Add database function: tstamp_interpolate

etc/db/upgrades/upgrade11-v0.2.1-tstamp-functions.sql

@@ -73,6 +73,41 @@ BEGIN
 		IS 'Get the timestamp of an existing shotpoint.';
 
 
+	CREATE OR REPLACE FUNCTION tstamp_interpolate(s numeric, p numeric) RETURNS timestamptz
+	AS $inner$
+	DECLARE
+		ts0 timestamptz;
+		ts1 timestamptz;
+		pt0 numeric;
+		pt1 numeric;
+	BEGIN
+
+		SELECT tstamp, point
+		INTO ts0, pt0
+		FROM raw_shots
+		WHERE sequence = s AND point < p
+		ORDER BY point DESC LIMIT 1;
+
+
+		SELECT tstamp, point
+		INTO ts1, pt1
+		FROM raw_shots
+		WHERE sequence = s AND point > p
+		ORDER BY point ASC LIMIT 1;
+
+		RETURN (ts1-ts0)/abs(pt1-pt0)*abs(p-pt0)+ts0;
+
+	END;
+	$inner$ LANGUAGE PLPGSQL;
+
+	COMMENT ON FUNCTION tstamp_interpolate(numeric, numeric)
+		IS 'Interpolate a timestamp given sequence and point values.
+
+It will try to find the points immediately before and after in the sequence and interpolate into the gap, which may consist of multiple missed shots.
+
+If called on an existing shotpoint it will return an interpolated timestamp as if the shotpoint did not exist, as opposed to returning its actual timestamp.
+
+Returns NULL if it is not possible to interpolate.';
 
 END;
 $$ LANGUAGE plpgsql;