The WDF is the "within-document frequency" value for a particular
term. It's intended to provide an indication of how frequent a term is
within a document, (for use in computing relevance). Xapian's term
generator already computes WDF values when we use that, (which we do
for indexing all mail content).
We don't use the term generator when adding single terms for things
that don't actually appear in the mail document, (such as tags, the
filename, etc.). In this case, the WDF value for these terms doesn't
matter much.
But Xapian's flint backend can be more efficient with changes to terms
that don't affect the document "length". So there's a performance
advantage for manipulating tags (with the flint backend) if the WDF of
these terms is 0.
Previously, many checks were deep in the library just before a cast
operation. These have now been replaced with internal errors and new
checks have instead been added at the beginning of all top-levelentry
points requiring a read-write database.
The new checks now also use a single function for checking and
printing the error message. This will give us a convenient location to
extend the check, (such as based on database version as well).
This new directory ojbect provides all the infrastructure needed to
detect when files or directories are deleted or renamed. There's still
code needed on top of this (within "notmuch new") to actually do that
detection.
