# Licensed under a 3-clause BSD style license - see LICENSE.rst
"""
This module contains a helper function to fill erfa.astrom struct and a
ScienceState, which allows to speed up coordinate transformations at the
expense of accuracy.
"""

import functools
import warnings

import erfa
import numpy as np

import astropy.units as u
from astropy.time import Time
from astropy.utils.exceptions import AstropyWarning
from astropy.utils.state import ScienceState

from .builtin_frames.utils import (
    get_cip,
    get_jd12,
    get_polar_motion,
    pav2pv,
    prepare_earth_position_vel,
)
from .matrix_utilities import rotation_matrix

__all__ = []


def _refco(frame_or_coord):
    if not hasattr(frame_or_coord, "pressure"):
        # This is not an AltAz frame, so don't bother computing refraction
        return 0.0, 0.0
    return erfa.refco(
        frame_or_coord.pressure.to_value(u.hPa),
        frame_or_coord.temperature.to_value(u.deg_C),
        frame_or_coord.relative_humidity.value,
        frame_or_coord.obswl.to_value(u.micron),
    )


class ErfaAstrom:
    """
    The default provider for astrometry values.
    A utility class to extract the necessary arguments for
    erfa functions from frame attributes, call the corresponding
    erfa functions and return the astrom object.
    """

    @staticmethod
    def apco(frame_or_coord):
        """
        Wrapper for ``erfa.apco``, used in conversions AltAz <-> ICRS and CIRS <-> ICRS.

        Parameters
        ----------
        frame_or_coord : ``astropy.coordinates.BaseCoordinateFrame`` or ``astropy.coordinates.SkyCoord``
            Frame or coordinate instance in the corresponding frame
            for which to calculate the calculate the astrom values.
            For this function, an AltAz or CIRS frame is expected.
        """
        lon, lat, height = frame_or_coord.location.to_geodetic("WGS84")
        obstime = frame_or_coord.obstime

        jd1_tt, jd2_tt = get_jd12(obstime, "tt")
        xp, yp = get_polar_motion(obstime)
        sp = erfa.sp00(jd1_tt, jd2_tt)
        x, y, s = get_cip(jd1_tt, jd2_tt)
        era = erfa.era00(*get_jd12(obstime, "ut1"))
        earth_pv, earth_heliocentric = prepare_earth_position_vel(obstime)

        # refraction constants
        refa, refb = _refco(frame_or_coord)

        return erfa.apco(
            jd1_tt,
            jd2_tt,
            earth_pv,
            earth_heliocentric,
            x,
            y,
            s,
            era,
            lon.to_value(u.radian),
            lat.to_value(u.radian),
            height.to_value(u.m),
            xp,
            yp,
            sp,
            refa,
            refb,
        )

    @staticmethod
    def apcs(frame_or_coord):
        """
        Wrapper for ``erfa.apcs``, used in conversions GCRS <-> ICRS.

        Parameters
        ----------
        frame_or_coord : ``astropy.coordinates.BaseCoordinateFrame`` or ``astropy.coordinates.SkyCoord``
            Frame or coordinate instance in the corresponding frame
            for which to calculate the calculate the astrom values.
            For this function, a GCRS frame is expected.
        """
        jd1_tt, jd2_tt = get_jd12(frame_or_coord.obstime, "tt")
        obs_pv = pav2pv(
            frame_or_coord.obsgeoloc.get_xyz(xyz_axis=-1).value,
            frame_or_coord.obsgeovel.get_xyz(xyz_axis=-1).value,
        )
        earth_pv, earth_heliocentric = prepare_earth_position_vel(
            frame_or_coord.obstime
        )
        return erfa.apcs(jd1_tt, jd2_tt, obs_pv, earth_pv, earth_heliocentric)

    @staticmethod
    def apio(frame_or_coord):
        """
        Slightly modified equivalent of ``erfa.apio``, used in conversions AltAz <-> CIRS.

        Since we use a topocentric CIRS frame, we have dropped the steps needed to calculate
        diurnal aberration.

        Parameters
        ----------
        frame_or_coord : ``astropy.coordinates.BaseCoordinateFrame`` or ``astropy.coordinates.SkyCoord``
            Frame or coordinate instance in the corresponding frame
            for which to calculate the calculate the astrom values.
            For this function, an AltAz frame is expected.
        """
        # Calculate erfa.apio input parameters.
        # TIO locator s'
        sp = erfa.sp00(*get_jd12(frame_or_coord.obstime, "tt"))

        # Earth rotation angle.
        theta = erfa.era00(*get_jd12(frame_or_coord.obstime, "ut1"))

        # Longitude and latitude in radians.
        lon, lat, height = frame_or_coord.location.to_geodetic("WGS84")
        elong = lon.to_value(u.radian)
        phi = lat.to_value(u.radian)

        # Polar motion, rotated onto local meridian
        xp, yp = get_polar_motion(frame_or_coord.obstime)

        # we need an empty astrom structure before we fill in the required sections
        astrom = np.zeros(frame_or_coord.obstime.shape, dtype=erfa.dt_eraASTROM)

        # Form the rotation matrix, CIRS to apparent [HA,Dec].
        r = (
            rotation_matrix(elong, "z", unit=u.radian)
            @ rotation_matrix(-yp, "x", unit=u.radian)
            @ rotation_matrix(-xp, "y", unit=u.radian)
            @ rotation_matrix(theta + sp, "z", unit=u.radian)
        )

        # Solve for local Earth rotation angle.
        a = r[..., 0, 0]
        b = r[..., 0, 1]
        eral = np.arctan2(b, a)
        astrom["eral"] = eral

        # Solve for polar motion [X,Y] with respect to local meridian.
        c = r[..., 0, 2]
        astrom["xpl"] = np.arctan2(c, np.sqrt(a * a + b * b))
        a = r[..., 1, 2]
        b = r[..., 2, 2]
        astrom["ypl"] = -np.arctan2(a, b)

        # Adjusted longitude.
        astrom["along"] = erfa.anpm(eral - theta)

        # Functions of latitude.
        astrom["sphi"] = np.sin(phi)
        astrom["cphi"] = np.cos(phi)

        # Omit two steps that are zero for a geocentric observer:
        # Observer's geocentric position and velocity (m, m/s, CIRS).
        # Magnitude of diurnal aberration vector.

        # Refraction constants.
        astrom["refa"], astrom["refb"] = _refco(frame_or_coord)

        return astrom


class ErfaAstromInterpolator(ErfaAstrom):
    """
    A provider for astrometry values that does not call erfa
    for each individual timestamp but interpolates linearly
    between support points.

    For the interpolation, float64 MJD values are used, so time precision
    for the interpolation will be around a microsecond.

    This can dramatically speed up coordinate transformations,
    e.g. between CIRS and ICRS,
    when obstime is an array of many values (factors of 10 to > 100 depending
    on the selected resolution, number of points and the time range of the values).

    The precision of the transformation will still be in the order of microseconds
    for reasonable values of time_resolution, e.g. ``300 * u.s``.

    Users should benchmark performance and accuracy with the default transformation
    for their specific use case and then choose a suitable ``time_resolution``
    from there.

    This class is intended be used together with the ``erfa_astrom`` science state,
    e.g. in a context manager like this

    Example
    -------
    >>> from astropy.coordinates import SkyCoord, CIRS
    >>> from astropy.coordinates.erfa_astrom import erfa_astrom, ErfaAstromInterpolator
    >>> import astropy.units as u
    >>> from astropy.time import Time
    >>> import numpy as np

    >>> obstime = Time('2010-01-01T20:00:00') + np.linspace(0, 4, 1000) * u.hour
    >>> crab = SkyCoord(ra='05h34m31.94s', dec='22d00m52.2s')
    >>> with erfa_astrom.set(ErfaAstromInterpolator(300 * u.s)):
    ...    cirs = crab.transform_to(CIRS(obstime=obstime))
    """

    @u.quantity_input(time_resolution=u.day)
    def __init__(self, time_resolution):
        if time_resolution.to_value(u.us) < 10:
            warnings.warn(
                f"Using {self.__class__.__name__} with `time_resolution`"
                " below 10 microseconds might lead to numerical inaccuracies"
                " as the MJD-based interpolation is limited by floating point "
                " precision to about a microsecond of precision",
                AstropyWarning,
            )
        self.mjd_resolution = time_resolution.to_value(u.day)

    def _get_support_points(self, obstime):
        """
        Calculate support points for the interpolation.

        We divide the MJD by the time resolution (as single float64 values),
        and calculate ceil and floor.
        Then we take the unique and sorted values and scale back to MJD.
        This will create a sparse support for non-regular input obstimes.
        """
        mjd_scaled = np.ravel(obstime.mjd / self.mjd_resolution)

        # unique already does sorting
        mjd_u = np.unique(np.concatenate([np.floor(mjd_scaled), np.ceil(mjd_scaled)]))

        return Time(
            mjd_u * self.mjd_resolution,
            format="mjd",
            scale=obstime.scale,
        )

    @staticmethod
    def _prepare_earth_position_vel(support, obstime):
        """
        Calculate Earth's position and velocity.

        Uses the coarser grid ``support`` to do the calculation, and interpolates
        onto the finer grid ``obstime``.
        """
        pv_support, heliocentric_support = prepare_earth_position_vel(support)

        # do interpolation
        earth_pv = np.empty(obstime.shape, dtype=erfa.dt_pv)
        earth_heliocentric = np.empty(obstime.shape + (3,))
        for dim in range(3):
            for key in "pv":
                earth_pv[key][..., dim] = np.interp(
                    obstime.mjd, support.mjd, pv_support[key][..., dim]
                )
            earth_heliocentric[..., dim] = np.interp(
                obstime.mjd, support.mjd, heliocentric_support[..., dim]
            )

        return earth_pv, earth_heliocentric

    def apco(self, frame_or_coord):
        """
        Wrapper for ``erfa.apco``, used in conversions AltAz <-> ICRS and CIRS <-> ICRS.

        Parameters
        ----------
        frame_or_coord : ``astropy.coordinates.BaseCoordinateFrame`` or ``astropy.coordinates.SkyCoord``
            Frame or coordinate instance in the corresponding frame
            for which to calculate the calculate the astrom values.
            For this function, an AltAz or CIRS frame is expected.
        """
        lon, lat, height = frame_or_coord.location.to_geodetic("WGS84")
        obstime = frame_or_coord.obstime
        support = self._get_support_points(obstime)
        interp = functools.partial(np.interp, obstime.mjd, support.mjd)
        jd1_tt, jd2_tt = get_jd12(obstime, "tt")

        # get the position and velocity arrays for the observatory.  Need to
        # have xyz in last dimension, and pos/vel in one-but-last.
        earth_pv, earth_heliocentric = self._prepare_earth_position_vel(
            support, obstime
        )

        xp, yp = map(interp, get_polar_motion(support))
        sp = erfa.sp00(jd1_tt, jd2_tt)
        x, y, s = map(interp, get_cip(*get_jd12(support, "tt")))
        era = erfa.era00(*get_jd12(obstime, "ut1"))

        # refraction constants
        refa, refb = _refco(frame_or_coord)

        return erfa.apco(
            jd1_tt,
            jd2_tt,
            earth_pv,
            earth_heliocentric,
            x,
            y,
            s,
            era,
            lon.to_value(u.radian),
            lat.to_value(u.radian),
            height.to_value(u.m),
            xp,
            yp,
            sp,
            refa,
            refb,
        )

    def apcs(self, frame_or_coord):
        """
        Wrapper for ``erfa.apci``, used in conversions GCRS <-> ICRS.

        Parameters
        ----------
        frame_or_coord : ``astropy.coordinates.BaseCoordinateFrame`` or ``astropy.coordinates.SkyCoord``
            Frame or coordinate instance in the corresponding frame
            for which to calculate the calculate the astrom values.
            For this function, a GCRS frame is expected.
        """
        obstime = frame_or_coord.obstime
        support = self._get_support_points(obstime)

        # get the position and velocity arrays for the observatory.  Need to
        # have xyz in last dimension, and pos/vel in one-but-last.
        earth_pv, earth_heliocentric = self._prepare_earth_position_vel(
            support, obstime
        )
        pv = pav2pv(
            frame_or_coord.obsgeoloc.get_xyz(xyz_axis=-1).value,
            frame_or_coord.obsgeovel.get_xyz(xyz_axis=-1).value,
        )

        jd1_tt, jd2_tt = get_jd12(obstime, "tt")
        return erfa.apcs(jd1_tt, jd2_tt, pv, earth_pv, earth_heliocentric)


class erfa_astrom(ScienceState):
    """
    ScienceState to select with astrom provider is used in
    coordinate transformations.
    """

    _value = ErfaAstrom()

    @classmethod
    def validate(cls, value):
        if not isinstance(value, ErfaAstrom):
            raise TypeError(f"Must be an instance of {ErfaAstrom!r}")
        return value
