hi all,
I am now hacking the BooleanScorer2 to let it keep the docID() of the
leaf scorer(mostly possible TermScorer) the same as the toplevel Scorer.
Why I want to do this is: When I Collect a doc, I want to know which term
is matched(especially for BooleanClause whose Occur is SHOULD). we have
discussed some solutions, such as adding bit masks in disjunction scorers.
with this method, when we finds a matched doc, we can recursively find
which leaf scorer is matched. But we think it's not very efficient and not
convenient to use(this is my proposal but not agreed by others in our
team). and then we came up with another one: Modifying DisjunctionSumScorer.
we analysed the codes and found that the only Scorers used by
BooleanScorer2 that will make the children scorers' docID() not equal to
parent is an anonymous class inherited from DisjunctionSumScorer. All other
ones including SingleMatchScorer, countingConjunctionSumScorer(anonymous),
dualConjuctionSumScorer, ReqOptSumScorer and ReqExclScorer are fit our need.
The implementation algorithm of DisjunctionSumScorer use a heap to find
the smallest doc. after finding a matched doc, the currentDoc is the
matched doc and all the scorers in the heap will call nextDoc() so all of
the scorers' current docID the nextDoc of currentDoc. if there are N level
DisjunctionSumScorer, the leaf scorer's current doc is the nth next docId
of the root of the scorer tree.
So we modify the DisjuctionSumScorer and let it behavior as we expected.
And then I wrote some TestCase and it works well. And also I wrote some
random generated TermScorer and compared the nextDoc(),score() and
advance(int) method of original DisjunctionSumScorer and modified one.
nextDoc() and score() and exactly the same. But for advance(int target), we
found some interesting and strange things.
at the beginning, I think if target is less than current docID, it will
just return current docID and do nothing. this assumption let my algorithm
go wrong. Then I read the codes of TermScorer and found each call of
advance(int) of TermScorer will call nextDoc() no matter whether current
docID is larger than target or not.
So I am confused and then read the javadoc of DocIdSetIterator:
 javadoc of DocIdSetIterator.advance(int
target)
int org.apache.lucene.search.DocIdSetIterator.advance(int target) throws
IOException
Advances to the first beyond (see NOTE below) the current whose document
number is greater than or equal
to target. Returns the current document number or NO_MORE_DOCS if there
are no more docs in the set.
Behaves as if written:
int advance(int target) {
int doc;
while ((doc = nextDoc()) < target) {
}
return doc;
}
Some implementations are considerably more efficient than that.
NOTE: when target < current implementations may opt not to advance beyond
their current docID().
NOTE: this method may be called with NO_MORE_DOCS for efficiency by some
Scorers. If your
implementation cannot efficiently determine that it should exhaust, it is
recommended that you check for
that value in each call to this method.
NOTE: after the iterator has exhausted you should not call this method, as
it may result in unpredicted
behavior.

Then I modified my algorithm again and found that
DisjunctionSumScorer.advance(int target) has some strange behavior. most of
the cases, it will return currentDoc if target < currentDoc. but in some
boundary condition, it will not.
it's not a bug but let me sad. I thought my algorithm has some bug because
it's advance method is not exactly the same as original
DisjunctionSumScorer's.
codes of DisjunctionSumScorer
@Override
public int advance(int target) throws IOException {
if (scorerDocQueue.size() < minimumNrMatchers) {
return currentDoc = NO_MORE_DOCS;
}
if (target <= currentDoc) {
return currentDoc;
}
....

for most case if (target <= currentDoc) it will return currentDoc;
but if previous advance will make sub scorers exhausted, then if may return
NO_MORE_DOCS
an example is:
currentDoc=1
minimumNrMatchers=1
subScorers:
TermScorer: docIds: [1, 2, 6]
TermScorer: docIds: [2, 4]
after first call advance(5)
currentDoc=6
only first scorer is now in the heap, scorerDocQueue.size()==1
then call advance(6)
because scorerDocQueue.size() < minimumNrMatchers, it just return
NO_MORE_DOCS
My question is why the advance(int target) method is defined like this? for
the reason of efficient or any other reasons?
