Refactor graph copy functions

[Affie]

Look at draft of copyGraph functions in attic/sandbox.jl
The idea is to have only one function to be called by functions such as buildSubgraphFromLabels

Will at least look at it for v0.6.0 and decide if it should be added.

[Affie]

related to #250
Will refactor and consolidate in the v0.6.x cycle

[dehann]

agree, buildSubgraphFromLabels should be using common DFG functions to minimize code maintenance. Might need a keyword or two for more use cases.

[Affie]

Current state

I don't quite get the there are currently 5 subgraph functions comment, but lets look at all related functions:

Currently we have _copyIntoGraph! as the core function.

• getSubgraph (basically an alias for _copyIntoGraph!)
• getSubgraphAroundNode (getNeighborhood -> _copyIntoGraph!)
  • can possibly be consolidated with getSubgraph
  • Maybe use build verb (buildSubgraph) and then use multiple dispatch for "AroundNode" part.

We then have 2 "IIF" functions, but I think its mostly because of a misunderstanding.

• buildSubgraphFromLabels! (I don't think this function should exist at all, but I guess that's why we have 2)
• buildCliqSubgraph! (This doesn't use _copyIntoGraph! yet, but its planned with its replacement deepcopyGraph!. see snipped bellow.

buildCliqSubgraph! builds a very unique subgraph with frontals and separators handled differently (so I believe it has a place as its own function in IIF). Part of the motivation is to replace the "copy" with a "view" in the future, ie. viewGraph.

function buildCliqSubgraph!(dfg::AbstractDFG,
                            cliqSubFg::AbstractDFG,
                            frontals::Vector{Symbol},
                            separators::Vector{Symbol};
                            solvable::Int=0)


  allvars = union(frontals,separators)

  #get list of factors to possibly add, ie. frontal neighbors
  addfac = Symbol[]
  for sym in frontals
    union!(addfac, getNeighbors(dfg,sym))
  end

  #filter to only the factors in the clique
  allfacs = Symbol[]
  for sym in addfac
    vos = getVariableOrder(dfg, sym)
    if vos ⊆ allvars   
      union!(allfacs, [sym])
    end
  end

  # add all the factors and variables to the clique subgraph
  deepcopyGraph!(cliqSubFg, dfg, union(allvars, allfacs))

  return cliqSubFg
end

Summary

I would suggest and will implement if there are no objections:

• A core copy function. _copyIntoGraph! improved and renamed to copyGraph! with "aliases" such as deepcopyGraph! and Base.convert. Drivers can overwrite this function for efficiency. see 569c3a3
• DFG function buildSubgraph generic subgraph helper methods for neighbourhoods etc. will call copyGraph! to do the work with labels created from neighbors and labels.
• IIF specialized buildCliqSubgraph! function that builds a subgraph from frontals and separators.