[redland] Way to get iterator of unique (distinct) sources?

[Dave Beckett]

>>>Seth Ladd said:
> Hello,
> 
> Is there a way to say "Give me a list of all distinct Subjects in this 
> model"?  I've tried librdf_model_get_sources(model, NULL, NULL) but that 
> seems to be returning an empty iterator (even though I know there are 
> statements in the model).

The get_sources method on model isn't a search; the two arguments
must both be present.  As such the above won't work at all and will
always return nothing.

As to the question of getting all *distinct* subjects of a graph; ie
the resources, there is no method call yet that will do it.  I have
considered adding it to the API, still might.

However, to efficiently do this from a general set of triples, I was
considering adding later as an option to the iterators/streams.  They
can take filtering/mapping operations on the streams of items so it
would make sense to do this by using such a callback function with a
hash keyed off the item.  (At the C level, an in-memory librdf_hash
could be used).

Or it could be done at the higher level language layer such as by
using perl hashes or python, java, etc. equivalents

Dave