From fb2ac576eadb78f8082ec4f793838a2c7b3a4e78 Mon Sep 17 00:00:00 2001
From: rstiskalek <richard.stiskalek@protonmail.com>
Date: Mon, 23 Oct 2023 16:01:26 +0100
Subject: [PATCH] Add great circle distance

---
 csiborgtools/__init__.py |  3 ++-
 csiborgtools/utils.py    | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/csiborgtools/__init__.py b/csiborgtools/__init__.py
index b791fa6..7cad633 100644
--- a/csiborgtools/__init__.py
+++ b/csiborgtools/__init__.py
@@ -16,7 +16,8 @@ from csiborgtools import clustering, field, match, read, summary
 
 from .utils import (center_of_mass, delta2ncells, number_counts,                 # noqa
                     periodic_distance, periodic_distance_two_points,             # noqa
-                    binned_statistic, cosine_similarity, fprint)                 # noqa
+                    binned_statistic, cosine_similarity, fprint,                 # noqa
+                    great_circle_distance)                                       # noqa
 
 
 # Arguments to csiborgtools.read.Paths.
diff --git a/csiborgtools/utils.py b/csiborgtools/utils.py
index ccbf721..7cc5c72 100644
--- a/csiborgtools/utils.py
+++ b/csiborgtools/utils.py
@@ -148,6 +148,26 @@ def radec_to_cartesian(X):
         ]).T
 
 
+@jit(nopython=True, fastmath=True, boundscheck=False)
+def great_circle_distance(x1, x2):
+    """
+    Great circle distance between two points on a sphere, defined by RA and
+    dec, both in degrees.
+    """
+    ra1, dec1 = x1
+    ra2, dec2 = x2
+
+    ra1 *= numpy.pi / 180
+    dec1 *= numpy.pi / 180
+    ra2 *= numpy.pi / 180
+    dec2 *= numpy.pi / 180
+
+    return 180 / numpy.pi * numpy.arccos(
+        numpy.sin(dec1) * numpy.sin(dec2)
+        + numpy.cos(dec1) * numpy.cos(dec2) * numpy.cos(ra1 - ra2)
+        )
+
+
 def cosine_similarity(x, y):
     r"""
     Calculate the cosine similarity between two Cartesian vectors. Defined