/// <summary> Score a candidate doc for all slop-valid position-combinations (matches)
/// encountered while traversing/hopping the PhrasePositions.
/// <br/> The score contribution of a match depends on the distance:
/// <br/> - highest score for distance=0 (exact match).
/// <br/> - score gets lower as distance gets higher.
/// <br/>Example: for query "a b"~2, a document "x a b a y" can be scored twice:
/// once for "a b" (distance=0), and once for "b a" (distance=2).
/// <br/>Possibly not all valid combinations are encountered, because for efficiency
/// we always propagate the least PhrasePosition. This allows to base on
/// PriorityQueue and move forward faster.
/// As result, for example, document "a b c b a"
/// would score differently for queries "a b c"~4 and "c b a"~4, although
/// they really are equivalent.
/// Similarly, for doc "a b c b a f g", query "c b"~2
/// would get same score as "g f"~2, although "c b"~2 could be matched twice.
/// We may want to fix this in the future (currently not, for performance reasons).
/// </summary>
protected internal override float PhraseFreq(IState state)
{
int end = InitPhrasePositions(state);
float freq = 0.0f;
bool done = (end < 0);
while (!done)
{
PhrasePositions pp = pq.Pop();
int start = pp.position;
int next = pq.Top().position;
bool tpsDiffer = true;
for (int pos = start; pos <= next || !tpsDiffer; pos = pp.position)
{
if (pos <= next && tpsDiffer)
{
start = pos; // advance pp to min window
}
if (!pp.NextPosition(state))
{
done = true; // ran out of a term -- done
break;
}
PhrasePositions pp2 = null;
tpsDiffer = !pp.repeats || (pp2 = TermPositionsDiffer(pp)) == null;
if (pp2 != null && pp2 != pp)
{
pp = Flip(pp, pp2); // flip pp to pp2
}
}
int matchLength = end - start;
if (matchLength <= slop)
{
freq += Similarity.SloppyFreq(matchLength); // score match
}
if (pp.position > end)
{
end = pp.position;
}
pq.Add(pp); // restore pq
}
return(freq);
}
