#!/usr/bin/env python3
#
# Copyright (c) 2017 Weitian LI <weitian@aaronly.me>
# MIT license
#

"""
Calculate the 2D cylindrical-averaged power spectrum from the
3D image spectral cube.

References
----------
.. [liu2014]
   Liu, Parsons & Trott 2014, PhRvD, 90, 023018
   http://adsabs.harvard.edu/abs/2014PhRvD..90b3018L
   Appendix.A

.. [morales2004]
   Morales & Hewitt 2004, ApJ, 615, 7
   http://adsabs.harvard.edu/abs/2004ApJ...615....7M
   Sec.3

.. [matlab-psd-fft]
   MATLAB - Power Spectral Density Estimates Using FFT
   https://cn.mathworks.com/help/signal/ug/power-spectral-density-estimates-using-fft.html

.. [matlab-answer-psd]
   MATLAB Answers - How to create power spectral density from FFT
   https://cn.mathworks.com/matlabcentral/answers/43548-how-to-create-power-spectral-density-from-fft-fourier-transform
"""

import os
import sys
import argparse
import logging

import numpy as np
from scipy import fftpack
from scipy import signal
from astropy.io import fits
from astropy.wcs import WCS
from astropy.cosmology import FlatLambdaCDM
import astropy.constants as ac


logging.basicConfig(level=logging.INFO,
                    format="%(asctime)s [%(levelname)s] %(message)s",
                    datefmt="%Y-%m-%dT%H:%M:%S")
logger = logging.getLogger(os.path.basename(sys.argv[0]))


# HI line frequency
freq21cm = 1420.405751  # [MHz]
# Adopted cosmology
H0 = 71.0  # [km/s/Mpc]
OmegaM0 = 0.27
cosmo = FlatLambdaCDM(H0=H0, Om0=OmegaM0)


def freq2z(freq):
    z = freq21cm / freq - 1.0
    return z


def get_frequencies(wcs, nfreq):
    pix = np.zeros(shape=(nfreq, 3), dtype=np.int)
    pix[:, -1] = np.arange(nfreq)
    world = wcs.wcs_pix2world(pix, 0)
    freqMHz = world[:, -1] / 1e6
    return freqMHz


class PS2D:
    """
    2D cylindrically averaged power spectrum

    NOTE
    ----
    * Cube dimensions: [nfreq, height, width] <-> [Z, Y, X]
    * Cube data unit: [K] (brightness temperature)
    """
    def __init__(self, cube, pixelsize, frequencies,
                 window_name=None, window_width="extended"):
        logger.info("Initializing PS2D instance ...")
        self.cube = cube
        self.pixelsize = pixelsize  # [arcsec]
        logger.info("Loaded data cube: %dx%d (cells) * %d (channels)" %
                    (self.Nx, self.Ny, self.Nz))
        logger.info("Image pixel size: %.2f [arcsec]" % pixelsize)

        self.frequencies = np.asarray(frequencies)  # [MHz]
        self.nfreq = len(self.frequencies)
        self.dfreq = self.frequencies[1] - self.frequencies[0]  # [MHz]
        if self.nfreq != self.Nz:
            raise RuntimeError("data cube and frequencies do not match!")
        logger.info("Frequency band: %.2f-%.2f [MHz]" %
                    (self.frequencies.min(), self.frequencies.max()))
        logger.info("Frequency channel width: %.2f [MHz], %d channels" %
                    (self.dfreq, self.nfreq))

        # Central frequency and redshift
        self.freqc = self.frequencies.mean()
        self.zc = freq2z(self.freqc)
        logger.info("Central frequency %.2f [MHz] <-> redshift %.4f" %
                    (self.freqc, self.zc))

        # Transverse comoving distance at zc; unit: [Mpc]
        self.DMz = cosmo.comoving_transverse_distance(self.zc).value
        self.window_name = window_name
        self.window_width = window_width
        self.window = self.gen_window(name=window_name, width=window_width)

    def gen_window(self, name=None, width="extended"):
        if name is None:
            return None

        window_func = getattr(signal.windows, name)
        if width == "extended":
            w = window_func(self.nfreq, sym=False)
            ex = 1.0 / (w.sum() / self.nfreq)
            width_pix = int(ex * self.nfreq)
        else:
            width_pix = self.nfreq

        window = window_func(width_pix, sym=False)
        if len(window) > self.nfreq:
            # cut the filter
            midx = int(len(window) / 2)  # index of the peak element
            nleft = int(self.nfreq / 2)  # number of element on the left
            nright = int((self.nfreq-1) / 2)  # number of element on the right
            window = window[(midx-nleft):(midx+nright+1)]
        logger.info("Generated window: %s (%s/%d)" % (name, width, width_pix))
        return window

    def pad_cube(self):
        """
        Pad the image cube to be square in spatial dimensions.
        """
        if self.Nx != self.Ny:
            logger.info("Padding image to be square ...")
            raise NotImplementedError

    def calc_ps3d(self):
        """
        Calculate the 3D power spectrum of the image cube.

        The power spectrum is properly normalized to have dimension
        of [K^2 Mpc^3].
        """
        if self.window is not None:
            logger.info("Applying window along frequency axis ...")
            cube2 = self.cube * self.window[:, np.newaxis, np.newaxis]
        else:
            cube2 = self.cube.astype(np.float)

        logger.info("Calculating 3D FFT ...")
        cubefft = fftpack.fftshift(fftpack.fftn(cube2))

        logger.info("Calculating 3D PS ...")
        ps3d = np.abs(cubefft) ** 2  # [K^2]
        # Normalization
        norm1 = 1 / (self.Nx * self.Ny * self.Nz)
        norm2 = 1 / (self.fs_xy**2 * self.fs_z)  # [Mpc^3]
        norm3 = 1 / (2*np.pi)**3
        self.ps3d = ps3d * norm1 * norm2 * norm3  # [K^2 Mpc^3]
        return self.ps3d

    def calc_ps2d(self):
        """
        Calculate the 2D power spectrum by cylindrically binning
        the above 3D power spectrum.
        """
        logger.info("Calculating 2D power spectrum ...")
        n_k_perp = len(self.k_perp)
        n_k_los = len(self.k_los)
        ps2d = np.zeros(shape=(n_k_los, n_k_perp))  # (k_los, k_perp)

        eps = 1e-8
        ic_xy = (np.abs(self.k_xy) < eps).nonzero()[0][0]
        ic_z = (np.abs(self.k_z) < eps).nonzero()[0][0]
        p_xy = np.arange(self.Nx) - ic_xy
        p_z = np.abs(np.arange(self.Nz) - ic_z)
        mx, my = np.meshgrid(p_xy, p_xy)
        rho, phi = self.cart2pol(mx, my)
        rho = np.around(rho).astype(np.int)

        logger.info("Cylindrically averaging 3D power spectrum ...")
        for r in range(n_k_perp):
            ix, iy = (rho == r).nonzero()
            for s in range(n_k_los):
                iz = (p_z == s).nonzero()[0]
                cells = np.concatenate([self.ps3d[z, iy, ix] for z in iz])
                ps2d[s, r] = cells.mean()

        self.ps2d = ps2d
        return ps2d

    def save(self, outfile, clobber=False):
        """
        Save the calculated 2D power spectrum as a FITS image.
        """
        hdu = fits.PrimaryHDU(data=self.ps2d, header=self.header)
        try:
            hdu.writeto(outfile, overwrite=clobber)
        except TypeError:
            hdu.writeto(outfile, clobber=clobber)
        logger.info("Wrote 2D power spectrum to file: %s" % outfile)

    @property
    def Nx(self):
        """
        Number of cells/pixels along the X axis.
        Cube shape/dimensions: [Z, Y, X]
        """
        return self.cube.shape[2]

    @property
    def Ny(self):
        return self.cube.shape[1]

    @property
    def Nz(self):
        return self.cube.shape[0]

    @property
    def d_xy(self):
        """
        The sampling interval along the (X, Y) spatial dimensions,
        translated from the pixel size.
        Unit: [Mpc]

        Reference: Ref.[liu2014].Eq.(A7)
        """
        pixelsize = self.pixelsize / 3600  # [arcsec] -> [deg]
        d_xy = self.DMz * np.deg2rad(pixelsize)
        return d_xy

    @property
    def d_z(self):
        """
        The sampling interval along the Z line-of-sight dimension,
        translated from the frequency channel width.
        Unit: [Mpc]

        Reference: Ref.[liu2014].Eq.(A9)
        """
        dfreq = self.dfreq  # [MHz]
        c = ac.c.si.value  # [m/s]
        Ez = cosmo.efunc(self.zc)
        Hz = Ez * H0 * 1000.0  # [m/s/Mpc]
        d_z = c * (1+self.zc)**2 * dfreq / Hz / freq21cm
        return d_z

    @property
    def fs_xy(self):
        """
        The sampling frequency along the (X, Y) spatial dimensions:
            Fs = 1/T (inverse of interval)
        Unit: [Mpc^-1]
        """
        return 1/self.d_xy

    @property
    def fs_z(self):
        """
        The sampling frequency along the Z line-of-sight dimension.
        Unit: [Mpc^-1]
        """
        return 1/self.d_z

    @property
    def df_xy(self):
        """
        The spatial frequency bin size (i.e., resolution) along the
        (X, Y) dimensions.
        Unit: [Mpc^-1]
        """
        return self.fs_xy / self.Nx

    @property
    def df_z(self):
        """
        The spatial frequency bin size (i.e., resolution) along the
        line-of-sight (Z) direction.
        Unit: [Mpc^-1]
        """
        return self.fs_z / self.Nz

    @property
    def dk_xy(self):
        """
        The k-space (spatial) frequency bin size (i.e., resolution).
        """
        return 2*np.pi * self.df_xy

    @property
    def dk_z(self):
        return 2*np.pi * self.df_z

    @property
    def k_xy(self):
        """
        The k-space coordinates along the (X, Y) spatial dimensions,
        which describe the spatial frequencies.

        NOTE:
        k = 2*pi * f, where "f" is the spatial frequencies, and the
        Fourier dual to spatial transverse distances x/y.

        Unit: [Mpc^-1]
        """
        f_xy = fftpack.fftshift(fftpack.fftfreq(self.Nx, d=self.d_xy))
        k_xy = 2*np.pi * f_xy
        return k_xy

    @property
    def k_z(self):
        f_z = fftpack.fftshift(fftpack.fftfreq(self.Nz, d=self.d_z))
        k_z = 2*np.pi * f_z
        return k_z

    @property
    def k_perp(self):
        """
        Comoving wavenumbers perpendicular to the LoS

        NOTE: The Nyquist frequency just located at the first element
              after fftshift when the length is even, and it is negative.
        """
        k_x = self.k_xy
        return k_x[k_x >= 0]

    @property
    def k_los(self):
        """
        Comoving wavenumbers along the LoS
        """
        k_z = self.k_z
        return k_z[k_z >= 0]

    @staticmethod
    def cart2pol(x, y):
        """
        Convert Cartesian coordinates to polar coordinates.
        """
        rho = np.sqrt(x**2 + y**2)
        phi = np.arctan2(y, x)
        return (rho, phi)

    @property
    def header(self):
        dk_xy = self.dk_xy
        dk_z = self.dk_z
        hdr = fits.Header()
        hdr["HDUNAME"] = ("PS2D", "block name")
        hdr["CONTENT"] = ("2D cylindrically averaged power spectrum",
                          "data product")
        hdr["BUNIT"] = ("K^2 Mpc^3", "data unit")
        # Physical coordinates: IRAF LTM/LTV
        # Li{Image} = LTMi_i * Pi{Physical} + LTVi
        # Reference: ftp://iraf.noao.edu/iraf/web/projects/fitswcs/specwcs.html
        hdr["LTV1"] = 0.0
        hdr["LTM1_1"] = 1.0 / dk_xy
        hdr["LTV2"] = 0.0
        hdr["LTM2_2"] = 1.0 / dk_z
        # WCS physical coordinates
        hdr["WCSTY1P"] = "PHYSICAL"
        hdr["CTYPE1P"] = ("k_perp", "wavenumbers perpendicular to LoS")
        hdr["CRPIX1P"] = (0.5, "reference pixel")
        hdr["CRVAL1P"] = (0.0, "coordinate of the reference pixel")
        hdr["CDELT1P"] = (dk_xy, "coordinate delta/step")
        hdr["CUNIT1P"] = ("Mpc^-1", "coordinate unit")
        hdr["WCSTY2P"] = "PHYSICAL"
        hdr["CTYPE2P"] = ("k_los", "wavenumbers along LoS")
        hdr["CRPIX2P"] = (0.5, "reference pixel")
        hdr["CRVAL2P"] = (0.0, "coordinate of the reference pixel")
        hdr["CDELT2P"] = (dk_z, "coordinate delta/step")
        hdr["CUNIT2P"] = ("Mpc^-1", "coordinate unit")
        # Data information
        hdr["PixSize"] = (self.pixelsize, "[arcsec] data cube pixel size")
        hdr["Z_C"] = (self.zc, "data cube central redshift")
        hdr["Freq_C"] = (self.freqc, "[MHz] data cube central frequency")
        hdr["Freq_Min"] = (self.frequencies.min(),
                           "[MHz] data cube minimum frequency")
        hdr["Freq_Max"] = (self.frequencies.max(),
                           "[MHz] data cube maximum frequency")
        # Command history
        hdr.add_history(" ".join(sys.argv))
        return hdr


def main():
    parser = argparse.ArgumentParser(
        description="Calculate 2D power spectrum from 3D image cube")
    parser.add_argument("-C", "--clobber", dest="clobber",
                        action="store_true",
                        help="overwrite existing file")
    parser.add_argument("-p", "--pixelsize", dest="pixelsize",
                        type=float, required=True,
                        help="image cube pixel size; unit: [arcsec]")
    parser.add_argument("--window", dest="window",
                        choices=["nuttall"],
                        help="apply window along frequency axis " +
                        "(default: None)")
    parser.add_argument("-i", "--infile", dest="infile", required=True,
                        help="input FITS image cube")
    parser.add_argument("-o", "--outfile", dest="outfile", required=True,
                        help="output 2D power spectrum FITS file")
    args = parser.parse_args()

    with fits.open(args.infile) as f:
        cube = f[0].data
        wcs = WCS(f[0].header)
    nfreq = cube.shape[0]
    frequencies = get_frequencies(wcs, nfreq)
    ps2d = PS2D(cube=cube, pixelsize=args.pixelsize, frequencies=frequencies,
                window_name=args.window)
    ps2d.calc_ps3d()
    ps2d.calc_ps2d()
    ps2d.save(outfile=args.outfile, clobber=args.clobber)


if __name__ == "__main__":
    main()