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[[duplicate-edge]]
== Duplicate Edge Detection

Whether part of a graph maintenance process or for some other analysis need, it is sometimes necessary to detect
if there is more than one edge between two vertices. The following examples will assume that an edge with the same
label and direction will be considered "duplicate".

The "modern" graph does not have any duplicate edges that fit that definition, so the following example adds one
that is duplicative of the "created" edge between vertex "1" and "3".

[gremlin-groovy,modern]
----
g.V(1).as("a").V(3).addE("created").from("a").iterate()
g.V(1).outE("created")
----

One way to find the duplicate edges would be to do something like this:

[gremlin-groovy,existing]
----
g.V().outE().
  project("a","b").                         <1>
    by().by(inV().path().by().by(label)).
  group().                                  <2>
    by("b").
    by(select("a").fold()).
  unfold().                                 <3>
  select(values).                           <4>
  where(count(local).is(gt(1)))
----

<1> The "a" and "b" from the `project` contain the edge and the path respectively. The path consists of a the outgoing
vertex, an edge, and the incoming vertex. The use of `by().by(label))` converts the edge to its label (recall that `by`
are applied in round-robin fashion), so the path will look something like: `[v[1],created,v[3]]`.
<2> Group by the path from "b" and construct a list of edges from "a". Any value in this `Map` that has a list of edges
greater than one means that there is more than one edge for that edge label between those two vertices (i.e. the `Map`
key).
<3> Unroll the key-value pairs in the `Map` of paths-edges.
<4> Only the values from the `Map` are needed and as mentioned earlier, those lists with more than one edge would
contain  duplicate.

This method find the duplicates, but does require more memory than other approaches. A slightly more complex approach
that uses less memory might look like this:

[gremlin-groovy,existing]
----
g.V().as("ov").
  outE().as("e").
  inV().as("iv").
  inE().                                          <1>
  where(neq("e")).                                <2>
  where(eq("e")).by(label).
  where(outV().as("ov")).
  group().
    by(select("ov","e","iv").by().by(label)).     <3>
  unfold().                                       <4>
  select(values).
  where(count(local).is(gt(1)))
----

<1> To this point in the traversal, the outgoing edges of a vertex are being iterated with the current edge labeled
as "e". For "e", Gremlin traverses to the incoming vertex and back on in edges of that vertex.
<2> Those incoming edges are filtered with the following `where` steps. The first ensures that it does not traverse
back over "e" (i.e. the current edge). The second determines if the edge label is equivalent (i.e. the test for the
working definition of "duplicate"). The third determines if the outgoing vertex matches the one that started the
path labeled as "ov".
<3> This line is quite similar to the output achieved in the previous example at step 2. A `Map` is produced that uses
the outgoing vertex, the edge label, and the incoming vertex as the key, with the list of edges for that path as the
value.
<4> The rest of the traversal is the same as the previous one.

Note that the above traversal could also be written using `match` step:

[gremlin-groovy,existing]
----
g.V().match(
    __.as("ov").outE().as("e"),
    __.as("e").inV().as("iv"),
    __.as("iv").inE().as("ie"),
    __.as("ie").outV().as("ov")).
      where("ie",neq("e")).
      where("ie",eq("e")).by(label).
    select("ie").
    group().
      by(select("ov","e","iv").by().by(label)).
    unfold().select(values).
      where(count(local).is(gt(1)))
----

A third way to approach this problem would be to force a link:https://en.wikipedia.org/wiki/Depth-first_search[depth-first search].
The previous examples invoke traversal strategies that force a link:https://en.wikipedia.org/wiki/Breadth-first_search[breadth first search]
as a performance optimization.

[gremlin-groovy,existing]
----
g.withoutStrategies(LazyBarrierStrategy, PathRetractionStrategy).V().as("ov").   <1>
  outE().as("e1").
  inV().as("iv").
  inE().
  where(neq("e1")).
  where(outV().as("ov")).as("e2").                                               <2>
  where("e1", eq("e2")).by(label)                                                <3>
----

<1> Remove strategies that will optimize for breadth first searches and thus allow Gremlin to go depth first.
<2> To this point, the traversal is very much like the previous one. Review step 2 in the previous example to see the
parallels here.
<3> The final `where` simply looks for edges that match on label, which would then meet the working definition of
"duplicate".

The basic pattern at play here is to compare the path of the outgoing vertex, its outgoing edge label and the incoming
vertex. This model can obviously be contracted or expanded as needed to fit different definitions of "duplicate". For
example, a "duplicate" definition could extended to the label and properties of the edge. For purposes of
demonstration, an additional edge is added to the "modern" graph:

[gremlin-groovy,modern]
----
g.V(1).as("a").V(3).addE("created").property("weight",0.4d).from("a").iterate()
g.V(1).as("a").V(3).addE("created").property("weight",0.5d).from("a").iterate()
g.V(1).outE("created").elementMap()
----

To identify the duplicate with this revised definition, the previous traversal can be modified to:

[gremlin-groovy,existing]
----
g.withoutStrategies(LazyBarrierStrategy, PathRetractionStrategy).V().as("ov").
  outE().as("e1").
  inV().as("iv").
  inE().
  where(neq("e1")).
  where(outV().as("ov")).as("e2").
  where("e1", eq("e2")).by(label).
  where("e1", eq("e2")).by("weight").elementMap()
----
