the opts approach used in #263, but open to suggestions.

[mx2323]

how does this address #201? im looking for that functionality but dont see how this maps to the ability to incrementally layout a new node to a graph.

[izhan]

@mx2323 I haven't tested it yet, but after calling layout(), take the output ordering of nodes within the same rank (done by looking at their x + y coordinates) and generate pairwise constraints. further calls to layout() w/ these constraints should not create any shuffling, even after adding a new node

[branpk]

Are you sure this addresses #112? My understanding is that this adds node order constraints but that doesn't necessary help with outgoing edges. For example:

Swapping C and D or A and B won't fix the edges by itself. The correct way is to add an extra crossing between either A's outgoing edges or B's outgoing edges.

[izhan]

@branpk ah good point, this won't necessarily solve outgoing edges. My understanding is that this PR fixes the second ask of #112 (within same rank):

In fact, it would be the best if i could constraint nodes order too (position from left to right in top down graph), but as I understand, it is not possible at the moment

Thanks for the callout, hope that clarifies!

[zygeilit]

@izhan excellent, solved my question. nice

[danyflorez]

Does anyone knows a hack to accomplish this?

[ZahidChandio]

Could someone please review and merge the open PRs? It would also be great if we could get responses to the open issues. Thank you!