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https://issues.apache.org/jira/browse/GIRAPH-547?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13590936#comment-13590936
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Alessandro Presta commented on GIRAPH-547:
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Another follow-up from a conversation with Maja: we could add a
MutableVertexEdges interface which can return the mutable iterator.
That way, we can optimize in-place modification in cases like primitive arrays
(where you have random access) and Java collections (where the mutable edge
iterable is the same as the immutable one).
I will work on this after the API redesign.
> Allow in-place modification of edges
> ------------------------------------
>
> Key: GIRAPH-547
> URL: https://issues.apache.org/jira/browse/GIRAPH-547
> Project: Giraph
> Issue Type: New Feature
> Reporter: Alessandro Presta
> Assignee: Alessandro Presta
>
> This is a somewhat long term item.
> Because of some optimized edge storage implementations (byte array, primitive
> array), we have a contract with the user that Edge objects returned by
> getEdges() are read-only.
> One concrete example where in-place modification would be useful: in the
> weighted version of PageRank, you can store the weight sum and normalize each
> message sent, or you could more efficiently normalize the out-edges once in
> superstep 0.
> The Pregel paper describes an OutEdgeIterator that allows for in-place
> modification of edges. I can see how that would be easy to implement in C++,
> where there is no need to reuse objects.
> Giraph "unofficially" supports this if one is using generic collections to
> represent edges (e.g. ArrayList or HashMap).
> It may be trickier in some optimized implementations, but in principle it
> should be doable.
> One way would be to have some special MutableEdge implementation which calls
> back to the edge data structure in order to save modifications:
> {code}
> for (Edge<I, E> edge : getEdges()) {
> edge.setValue(newValue);
> }
> {code}
> Another option would be to add a special set() method to our edge iterator,
> where one can replace the current edge:
> {code}
> for (EdgeIterator<I, E> it = getEdges().iterator(); it.hasNext();) {
> Edge<I, E> edge = it.next();
> edge.setValue(newValue);
> it.set(edge);
> }
> {code}
> We could actually implement the first version as syntactic sugar on top of
> the second version (the special MutableEdge would need a reference to the
> iterator in order to call set(this)).
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