GitLab

Fixes these UBSan errors from the unit test suite when
rasterizing a Boolean HEALPix mask image:

    ligo/skymap/tool/tests/test_bayestar.py src/bayestar_moc.c:109:27: runtime error: load of misaligned address 0x7febfedc6009 for type 'const int64_t', which requires 8 byte alignment
    0x7febfedc6009: note: pointer points here
     00 00 00  01 61 61 2d 00 00 00 00  00 01 66 61 2d 00 00 00  00 00 01 73 61 2d 00 00  00 00 00 01 38

Increase the range of validity of the solver used in
`ligo.skymap.distance.moments_to_parameters` for low-probability
pixels that are very prior dominated. Sky maps that have many such
pixels could have credible volumes repoted as infinity. The
incidence of such cases should now be decreased.

This eliminates LAL as a dependency for basic sky map I/O.

Improve the numerical stability of the method
`ligo.skymap.distance.conditional_ppf` by reparametrizing the equation
that is being solved. This method, which calculates the inverse of the
distance CDF, works by solving the equation `f(x) - p = 0` for `x`,
where `f(x)` is the distance CDF, and `p` is the desired probability.

The reparametrized equation is `log(1 - f(x)) - log(1 - p) = 0` if `p >
1/2` and `log(f(x)) - log(p) = 0` otherwise. This reparametrization is
effective because it improves the dynamic range in the tails of the
distribution. This same reparametrization had already proven effective
in the related method `ligo.skymap.distance.marginal_ppf`.

This change also fixes some rare corner cases where `marginal_ppf`
returned silly values becauses it uses `conditional_ppf` internally to
create its own initial guess. One example was the median distance for
the binary neutron star candidate S191205ah. Before this patch, the
result was negative and invalid:

    >>> from ligo.skymap.distance import marginal_ppf
    >>> from ligo.skymap.moc import uniq2pixarea
    >>> from ligo.skymap.io import read_sky_map
    >>> url = 'https://gracedb.ligo.org/apiweb/superevents/S191205ah/files/bayestar.multiorder.fits'
    >>> s = read_sky_map(url, moc=True)
    >>> marginal_ppf(0.5, s['PROBDENSITY'] * uniq2pixarea(s['UNIQ']),
    ...              s['DISTMU'], s['DISTSIGMA'], s['DISTNORM'])
    /Users/lpsinger/src/ligo.skymap/ligo/skymap/util/numpy.py:46:
    RuntimeWarning: invalid value encountered in marginal_ppf
      return func(*args, **kwargs)
    -223357.8508233767

After this patch, the result is positive and sensible:

    >>> marginal_ppf(0.5, s['PROBDENSITY'] * uniq2pixarea(s['UNIQ']),
    ...              s['DISTMU'], s['DISTSIGMA'], s['DISTNORM'])
    362.7485740018039

This reverts commit 500d788f.

This reverts commit ecece947.

This reverts commit 97060beb.