Zope-index

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- Insure proper unindex behavior if index_doc is called with a empty sequence.

- Use the standard Python doctest module instead of zope.testing.doctest

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- KeywordIndex: Minor optimization; use __nonzero__ instead of __len__
to avoid loading the full TreeSet.

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- KeywordIndex: Store docids in TreeSet rather than a Set when the
number of documents matching a word reaches a configurable
threshold (default 64). The rule is applied to individual words at
indexing time, but you can call the new optimize method to optimize
all the words in an index at once. Designed to fix LP 881950.

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- TextIndex: reuse the lexicon from the underlying Okapi / Cosine
index, if passed. (LP 232516)

- Lexicon: avoid raising an exception when indexing None. (LP 598776)

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- Improve test readability and reached 100% test coverage.

- Fix a broken optimization in okascore.c: it was passing a Python
float to the PyInt_AS_LONG() macro. This resulted in wrong scores,
especially on 64 bit platforms, where all scores typically ended up
being zero.

- Change okascore.c to produce the same results as its Python
equivalent, reducing the brittleness of the text index tests.

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- Port okascore.c optimization used in okapiiindex from Zope2 catalog
implementation. This module is compiled conditionally, based on
whether your environment has a working C compiler.

- Don't use ``len(self._docweight)`` in okapiindex _search_wids method
(obtaining the length of a BTree is very expensive at scale).
Instead use self.documentCount(). Also a Zope2 port.