Add function hp_index_to_lonlat() and hp_interpolate_weights().

src/mpim_grid/grid_healpix.cc

@@ -3,6 +3,7 @@ extern "C"
 {
 #endif
 #include "lib/healpix/healpix.h"
+#include "lib/healpix/interpolation.h"
 #ifdef __cplusplus
 }
 #endif
@@ -71,3 +72,34 @@ hp_generate_coords(HpOrder order, int nside, int64_t nvals, double *xvals, doubl
         }
     }
 }
+
+void
+hp_index_to_lonlat(HpOrder order, int nside, int64_t ipix, double *xval, double *yval)
+{
+  int64_t (*convert_order)(int64_t ipix, int nside);
+  // clang-format off
+  convert_order = (order == HpOrder::Ring) ? &healpixl_ring_to_xy :
+                  (order == HpOrder::Nest) ? &healpixl_nested_to_xy : &hp_xy_to_xy;
+  // clang-format on
+
+  auto cpix = convert_order(ipix, nside);
+  healpixl_to_radec(cpix, nside, 0.5, 0.5, xval, yval);
+}
+
+void
+hp_interpolate_weights(double lon, double lat, size_t *indices, double *weights, int nside, HpOrder order)
+{
+  int64_t (*convert_order)(int64_t ipix, int nside);
+  // clang-format off
+  convert_order = (order == HpOrder::Ring) ? &healpixl_xy_to_ring :
+                  (order == HpOrder::Nest) ? &healpixl_xy_to_nested : &hp_xy_to_xy;
+  // clang-format on
+
+  int64_t ringIndices[4];
+  interpolate_weights(lon, lat, ringIndices, weights, nside);
+
+  if (order == HpOrder::Ring)
+    for (int i = 0; i < 4; ++i) indices[i] = ringIndices[i];
+  else
+    for (int i = 0; i < 4; ++i) indices[i] = convert_order(healpixl_ring_to_xy(ringIndices[i], nside), nside);
+}

src/mpim_grid/grid_healpix.h

@@ -15,5 +15,7 @@ HpOrder hp_get_order(const char *orderName);
 int64_t hp_lonlat_to_healpixl(HpOrder order, int nside, double xval, double yval);
 void hp_generate_coords(HpOrder order, int nside, int64_t nvals, double *xvals, double *yvals, bool withBounds, double *xbounds,
                         double *ybounds);
+void hp_index_to_lonlat(HpOrder order, int nside, int64_t ipix, double *xval, double *yval);
+void hp_interpolate_weights(double lon, double lat, size_t *indices, double *weights, int nside, HpOrder order);
 
 #endif