update night in prenight when visits loaded, if needed

schedview/app/prenight/prenight.py

@@ -305,6 +305,20 @@ def _update_visits(self):
                 Time(self._almanac_events.loc["sunset", "UTC"]),
                 Time(self._almanac_events.loc["sunrise", "UTC"]),
             )
+            if len(visits) == 0:
+                self.logger.info("No visits on requested night, looking for a night with visits.")
+                # read all visits to so we can find a central one
+                visits = schedview.collect.opsim.read_opsim(self.opsim_output_fname)
+                if len(visits) > 0:
+                    self.logger.info("Changing night to one with visits.")
+                    self.night = schedview.compute.astro.compute_central_night(visits)
+                    # Setting night will trigger a call to _update_visits
+                    # by way of _almanac_events, so we should not continue
+                    # this execution..
+                    return
+                else:
+                    self.logger.info("No visits found in this opsim database.")
+
             self._visits = visits
             self.logger.info("Finish updating visits DataFrame.")
             self.logger.info("Starting to update visits ColumnDataSource.")

schedview/compute/astro.py

@@ -99,3 +99,33 @@ def night_events(night_date=None, site=None, timezone="Chile/Continental"):
     time_df.index.name = "event"
 
     return time_df
+
+
+def compute_central_night(visits, site=None, timezone="Chile/Continental"):
+    """Compute the central night of a set of visits.
+
+    Parameters
+    ----------
+    visits : `pandas.DataFrame`
+        A DataFrame of visits.
+    site : `astropy.coordinates.earth.EarthLocation`
+        The observatory location. Defaults to Rubin observatory.
+    timezone: `str`
+        The timezone name. Defaults to 'Chile/Continental'
+
+    Returns
+    -------
+    central_night : `datetime.date`
+        The central night of the visits.
+    """
+    central_mjd = visits["observationStartMJD"].median()
+    candidate_night = Time(central_mjd, format="mjd", scale="utc").datetime.date()
+
+    # The mjd rollover can occur during the night, so the above might be offset
+    # by a night. Make sure the night we have is the one with a central mjd
+    # closest to the median visit mjd.
+    candidate_middle_mjd = night_events(candidate_night, site, timezone).loc["night_middle", "MJD"]
+    mjd_shift = np.round(central_mjd - candidate_middle_mjd)
+    central_night = Time(central_mjd + mjd_shift, format="mjd", scale="utc").datetime.date()
+
+    return central_night

tests/test_compute_astro.py

@@ -0,0 +1,25 @@
+import unittest
+
+import numpy as np
+import pandas as pd
+
+from schedview.compute.astro import compute_central_night, night_events
+
+TEST_MJDS = [60000.6, 60000.8, 60001.2]
+
+
+class TestComputeAstro(unittest.TestCase):
+    def test_compute_central_night(self):
+        for mjd in TEST_MJDS:
+            visits = pd.DataFrame(
+                {
+                    "observationStartMJD": [mjd - 0.1, mjd, mjd + 0.1],
+                    "fieldRA": [0, 0, 0],
+                    "fieldDec": [0, 0, 0],
+                    "filter": ["r", "i", "z"],
+                }
+            )
+            computed_night = compute_central_night(visits)
+            computed_night_events = night_events(computed_night)
+            computed_night_middle_mjd = computed_night_events.loc["night_middle", "MJD"]
+            assert np.abs(computed_night_middle_mjd - mjd) <= 0.5