Neighborhood methods allow vertices to perform an aggregation on their first-hop neighborhood.
`groupReduceOnEdges()` can be used to compute an aggregation on the neighboring edges of a vertex, while `groupReduceOnNeighbors()` has access on both the neighboring edges and vertices. The neighborhood scope is defined by the `EdgeDirection` parameter, which takes the values `IN`, `OUT` or `ALL`. `IN` will gather all in-coming edges (neighbors) of a vertex, `OUT` will gather all out-going edges (neighbors), while `ALL` will gather all edges (neighbors).
`groupReduceOnEdges()` can be used to compute an aggregation on the neighboring edges of a vertex,
while `groupReduceOnNeighbors()` has access to both the neighboring edges and vertices. The neighborhood scope
is defined by the `EdgeDirection` parameter, which takes the values `IN`, `OUT` or `ALL`. `IN` will gather all in-coming edges (neighbors) of a vertex, `OUT` will gather all out-going edges (neighbors), while `ALL` will gather all edges (neighbors).
The `groupReduceOnEdges()` and `groupReduceOnNeighbors()` methods return zero, one or more values per vertex.
When returning a single value per vertex, `reduceOnEdges()` or `reduceOnNeighbors()` should be called as they are more efficient.
When the user-defined function to be applied on the neighborhood is associative and commutative, it is highly advised to use the `reduceOnEdges()` and `reduceOnNeighbors()` methods. These methods exploit combiners internally, significantly improving performance.
For example, assume that you want to select the minimum weight of all out-edges for each vertex in the following graph:
The following code will collect the in-edges for each vertex and apply the `SumInNeighbors()` user-defined function on each of the resulting neighborhoods:
When the aggregation computation does not require access to the vertex value (for which the aggregation is performed), it is advised to use the more efficient `EdgesFunction` and `NeighborsFunction` for the user-defined functions. When access to the vertex value is required, one should use `EdgesFunctionWithVertexValue` and `NeighborsFunctionWithVertexValue` instead.
