/*============================================================================
  WCSLIB 8.3 - an implementation of the FITS WCS standard.
  Copyright (C) 1995-2024, Mark Calabretta

  This file is part of WCSLIB.

  WCSLIB is free software: you can redistribute it and/or modify it under the
  terms of the GNU Lesser General Public License as published by the Free
  Software Foundation, either version 3 of the License, or (at your option)
  any later version.

  WCSLIB is distributed in the hope that it will be useful, but WITHOUT ANY
  WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
  FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for
  more details.

  You should have received a copy of the GNU Lesser General Public License
  along with WCSLIB.  If not, see http://www.gnu.org/licenses.

  Author: Mark Calabretta, Australia Telescope National Facility, CSIRO.
  http://www.atnf.csiro.au/people/Mark.Calabretta
  $Id: HPXcvt.c,v 8.3 2024/05/13 16:33:01 mcalabre Exp $
*=============================================================================
* Usage: HPXcvt [-C<col>] [-c<sys>] [-n|-r] [-q<quad>] [-x<n|s>]
*               [<infile> [<outfile>]]
*-----------------------------------------------------------------------------
* HPXcvt reorganises HEALPix data into a 2-D FITS image.  Refer to the usage
* notes below.
*---------------------------------------------------------------------------*/

char usage[] =
"Usage: HPXcvt [-C<col>] [-c<sys>] [-n|-r] [-q<quad>] [-x<n|s>]\n"
"              [<infile> [<outfile>]]\n"
"\n"
"HPXcvt reorganises HEALPix data into a 2-D FITS image with HPX coordinate\n"
"system.  The input data may be stored in a FITS file as a primary image\n"
"or image extension, or as a binary table extension.  Both NESTED and RING\n"
"pixel indices are supported.  The input and output files may be omitted or\n"
"specified as \"-\" to indicate stdin and stdout respectively.\n"
"\n"
"Options:\n"
"  -C<col>      Binary table column number from which to read data,\n"
"               default 1 (n.b. WMAP exclusion masks are in column 2).\n"
"\n"
"  -c<sys>      Specify the coordinate system to be used to label the\n"
"               output map if the COORDSYS keyword is absent from the input\n"
"               FITS header.  Recognised values are g (galactic),\n"
"               e (ecliptic) or q (equatorial).\n"
"\n"
"  -n|-r        Assume n(ested) or r(ing) organization if the ORDERING\n"
"               keyword is absent from the input FITS header.\n"
"\n"
"  -q<quad>     Recentre longitude at quad(mod 4) x 90 degrees, where\n"
"               quad(rant) is an integer.\n"
"\n"
"  -x<n|s>      Use a north-polar or south-polar layout (XPH).\n";

#include <ctype.h>
#include <errno.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <fitsio.h>

#define HEALPIX_NULLVAL (-1.6375e30)

struct healpix {
  char  *infile;	// Input file.
  char  *outfile;	// Output file.
  int   col;		// Input binary table column number.
  char  crdsys;		// G(alactic), E(cliptic), or (e)Q(uatorial).
  char  ordering;	// R(ing) or N(ested).
  char  layout;		// Required output layout,
                        //   0: equatorial (default),
                        //   1: north, or
                        //   2: south.
  char  quad;		// Recentre longitude on quadrant (modulo 4).
  int   nside;		// Dimension of a base-resolution pixel.
  int   padding;	// (Dummy inserted for alignment purposes.)
  long  npix;		// Total number of pixels in the data array.
  float *data;		// Pointer to memory allocated to hold data.
};

int HEALPixIn(struct healpix *hpxdat);
int HPXout(struct healpix *hpxdat);
int NESTidx(int nside, int facet, int rotn, int row, long *healidx);
int RINGidx(int nside, int facet, int rotn, int row, long *healidx);
int HPXhdr(fitsfile *fptr, struct healpix *hpxdat);


int main(int argc, char **argv)

{
  struct healpix hpxdat;
  hpxdat.col      = 1;
  hpxdat.crdsys   = '?';
  hpxdat.ordering = '?';
  hpxdat.layout   = 0;
  hpxdat.quad     = 0;

  // Parse options.
  int i;
  for (i = 1; i < argc && argv[i][0] == '-'; i++) {
    int crdsys, layout, quad;
    if (!argv[i][1]) break;

    switch (argv[i][1]) {
    case 'C':
      hpxdat.col = atoi(argv[i]+2);
      if (hpxdat.col < 1) hpxdat.col = 1;
      break;
    case 'c':
      crdsys = toupper(argv[i][2]);
      switch (crdsys) {
      case 'G':
      case 'E':
      case 'Q':
        hpxdat.crdsys = (char)crdsys;
      };
      break;
    case 'n':
      hpxdat.ordering = 'N';
      break;
    case 'q':
      quad = atoi(argv[i]+2)%4;
      if (quad < 0) quad += 4;
      hpxdat.quad = (char)quad;
      break;
    case 'r':
      hpxdat.ordering = 'R';
      break;
    case 'x':
      layout = toupper(argv[i][2]);
      switch (layout) {
      case '\0':
      case 'N':
        hpxdat.layout = 1;
        break;
      case 'S':
        hpxdat.layout = 2;
        break;
      };
      break;
    default:
      fprintf(stderr, "%s", usage);
      return 1;
    }
  }

  if (i < argc) {
    hpxdat.infile = argv[i++];

    if (i < argc) {
      hpxdat.outfile = argv[i++];

      if (i < argc) {
        fprintf(stderr, "%s", usage);
        return 1;
      }
    } else {
      hpxdat.outfile = "-";
    }
  } else {
    hpxdat.infile = "-";
  }

  // Check accessibility of the input file.
  if (strcmp(hpxdat.infile, "-") && access(hpxdat.infile, R_OK) == -1) {
    printf("HPXcvt: Cannot access %s.\n", hpxdat.infile);
    return 1;
  }


  // Get the HEALPix data as a vector.
  int status = HEALPixIn(&hpxdat);
  if (status) {
    status = 2;
    goto cleanup;
  }

  if (hpxdat.ordering == '?') {
    fprintf(stderr, "WARNING: ORDERING keyword absent, assuming RING.\n");
    hpxdat.ordering = 'r';
  }

  printf("HPXcvt: Read 12 * %d^2  = %ld pixels with %s indexing.\n",
    hpxdat.nside, hpxdat.npix, (hpxdat.ordering == 'N') ? "nested" : "ring");

  // Map and write it out as a FITS image.
  status = HPXout(&hpxdat);
  if (status) {
    status = 3;
  }

cleanup:
  if (hpxdat.data) free(hpxdat.data);

  return status;
}

//----------------------------------------------------------------------------

int HEALPixIn(struct healpix *hpxdat)

{
  int status = 0;
  hpxdat->data = 0x0;

  // Open the FITS file and move to the first HDU with NAXIS != 0.
  fitsfile *fptr = 0x0;
  if (fits_open_data(&fptr, hpxdat->infile, READONLY, &status)) goto fitserr;

  // Is this the primary HDU or an extension?
  int hdutype;
  if (fits_get_hdu_type(fptr, &hdutype, &status)) goto fitserr;
  if (!(hdutype == IMAGE_HDU || hdutype == BINARY_TBL)) {
    fprintf(stderr, "ERROR: %s does not contain HEALPix data.\n",
            hpxdat->infile);
    return 1;
  }


  // Get the image size.
  long naxis;
  if (fits_read_key_lng(fptr, "NAXIS", &naxis, 0x0, &status)) goto fitserr;

  int  nfound;
  long *naxes = 0x0;
  naxes = malloc(naxis*sizeof(long));
  if (fits_read_keys_lng(fptr, "NAXIS", 1, (int)naxis, naxes, &nfound,
                         &status)) goto cleanup;

  LONGLONG nelem = 0, npix = 0, nrow = 0;
  if (hdutype == IMAGE_HDU) {
    // Look for the first non-degenerate image axis.
    for (int iaxis = 0; iaxis < nfound; iaxis++) {
      if (naxes[iaxis] > 1) {
        // Assume for now that it is the total number of pixels.
        npix = naxes[iaxis];
        break;
      }
    }

  } else if (hdutype == BINARY_TBL) {
    // Binary tables are simpler.
    if (nfound > 1) nrow = naxes[1];

    // (Note that fits_get_coltypell() is not available in cfitsio 2.x.)
    long repeat;
    if (fits_get_coltype(fptr, hpxdat->col, 0x0, &repeat, 0x0, &status)) {
      goto cleanup;
    }
    nelem = (LONGLONG)repeat;
  }

  if (!npix && !nrow) {
    fprintf(stderr, "ERROR: Could not determine image size.\n");
    goto cleanup;
  }


  // Number of pixels per side of each base-resolution pixel.
  long nside = 0;
  if (fits_read_key_lng(fptr, "NSIDE", &nside, 0x0, &status)) {
    // Some HEALPix files, e.g. SFD dust maps, don't record NSIDE.
    if (status != KEY_NO_EXIST) goto cleanup;
    status = 0;
  }

  // FIRSTPIX and LASTPIX, if present, record the 0-relative pixel numbers of
  // the first and last pixels stored in the data.
  long firstpix = -1;
  if (fits_read_key_lng(fptr, "FIRSTPIX", &firstpix, 0x0, &status)) {
    if (status != KEY_NO_EXIST) goto cleanup;
    status = 0;
  }

  long lastpix = -1;
  if (fits_read_key_lng(fptr, "LASTPIX", &lastpix, 0x0, &status)) {
    if (status != KEY_NO_EXIST) goto cleanup;
    status = 0;
  }

  if (!nside) {
    // Deduce NSIDE.
    if (lastpix >= 0) {
      // If LASTPIX is present without NSIDE we can only assume it's npix.
      nside = (int)(sqrt((double)((lastpix+1) / 12)) + 0.5);
    } else if (hdutype == IMAGE_HDU) {
      nside = (int)(sqrt((double)(npix / 12)) + 0.5);
    } else if (hdutype == BINARY_TBL) {
      nside = (int)(sqrt((double)((nrow * nelem) / 12)) + 0.5);
    }
  }

  hpxdat->nside = (int)nside;
  hpxdat->npix  = 12*nside*nside;

  // Ensure that FIRSTPIX and LASTPIX are set.
  if (firstpix < 0) firstpix = 0;
  if (lastpix  < 0) lastpix  = hpxdat->npix - 1;


  // Any sign of a coordinate system identifier?
  char crdsys[32];
  if (fits_read_key_str(fptr, "COORDSYS", crdsys, 0x0, &status)) {
    if (status != KEY_NO_EXIST) goto cleanup;
    status = 0;
  } else if (crdsys[0] == 'G') {
    hpxdat->crdsys = 'G';
  } else if (crdsys[0] == 'E') {
    hpxdat->crdsys = 'E';
  } else if (crdsys[0] == 'C') {
    // ("celestial")
    hpxdat->crdsys = 'Q';
  }

  // Nested or ring ordering?
  char ordering[32];
  if (fits_read_key_str(fptr, "ORDERING", ordering, 0x0, &status)) {
    // Some HEALPix files, e.g. SFD dust maps, don't record ORDERING.
    if (status != KEY_NO_EXIST) goto cleanup;
    status = 0;

  } else if (strcmp(ordering, "NESTED") == 0) {
    hpxdat->ordering = 'N';

  } else if (strcmp(ordering, "RING") == 0) {
    hpxdat->ordering = 'R';

  } else {
    fprintf(stderr, "WARNING: Invalid ORDERING keyword: %s.\n", ordering);
  }


  // Allocate memory and read the data.
  if ((hpxdat->data = malloc((hpxdat->npix)*sizeof(float))) == 0x0) {
    perror("HPXcvt");
    goto cleanup;
  }

  float nulval = HEALPIX_NULLVAL;
  float *datap = hpxdat->data;
  for (long ipix = 0; ipix < firstpix; ipix++) {
    *(datap++) = nulval;
  }

  LONGLONG firstelem = (LONGLONG)1;
  if (hdutype == IMAGE_HDU) {
    int anynul;
    if (fits_read_img_flt(fptr, 0l, firstelem, npix, nulval, datap, &anynul,
        &status)) goto cleanup;

  } else if (hdutype == BINARY_TBL) {
    for (LONGLONG irow = 0; irow < nrow; irow++) {
      int anynul;
      if (fits_read_col_flt(fptr, hpxdat->col, irow+1, firstelem, nelem,
          nulval, datap, &anynul, &status)) goto cleanup;
      datap += nelem;
    }
  }

  datap = hpxdat->data + (lastpix + 1);
  for (long ipix = (lastpix+1); ipix < hpxdat->npix; ipix++) {
    *(datap++) = nulval;
  }

cleanup:
  if (naxes) free(naxes);

fitserr:
  if (status) {
    if (hpxdat->data) free(hpxdat->data);
    hpxdat->data = 0x0;

    fits_report_error(stderr, status);
  }

  if (fptr) {
    int fstat = 0;
    fits_close_file(fptr, &fstat);
  }

  return status?1:0;
}

//----------------------------------------------------------------------------

int HPXout(struct healpix *hpxdat)

{
  // Number of facets on a side of each layout.
  const int NFACET[] = {5, 4, 4};

  // Arrays that define the facet location and rotation for each recognised
  // layout.  Bear in mind that these appear to be upside-down, i.e. the top
  // line contains facet numbers for the bottom row of the output image.
  // Facets numbered -1 are blank.

                              // Equatorial (diagonal) facet layout.
  const int FACETS[][5][5] = {{{ 6,  9, -1, -1, -1},
                               { 1,  5,  8, -1, -1},
                               {-1,  0,  4, 11, -1},
                               {-1, -1,  3,  7, 10},
                               {-1, -1, -1,  2,  6}},
                              // North polar (X) facet layout.
                              {{ 8,  4,  4, 11, -1},
                               { 5,  0,  3,  7, -1},
                               { 5,  1,  2,  7, -1},
                               { 9,  6,  6, 10, -1},
                               {-1, -1, -1, -1, -1}},
                              // South polar (X) facet layout.
                              {{ 1,  6,  6,  2, -1},
                               { 5,  9, 10,  7, -1},
                               { 5,  8, 11,  7, -1},
                               { 0,  4,  4,  3, -1},
                               {-1, -1, -1, -1, -1}}};

  // All facets of the equatorial layout are rotated by +45 degrees with
  // respect to the normal orientation, i.e. that with the equator running
  // horizontally.  The rotation recorded for the polar facets is the number
  // of additional positive (anti-clockwise) 90 degree turns with respect to
  // the equatorial layout.

                              // Equatorial (diagonal), no facet rotation.
  const int FROTAT[][5][5] = {{{ 0,  0,  0,  0,  0},
                               { 0,  0,  0,  0,  0},
                               { 0,  0,  0,  0,  0},
                               { 0,  0,  0,  0,  0},
                               { 0,  0,  0,  0,  0}},
                              // North polar (X) facet rotation.
                              {{ 3,  3,  0,  0,  0},
                               { 3,  3,  0,  0,  0},
                               { 2,  2,  1,  1,  0},
                               { 2,  2,  1,  1,  0},
                               { 0,  0,  0,  0,  0}},
                              // South polar (X) facet rotation.
                              {{ 1,  1,  2,  2,  0},
                               { 1,  1,  2,  2,  0},
                               { 0,  0,  3,  3,  0},
                               { 0,  0,  3,  3,  0},
                               { 0,  0,  0,  0,  0}}};

  // Facet halving codes.  0: the facet is whole (or wholly blank),
  // 1: blanked bottom-right, 2: top-right, 3: top-left, 4: bottom-left.
  // Positive values mean that the diagonal is included, otherwise not.

                              // Equatorial (diagonal), no facet halving.
  const int FHALVE[][5][5] = {{{ 0,  0,  0,  0,  0},
                               { 0,  0,  0,  0,  0},
                               { 0,  0,  0,  0,  0},
                               { 0,  0,  0,  0,  0},
                               { 0,  0,  0,  0,  0}},
                              // North polar (X) facet halving.
                              {{ 0,  1, -4,  0,  0},
                               {-3,  0,  0,  2,  0},
                               { 4,  0,  0, -1,  0},
                               { 0, -2,  3,  0,  0},
                               { 0,  0,  0,  0,  0}},
                              // South polar (X) facet halving.
                              {{ 0,  1, -4,  0,  0},
                               {-3,  0,  0,  2,  0},
                               { 4,  0,  0, -1,  0},
                               { 0, -2,  3,  0,  0},
                               { 0,  0,  0,  0,  0}}};

  int nside  = hpxdat->nside;
  int layout = hpxdat->layout;
  int nfacet = NFACET[layout];

  // Create the output FITS file.
  int  status = 0;
  long naxes[2];
  naxes[0] = nfacet * nside;
  naxes[1] = naxes[0];
  fitsfile *fptr;
  if (fits_create_file(&fptr, hpxdat->outfile, &status)) goto fitserr;
  if (fits_create_img(fptr, FLOAT_IMG, 2, naxes, &status)) goto fitserr;

  // Write WCS keyrecords.
  if ((status = HPXhdr(fptr, hpxdat))) goto fitserr;


  // Allocate arrays.
  long  *healidx = 0x0;
  float *row = 0x0;
  if ((healidx = malloc(nside * sizeof(long)))   == 0x0 ||
      (row     = malloc(nside * sizeof(float)))  == 0x0) {
    perror("HPXcvt");
    goto cleanup;
  }

  // Loop vertically facet-by-facet.
  LONGLONG fpixel = 1;
  LONGLONG group  = 0;
  LONGLONG nelem  = nside;
  for (int jfacet = 0; jfacet < nfacet; jfacet++) {
    // Loop row-by-row.
    for (int j = 0; j < nside; j++) {
      // Loop horizontally facet-by-facet.
      for (int ifacet = 0; ifacet < nfacet; ifacet++) {
        int facet = FACETS[layout][jfacet][ifacet];
        int rotn  = FROTAT[layout][jfacet][ifacet];
        int halve = FHALVE[layout][jfacet][ifacet];

        // Recentre longitude?
        if (hpxdat->quad && facet >= 0) {
          if (facet <= 3) {
            facet += hpxdat->quad;
            if (facet > 3) facet -= 4;
          } else if (facet <= 7) {
            facet += hpxdat->quad;
            if (facet > 7) facet -= 4;
          } else {
            facet += hpxdat->quad;
            if (facet > 11) facet -= 4;
          }
        }

        // Write out the data.
        if (facet < 0) {
          // A blank facet.
          if (fits_write_img_null(fptr, group, fpixel, nelem, &status)) {
            goto cleanup;
          }

        } else {
          if (hpxdat->ordering == 'N') {
            // Get nested indices.
            status = NESTidx(nside, facet, rotn, j, healidx);
          } else {
            // Get ring indices.
            status = RINGidx(nside, facet, rotn, j, healidx);
          }

          // Gather data into the output vector.
          long *healp = healidx;
          for (float *rowp = row; rowp < row + nside; rowp++) {
            *rowp = hpxdat->data[*(healp++)];
          }

          // Apply blanking to halved facets.
          float nulval = HEALPIX_NULLVAL;
          if (halve) {
            int i1, i2;
            if (abs(halve) == 1) {
              // Blank bottom-right.
              i1 = j;
              i2 = nside;
              if (halve > 0) i1++;
            } else if (abs(halve) == 2) {
              // Blank top-right.
              i1 = nside - j;
              i2 = nside;
              if (halve < 0) i1--;
            } else if (abs(halve) == 3) {
              // Blank top-left.
              i1 = 0;
              i2 = j;
              if (halve < 0) i2++;
            } else {
              // Blank bottom-left.
              i1 = 0;
              i2 = nside - j;
              if (halve > 0) i2--;
            }

            for (float *rowp = row + i1; rowp < row + i2; rowp++) {
              *rowp = nulval;
            }
          }

          // Write out this facet's contribution to this row of the map.
          if (fits_write_imgnull_flt(fptr, group, fpixel, nelem, row, nulval,
                                     &status)) {
            goto cleanup;
          }
        }

        fpixel += nelem;
      }
    }
  }

  // Write history.
  char history[72];
  sprintf(history, "Original input file: %s", hpxdat->infile);
  fits_write_history(fptr, history, &status);
  sprintf(history, "     Original NSIDE: %d", hpxdat->nside);
  fits_write_history(fptr, history, &status);
  sprintf(history, "  Original ordering: %s",
    (hpxdat->ordering == 'N') ? "NESTED" : "RING");
  if (hpxdat->ordering == 'r') strcat(history, " (assumed)");
  fits_write_history(fptr, history, &status);


  // Clean up.
  free(healidx);
  free(row);
  fits_close_file(fptr, &status);
  status = 0;
  return 0;

cleanup:
  if (healidx) free(healidx);
  if (row)     free(row);
fitserr:
  fits_report_error(stderr, status);
  if (fptr) {
    status = 0;
    fits_close_file(fptr, &status);
  }

  return 1;
}

//----------------------------------------------------------------------------

// (imap,jmap) are 0-relative pixel coordinates in the output map with origin
// at the bottom-left corner of the specified facet which is rotated by
// (45 + rotn * 90) degrees from its natural orientation; imap increases to
// the right and jmap upwards.

int NESTidx(int nside, int facet, int rotn, int jmap, long *healidx)

{
  // Nested index (0-relative) of the first pixel in this facet.
  int h = facet * nside * nside;

  int  nside1 = nside - 1;
  long *hp = healidx;
  for (int imap = 0; imap < nside; imap++, hp++) {
    // (i,j) are 0-relative pixel coordinates with origin in the southern
    // corner of the facet; i increases to the north-east and j to the
    // north-west.
    int i, j;
    if (rotn == 0) {
      i = nside1 - imap;
      j = jmap;
    } else if (rotn == 1) {
      i = nside1 - jmap;
      j = nside1 - imap;
    } else if (rotn == 2) {
      i = imap;
      j = nside1 - jmap;
    } else if (rotn == 3) {
      i = jmap;
      j = imap;
    }

    *hp = 0;
    int bit = 1;
    while (i || j) {
      if (i & 1) *hp |= bit;
      bit <<= 1;
      if (j & 1) *hp |= bit;
      bit <<= 1;
      i >>= 1;
      j >>= 1;
    }

    *hp += h;
  }

  return 0;
}

//----------------------------------------------------------------------------

// (imap,jmap) pixel coordinates are as described above for NESTidx().  This
// function computes the double-pixelisation index then converts it to the
// regular ring index.

int RINGidx(int nside, int facet, int rotn, int jmap, long *healidx)

{
  const int I0[] = { 1,  3, -3, -1,  0,  2,  4, -2,  1,  3, -3, -1};
  const int J0[] = { 1,  1,  1,  1,  0,  0,  0,  0, -1, -1, -1, -1};

  int n2side = 2 * nside;
  int n8side = 8 * nside;

  // Double-pixelisation index of the last pixel in the north polar cap.
  int npole = (n2side - 1) * (n2side - 1) - 1;

  // Double-pixelisation pixel coordinates of the centre of the facet.
  int i0 = nside * I0[facet];
  int j0 = nside * J0[facet];

  int  nside1 = nside - 1;
  long *hp = healidx;
  for (int imap = 0; imap < nside; imap++, hp++) {
    // (i,j) are 0-relative, double-pixelisation pixel coordinates.  The
    // origin is at the intersection of the equator and prime meridian,
    // i increases to the east (N.B.) and j to the north.
    int i = 0, j = 0;
    if (rotn == 0) {
      i = i0 + nside1 - (jmap + imap);
      j = j0 + jmap - imap;
    } else if (rotn == 1) {
      i = i0 + imap - jmap;
      j = j0 + nside1 - (imap + jmap);
    } else if (rotn == 2) {
      i = i0 + (imap + jmap) - nside1;
      j = j0 + imap - jmap;
    } else if (rotn == 3) {
      i = i0 + jmap - imap;
      j = j0 + jmap + imap - nside1;
    }

    // Convert i for counting pixels.
    if (i < 0) i += n8side;
    i++;

    if (j > nside) {
      // North polar regime.
      if (j == n2side) {
        *hp = 0;
      } else {
        // Number of pixels in a polar facet with this value of j.
        int npj = 2 * (n2side - j);

        // Index of the last pixel in the row above this.
        *hp = (npj - 1) * (npj - 1) - 1;

        // Number of pixels in this row in the polar facets before this.
        *hp += npj * (i/n2side);

        // Pixel number in this polar facet.
        *hp += i%n2side - (j - nside) - 1;
      }

    } else if (j >= -nside) {
      // Equatorial regime.
      *hp = npole + n8side * (nside - j) + i;

    } else {
      // South polar regime.
      *hp = 24 * nside * nside + 1;

      if (j > -n2side) {
        // Number of pixels in a polar facet with this value of j.
        int npj = 2 * (j + n2side);

        // Total number of pixels in this row or below it.
        *hp -= (npj + 1) * (npj + 1);

        // Number of pixels in this row in the polar facets before this.
        *hp += npj * (i/n2side);

        // Pixel number in this polar facet.
        *hp += i%n2side + (nside + j) - 1;
      }
    }

    // Convert double-pixelisation index to regular.
    *hp -= 1;
    *hp /= 2;
  }

  return 0;
}

//----------------------------------------------------------------------------

int HPXhdr(fitsfile *fptr, struct healpix *hpxdat)

{
  int status = 0;

  fits_update_key_log(fptr, "EXTEND", 0,
    "No FITS extensions are present", &status);
  fits_write_date(fptr, &status);

  // Set pixel transformation parameters.
  float crpix1, crpix2;
  if (hpxdat->layout == 0) {
    crpix1 = (5 * hpxdat->nside + 1) / 2.0f;
  } else {
    crpix1 = (4 * hpxdat->nside + 1) / 2.0f;
  }
  crpix2 = crpix1;

  fits_write_key(fptr, TFLOAT, "CRPIX1", &crpix1,
    "Coordinate reference pixel", &status);
  fits_write_key(fptr, TFLOAT, "CRPIX2", &crpix2,
    "Coordinate reference pixel", &status);

  float cos45 = (float)sqrt(2.0) / 2.0f;
  if (hpxdat->layout == 0) {
    fits_write_key_flt(fptr, "PC1_1",  cos45, -8,
      "Transformation matrix element", &status);
    fits_write_key_flt(fptr, "PC1_2",  cos45, -8,
      "Transformation matrix element", &status);
    fits_write_key_flt(fptr, "PC2_1", -cos45, -8,
      "Transformation matrix element", &status);
    fits_write_key_flt(fptr, "PC2_2",  cos45, -8,
      "Transformation matrix element", &status);
  }

  double cdelt1 = -90.0 / hpxdat->nside / sqrt(2.0);
  double cdelt2 = -cdelt1;
  fits_write_key_dbl(fptr, "CDELT1", cdelt1, -8,
    "[deg] Coordinate increment", &status);
  fits_write_key_dbl(fptr, "CDELT2", cdelt2, -8,
    "[deg] Coordinate increment", &status);


  // Celestial transformation parameters.
  char *pcode;
  if (hpxdat->layout == 0) {
    pcode = "HPX";
  } else {
    pcode = "XPH";
  }

  char *ctype1, *ctype2, *descr1, *descr2;
  if (hpxdat->crdsys == 'G') {
    // Galactic.
    ctype1 = "GLON";
    ctype2 = "GLAT";
    descr1 = "Galactic longitude";
    descr2 = "Galactic  latitude";
  } else if (hpxdat->crdsys == 'E') {
    // Ecliptic, who-knows-what.
    ctype1 = "ELON";
    ctype2 = "ELAT";
    descr1 = "Ecliptic longitude";
    descr2 = "Ecliptic  latitude";
  } else if (hpxdat->crdsys == 'Q') {
    // Equatorial, who-knows-what.
    ctype1 = "RA--";
    ctype2 = "DEC-";
    descr1 = "Right ascension";
    descr2 = "Declination";
  } else {
    // Unknown.
    ctype1 = "XLON";
    ctype2 = "XLAT";
    descr1 = "Longitude";
    descr2 = " Latitude";
  }

  char cval[16];
  sprintf(cval, "%s-%s", ctype1, pcode);

  char comment[64];
  sprintf(comment, "%s in an %s projection", descr1, pcode);
  fits_write_key_str(fptr, "CTYPE1", cval, comment, &status);
  sprintf(cval, "%s-%s", ctype2, pcode);
  sprintf(comment, "%s in an %s projection", descr2, pcode);
  fits_write_key_str(fptr, "CTYPE2", cval, comment, &status);


  float crval1 = 0.0f + 90.0f * hpxdat->quad, crval2;
  if (hpxdat->layout == 0) {
    crval2 =   0.0f;
  } else if (hpxdat->layout == 1) {
    crval1 += 180.0f;
    crval2  =  90.0f;
  } else {
    crval1 += 180.0f;
    crval2  = -90.0f;
  }
  if (360.0f < crval1) crval1 -= 360.0f;

  sprintf(comment, "[deg] %s at the reference point", descr1);
  fits_write_key(fptr, TFLOAT, "CRVAL1", &crval1, comment, &status);
  sprintf(comment, "[deg] %s at the reference point", descr2);
  fits_write_key(fptr, TFLOAT, "CRVAL2", &crval2, comment, &status);

  if (hpxdat->layout) {
    float lonpole = 180.0f;
    sprintf(comment, "[deg] Native longitude of the celestial pole");
    fits_write_key(fptr, TFLOAT, "LONPOLE", &lonpole, comment, &status);
  }

  if (hpxdat->layout == 0) {
    fits_write_key_lng(fptr, "PV2_1", (LONGLONG)4,
      "HPX H parameter (longitude)", &status);
    fits_write_key_lng(fptr, "PV2_2", (LONGLONG)3,
      "HPX K parameter  (latitude)", &status);
  }

  // Commentary.
  fits_write_record(fptr, " ", &status);
  if (hpxdat->layout == 0) {
    fits_write_comment(fptr,
      "Celestial map with FITS-standard HPX coordinate system generated by",
      &status);
  } else {
    fits_write_comment(fptr,
      "Celestial map with XPH coordinate system (polar HPX) generated by",
      &status);
  }

  fits_write_comment(fptr,
    "'HPXcvt' which reorganises HEALPix data without interpolation as",
    &status);
  fits_write_comment(fptr,
    "described in \"Mapping on the HEALPix grid\" by Mark Calabretta and",
    &status);
  fits_write_comment(fptr,
    "Boud Roukema.  See http://www.atnf.csiro.au/people/Mark.Calabretta",
    &status);

  return status;
}
