add fallback_locations for via_ip

my/location/fallback/common.py

@@ -10,16 +10,16 @@ class FallbackLocation:
     lat: float
     lon: float
     dt: datetime
-    duration: int  # time in seconds for how long this is valid
+    duration: float  # time in seconds for how long this is valid
     accuracy: Optional[float] = None
     elevation: Optional[float] = None
     datasource: Optional[str] = None  # which module provided this, useful for debugging
 
     def to_location(self, end: bool = False) -> Location:
-        """
+        '''
         by default the start date is used for the location
         If end is True, the start date + duration is used
-        """
+        '''
         dt: datetime = self.dt
         if end:
             dt += timedelta(self.duration)
@@ -31,3 +31,33 @@ class FallbackLocation:
             elevation=self.elevation,
             datasource=self.datasource,
         )
+
+    @classmethod
+    def from_end_date(
+        cls,
+        lat: float,
+        lon: float,
+        dt: datetime,
+        end_dt: datetime,
+        accuracy: Optional[float] = None,
+        elevation: Optional[float] = None,
+        datasource: Optional[str] = None,
+    ) -> 'FallbackLocation':
+        '''
+        Create FallbackLocation from a start date and an end date
+        '''
+        if end_dt < dt:
+            raise ValueError('end_date must be after dt')
+        duration = (end_dt - dt).total_seconds()
+        return cls(
+            lat=lat,
+            lon=lon,
+            dt=dt,
+            duration=duration,
+            accuracy=accuracy,
+            elevation=elevation,
+            datasource=datasource,
+        )
+
+
+# TODO: create estimate location which uses other fallback_locations to estimate a location