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

"""
Create FITS image cube from a series of image slices.
"""

import os
import sys
import argparse
from datetime import datetime, timezone

import numpy as np
from astropy.io import fits
from astropy.wcs import WCS


class FITSCube:
    """
    FITS image cube.
    """
    def __init__(self, infile=None):
        if infile is not None:
            self.load(infile)

    def load(self, infile):
        with fits.open(infile) as f:
            self.data = f[0].data
            self.header = f[0].header
        print("Loaded FITS cube from file: %s" % infile)
        print("Cube dimensions: %dx%dx%d" %
              (self.width, self.height, self.nslice))
        # The Z-axis position of the first slice.
        self.zbegin = self.header["CRVAL3"]
        # The Z-axis step/spacing between slices.
        self.zstep = self.header["CDELT3"]

    def add_slices(self, slices, zbegin=0.0, zstep=1.0):
        """
        Create a FITS cube from input image slices.
        """
        self.slices = slices
        self.zbegin = zbegin
        self.zstep = zstep
        nslice = len(slices)
        header, image = self.open_image(slices[0])
        shape = (nslice, ) + image.shape
        data = np.zeros(shape, dtype=image.dtype)
        for i, s in enumerate(slices):
            print("[%d/%d] Adding image slice: %s ..." % (i+1, nslice, s))
            hdr, img = self.open_image(s)
            data[i, :, :] = img
        self.data = data
        self.header = header.copy(strip=True)
        print("Created FITS cube of dimensions: %dx%dx%d" %
              (self.width, self.height, self.nslice))

    @staticmethod
    def open_image(infile):
        """
        Open the slice image and return its header and 2D image data.

        NOTE
        ----
        The input slice image may have following dimensions:
        * NAXIS=2: [Y, X]
        * NAXIS=3: [FREQ=1, Y, X]
        * NAXIS=4: [STOKES=1, FREQ=1, Y, X]

        NOTE
        ----
        Only open slice image that has only ONE frequency and ONE Stokes
        parameter.

        Returns
        -------
        header : `~astropy.io.fits.Header`
        image : 2D `~numpy.ndarray`
            The 2D [Y, X] image part of the slice image.
        """
        with fits.open(infile) as f:
            header = f[0].header
            data = f[0].data
        if data.ndim == 2:
            # NAXIS=2: [Y, X]
            image = data
        elif data.ndim == 3 and data.shape[0] == 1:
            # NAXIS=3: [FREQ=1, Y, X]
            image = data[0, :, :]
        elif data.ndim == 4 and data.shape[0] == 1 and data.shape[1] == 1:
            # NAXIS=4: [STOKES=1, FREQ=1, Y, X]
            image = data[0, 0, :, :]
        else:
            raise ValueError("Slice '{0}' has invalid dimensions: {1}".format(
                infile, data.shape))
        return (header, image)

    @property
    def header(self):
        if not hasattr(self, "header_"):
            self.header_ = fits.Header()
        return self.header_

    @header.setter
    def header(self, value):
        self.header_ = value
        for key in ["CTYPE4", "CRPIX4", "CRVAL4", "CDELT4", "CUNIT4"]:
            try:
                del self.header_[key]
            except KeyError:
                pass

    @property
    def wcs(self):
        w = WCS(naxis=3)
        w.wcs.ctype = ["pixel", "pixel", "pixel"]
        w.wcs.crpix = np.array([self.header.get("CRPIX1", 1.0),
                                self.header.get("CRPIX2", 1.0),
                                1.0])
        w.wcs.crval = np.array([self.header.get("CRVAL1", 0.0),
                                self.header.get("CRVAL2", 0.0),
                                self.zbegin])
        w.wcs.cdelt = np.array([self.header.get("CDELT1", 1.0),
                                self.header.get("CDELT2", 1.0),
                                self.zstep])
        return w

    def write(self, outfile, clobber=False):
        header = self.header
        header.extend(self.wcs.to_header(), update=True)
        header["DATE"] = (datetime.now(timezone.utc).astimezone().isoformat(),
                          "File creation date")
        header.add_history(" ".join(sys.argv))
        hdu = fits.PrimaryHDU(data=self.data, header=header)
        try:
            hdu.writeto(outfile, overwrite=clobber)
        except TypeError:
            hdu.writeto(outfile, clobber=clobber)

    @property
    def width(self):
        __, __, w = self.data.shape
        return w

    @property
    def height(self):
        __, h, __ = self.data.shape
        return h

    @property
    def nslice(self):
        ns, __, __ = self.data.shape
        return ns

    @property
    def zvalues(self):
        """
        Calculate the Z-axis positions for all slices
        """
        nslice = self.nslice
        wcs = self.wcs
        pix = np.zeros(shape=(nslice, 3), dtype=np.int)
        pix[:, 2] = np.arange(nslice)
        world = wcs.wcs_pix2world(pix, 0)
        return world[:, 2]

    @property
    def unit(self):
        """
        Data cube unit.
        """
        return self.header.get("BUNIT")

    @unit.setter
    def unit(self, value):
        self.header["BUNIT"] = value

    @property
    def zunit(self):
        """
        Unit of the slice z-axis positions.
        """
        return self.header.get("CUNIT3")

    @zunit.setter
    def zunit(self, value):
        self.header["CUNIT3"] = value


def cmd_info(args):
    """
    Sub-command: "info", show FITS cube information
    """
    cube = FITSCube(args.infile)
    if cube.zunit:
        pzunit = " [%s]" % cube.zunit
    else:
        pzunit = ""
    zvalues = cube.zvalues
    print("Data cube unit: %s" % cube.unit)
    print("Image/slice size: %dx%d" % (cube.width, cube.height))
    print("Number of slices: %d" % cube.nslice)
    print("Slice step/spacing: %s%s" % (cube.zstep, pzunit))
    print("Slice positions: %s <-> %s%s" %
          (zvalues.min(), zvalues.max(), pzunit))


def cmd_create(args):
    """
    Sub-command: "create", create a FITS cube
    """
    if not args.clobber and os.path.exists(args.outfile):
        raise FileExistsError("output file already exists: %s" % args.outfile)
    cube = FITSCube()
    cube.add_slices(args.infiles, zbegin=args.zbegin, zstep=args.zstep)
    cube.zunit = args.zunit
    if args.unit:
        cube.unit = args.unit
    cube.write(args.outfile, clobber=args.clobber)
    print("Created FITS cube: %s" % args.outfile)


def main():
    parser = argparse.ArgumentParser(
        description="Create FITS cube from a series of image slices.")
    subparsers = parser.add_subparsers(dest="subparser_name",
                                       title="sub-commands",
                                       help="additional help")
    # sub-command: "info"
    parser_info = subparsers.add_parser("info", help="show FITS cube info")
    parser_info.add_argument("infile", help="FITS cube filename")
    parser_info.set_defaults(func=cmd_info)
    # sub-command: "create"
    parser_create = subparsers.add_parser("create", help="create a FITS cube")
    parser_create.add_argument("-C", "--clobber", dest="clobber",
                               action="store_true",
                               help="overwrite existing output file")
    parser_create.add_argument("-U", "--data-unit", dest="unit",
                               help="cube data unit (will overwrite the " +
                               "slice data unit)")
    parser_create.add_argument("-z", "--z-begin", dest="zbegin",
                               type=float, default=0.0,
                               help="Z-axis position of the first slice")
    parser_create.add_argument("-s", "--z-step", dest="zstep",
                               type=float, default=1.0,
                               help="Z-axis step/spacing between slices")
    parser_create.add_argument("-u", "--z-unit", dest="zunit",
                               help="Z-axis unit (e.g., cm, Hz)")
    parser_create.add_argument("-o", "--outfile", dest="outfile",
                               required=True,
                               help="output FITS cube filename")
    parser_create.add_argument("-i", "--infiles", dest="infiles",
                               nargs="+", required=True,
                               help="input image slices (in order)")
    parser_create.set_defaults(func=cmd_create)
    #
    args = parser.parse_args()
    args.func(args)


if __name__ == "__main__":
    main()