gstlal_inspiral_compress_ranking_stat: fix compress logic

- wasn't doing what was intended.

gstlal-inspiral/bin/gstlal_inspiral_compress_ranking_stat

@@ -54,7 +54,7 @@ def parse_command_line():
 	parser = OptionParser(
 		version = "Name: %%prog\n%s" % "" # FIXME
 	)
-	parser.add_option("-t", "--threshold", type = "float", default = 0.01, help = "Only keep horizon distance values that differ by this much, fractionally, from their neighbours (default = 0.01).")
+	parser.add_option("-t", "--threshold", type = "float", default = 0.03, help = "Only keep horizon distance values that differ by this much, fractionally, from their neighbours (default = 0.03).")
 	parser.add_option("-v", "--verbose", action = "store_true", help = "Be verbose.")
 	options, filenames = parser.parse_args()
 
@@ -115,34 +115,32 @@ for filename in filenames:
 
 	abs_ln_thresh = math.log1p(options.threshold)
 	for instrument, horizon_history in list(rankingstat.numerator.horizon_history.items()):
-		# GPS times
-		keys = horizon_history.keys()
-		# distances
-		values = horizon_history.values()
-
-		# indexes of distances that are non-zero and differ
-		# fractionally from both neighbours by less than the
-		# selected threshold.  always keep the first and last
-		# values
-		removals = [i for i in range(2, len(values) - 1) if values[i - 1] > 0. and values[i] > 0. and values[i + 1] > 0. and abs(math.log(values[i] / values[i - 1])) < abs_ln_thresh and abs(math.log(values[i] / values[i + 1])) < abs_ln_thresh]
-
-		# compress arrays
-		j = k = 0
-		for i in range(len(keys)):
-			if k < len(removals) and i == removals[k]:
-				k += 1
-			else:
-				keys[j] = keys[i]
-				values[j] = values[i]
-				j += 1
-		assert (i + 1) - j == len(removals), "%d %d %d" % (i, j, len(removals))
-		del keys[j:]
-		del values[j:]
+		# GPS time / distance pairs
+		items = horizon_history.items()
+
+		# compress array
+		j = 1
+		for i in range(1, len(items) - 1):
+			values = items[j - 1][1], items[i][1], items[i + 1][1]
+			# remove distances that are non-zero and differ
+			# fractionally from both neighbours by less than
+			# the selected threshold.  always keep the first
+			# and last values
+			if values[0] > 0. and values[1] > 0. and values[2] > 0. and abs(math.log(values[1] / values[0])) < abs_ln_thresh and abs(math.log(values[1] / values[2])) < abs_ln_thresh:
+				continue
+			# remove distances that are 0 and surrounded by 0
+			# on both sides (basically the same as the last
+			# test, but we can't take log(0)).
+			if values == (0., 0., 0.):
+				continue
+			items[j] = items[i]
+			j += 1
+		del items[j:]
 		if options.verbose:
 			print >>sys.stderr, "\"%s\":  %s horizon history reduced to %.3g%% of original size" % (filename, instrument, 100. * j / (i + 1.))
 
 		# replace
-		rankingstat.numerator.horizon_history[instrument] = type(horizon_history)(zip(keys, values))
+		rankingstat.numerator.horizon_history[instrument] = type(horizon_history)(items)
 
 	#
 	# re-insert into XML tree