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public/py/vendor_linux/skyfield/constellationlib.py

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+# -*- coding: utf-8 -*-
+"""Constellation identification.
+
+Constellation identification is tradionally performed with:
+
+http://cdsarc.u-strasbg.fr/viz-bin/Cat?VI/42
+https://iopscience.iop.org/article/10.1086/132034/pdf
+
+Given a position, a binary search can be used to index into its roughly
+12k data table for a matching declination, but the table then needs to
+be scanned linearly for the first subsequent segment that contains the
+target right ascension within its bounds.  The big problem for Skyfield:
+that algorithm can't be constructed from vectorized NumPy primitives.
+
+So we here take an approach that requires no linear search.  The sky is
+divided into a grid, with a grid line at every right ascension and
+declination that is mentioned in a constellation boundary.  This is a
+bit wasteful, as many values are only ever used for one boundary, but
+makes the search easy: do a binary search for right ascension, then for
+declination, then look up the indexes in the grid.  Memory requirement:
+
+  1.8k  right ascension table
+  1.6k  declination table
+ 46.6k  grid table
+    1k  constellation name abbreviations
+
+"""
+import zlib
+from numpy import searchsorted
+from pkgutil import get_data
+from .functions import load_bundled_npy
+from .timelib import Time, julian_date_of_besselian_epoch
+
+def load_constellation_map():
+    """Load Skyfield's constellation boundaries and return a lookup function.
+
+    Skyfield carries an internal map of constellation boundaries that is
+    optimized for quick position lookup.  Call this function to load the
+    map and return a function mapping position to constellation name.
+
+    >>> from skyfield.api import position_of_radec, load_constellation_map
+    >>> constellation_at = load_constellation_map()
+    >>> north_pole = position_of_radec(0, 90)
+    >>> constellation_at(north_pole)
+    'UMi'
+
+    If you pass an array of positions, you'll receive an array of names.
+
+    """
+    t1875 = Time(None, julian_date_of_besselian_epoch(1875))
+
+    arrays = load_bundled_npy('constellations.npz')
+    sorted_ra = arrays['sorted_ra']
+    sorted_dec = arrays['sorted_dec']
+    radec_to_index = arrays['radec_to_index']
+    indexed_abbreviations = arrays['indexed_abbreviations']
+
+    def constellation_at(position):
+        ra, dec, distance = position.radec(epoch=t1875)
+        i = searchsorted(sorted_ra, ra.hours)
+        j = searchsorted(sorted_dec, dec.degrees, side='right')
+        k = radec_to_index[i, j]
+        return indexed_abbreviations[k]
+
+    return constellation_at
+
+def load_constellation_names():
+    """Return a list of abbreviation-name tuples, like ``('Aql', 'Aquila')``.
+
+    You can pass the list to Python’s ``dict()`` to build a dictionary
+    that turns a constellation abbreviation into a full name:
+
+    >>> from skyfield.api import load_constellation_names
+    >>> d = dict(load_constellation_names())
+    >>> d['UMa']
+    'Ursa Major'
+
+    By swapping the order of the two items, you can map the other way,
+    from a full name back to an abbreviation:
+
+    >>> f = dict(reversed(item) for item in load_constellation_names())
+    >>> f['Ursa Major']
+    'UMa'
+
+    """
+    data = get_data('skyfield', 'data/constellations.gz')
+    data = zlib.decompress(data, zlib.MAX_WBITS+32).decode('ascii')
+    return [line.split(' ', 1) for line in data.splitlines()]