add functionality to sourmash signature to extract common hashes?

[ctb]

see #587 (comment) by @taylorreiter - "It would be useful if we could have a threshold for the intersection -- like if the hash occurs in 80% of signatures, give it to me in the intersection."

[ctb]

note that this functionality is not trivial, and enabling this kind of general query is in fact one of the works-in-progress that https://github.com/ctb/2017-sourmash-revindex and #604 is devoted to :)

[ctb]

(not trivial doesn't mean it's hard; we have several implementations of such a thing, including e.g. https://github.com/ctb/2017-sourmash-revindex/blob/master/hashes-to-numpy-2.py)

[ctb]

It would now (as of #946) be very easy to add a filtering option to LCA_Database to support this functionality.

[ctb]

similarly for #1808 and SqliteIndex, this is quite easy there.

[ctb]

is now generically available as a plugin: https://github.com/ctb/sourmash_plugin_commonhash

also ref #2383