Use OS tzdata for leap second calculation.

peregrine/gps_time.py

@@ -1,5 +1,78 @@
 import datetime
 
+# NTP timestamps are in units of seconds since the NTP epoch
+# See leap-seconds.list for further details.
+NTP_EPOCH = datetime.datetime(1900,1,1,0,0,0)
+
+def sec_since_NTP_epoch(dt):
+    """Return the number of seconds since the NTP epoch."""
+    return (dt-NTP_EPOCH).total_seconds()
+
+def load_leapsecond_table(f="/usr/share/zoneinfo/leap-seconds.list"):
+    """"
+    Loads leap second table from system.
+
+    The default file location is appropriate for Ubuntu and is provided by
+    the tzdata package.  Refer to the leap-seconds.list file for formatting
+    information.
+
+    Parameters
+    ----------
+    f : string
+      Path to a leap-seconds.list file
+
+    Returns: List of tuples in chronological order each containing an
+    epoch start time and number of leap seconds applicable for that epoch.
+    """
+
+    # Check the expiration date in the file
+    with open(f,'r') as fp:
+        for line in fp:
+            if line[0:2] != "#@":
+                continue
+
+            expiration_time = int(line.split('\t')[-1])
+            now = sec_since_NTP_epoch(datetime.datetime.now())
+
+            if expiration_time<now:
+                raise UserWarning("leap-seconds.list file has expired, update tzdata")
+
+    # Load the load the table
+    with open(f,'r') as fp:
+        table = []
+        for line in fp:
+            if line[0]=="#":
+                continue
+            raw = line.split('\t')
+            table.append( (int(raw[0]), int(raw[1])) )
+
+    # Add the expiration time to the end of the table
+    table.append( (expiration_time, None) )
+
+    return table
+
+def get_gpst_leap_seconds(dt):
+    """Returns the number of leap seconds at the time provided."""
+
+    # Load leap second data from system tzdata file
+    lstable = load_leapsecond_table()
+
+    t = sec_since_NTP_epoch(dt)
+
+    ls = None
+    for epoch_start,leapseconds in lstable:
+        if t>=epoch_start:
+            ls = leapseconds
+
+    # Raise warning if specified time is after expiry date of leap second table
+    if ls==None:
+        raise UserWarning("Specified datetime is after expiry time of leap second table.")
+
+    # GPS leap seconds relative to a Jan 1 1980, where TAI-UTC was 19 seconds.
+    gps_leap_seconds = ls - 19
+
+    return datetime.timedelta(seconds = gps_leap_seconds)
+
 def datetime_to_tow(t, mod1024 = True):
     """
     Convert a Python datetime object to GPS Week and Time Of Week.
@@ -39,12 +112,7 @@ def gpst_to_utc(t_gpst):
     -------
     t_utc : datetime
     """
-    t_utc = t_gpst - datetime.timedelta(seconds = 16)
-    if t_utc <= datetime.datetime(2012, 7, 1) or \
-       t_utc >= datetime.datetime(2015, 7, 1):
-        raise ValueError("Don't know how many leap seconds to use.  " +
-                         "Please implement something better in gpst_to_utc()")
-    return t_utc
+    return t_gpst - get_gpst_leap_seconds(t_gpst)
 
 def utc_to_gpst(t_utc):
     """
@@ -59,9 +127,7 @@ def utc_to_gpst(t_utc):
     -------
     t_gpst : datetime
     """
-    t_gpst = t_utc + datetime.timedelta(seconds = 16)
-    if t_utc <= datetime.datetime(2012, 7, 1) or \
-       t_utc >= datetime.datetime(2015, 7, 1):
-        raise ValueError("Don't know how many leap seconds to use.  " +
-                         "Please implement something better in utc_to_gpst()")
-    return t_gpst
+    # TODO: there may be an unhandled corner case here for conversions
+    # where the GPST-to-UTCT interval spans a leap second.
+
+    return t_utc + get_gpst_leap_seconds(t_utc)