gstlal-inspiral: filter sub-threshold triggers

- add hacks (some of which might prove to be permanent) to filter out
  sub-threshold triggers, preventing them from contaminating the ranking
  statistic PDFs

gstlal-inspiral/python/far.py

@@ -199,6 +199,21 @@ class RankingStat(snglcoinc.LnLikelihoodRatioMixin):
 		return new
 
 	def ln_lr_from_triggers(self, events, offsetvector):
+		#
+		# exclude triggers that are below the SNR threshold
+		#
+		# FIXME:  this alters the mapping from triggers to ln L
+		# density parameters.  it does not alter the definition of
+		# ln L for candidates with SNRs below the threshold.  that
+		# would perhaps be a more sound approach but would have to
+		# be done more carefully, to ensure the behaviour it
+		# introduces maintains the numerator and denominator as
+		# proper probability densities.  think about this some
+		# more.
+
+		events = tuple(event for event in events if event.snr > self.snr_min)
+		assert len(events) >= self.min_instruments, "coincidence engine failed to respect minimum instrument count requirement for candidates:  found candidate with %d < %d instruments" % (len(events), self.min_instruments)
+
 		#
 		# pick a random, but reproducible, trigger to provide a
 		# reference timestamp for, e.g, the \Delta t's between

gstlal-inspiral/python/lloidhandler.py

@@ -1037,7 +1037,8 @@ class Handler(simplehandler.Handler):
 			# so that the "how many instruments were on test"
 			# is aware of this buffer.
 			if not buf_is_gap:
-				self.rankingstat.denominator.triggerrates[instrument].add_ratebin(map(float, buf_seg), len(events))
+				snr_min = self.rankingstat.snr_min
+				self.rankingstat.denominator.triggerrates[instrument].add_ratebin(map(float, buf_seg), len([event for event in events if event.snr >= snr_min]))
 
 			# extract times when instruments were producing
 			# SNR.  used to define "on instruments" for coinc

gstlal-inspiral/python/stats/inspiral_lr.py

@@ -142,6 +142,27 @@ class LnLRDensity(snglcoinc.LnLRDensity):
 		# that they are for the correct templates.
 		if event.template_id not in self.template_ids:
 			raise ValueError("event from wrong template")
+		# ignore events below threshold.  the trigger generation
+		# mechanism now produces sub-threshold triggers to
+		# facilitiate the construction of sky maps in the online
+		# search.  it's therefore now our responsibility to exclude
+		# them from the PDFs in the ranking statistic here.
+		#
+		# FIXME:  this creates an inconsistency.  we exclude events
+		# from the PDFs, but we don't exclude them from the
+		# evaluation of those PDFs in the .__call__() method,
+		# instead we'll report that the candidate is impossible
+		# because it contains a trigger in a "forbidden" region of
+		# the parameter space (forbidden because we've ensured
+		# nothing populates those bins).  the .__call__() methods
+		# should probably be modified to exclude triggers with
+		# sub-threshold SNRs just as we've excluded them here, but
+		# I'm not yet sure.  instead, for the time being, we
+		# resolve the problem by excluding sub-threshold triggers
+		# from the "-from-triggers" method in far.py.  there is a
+		# FIXME related to this same issue in that code.
+		if event.snr < self.snr_min:
+			return
 		self.densities["%s_snr_chi" % event.ifo].count[event.snr, event.chisq / event.snr**2.] += 1.0
 
 	def copy(self):