#! /usr/bin/python3
# ===================================================================
# Given two lat/lon points on a sphere (the earth),
# calculate the great circle distance
# ===================================================================
# www.geeksforgeeks.org/program-distance-two-points-earth/
# wikipedia: en.wikipedia.org/wiki/Haversine_formula
# ===================================================================

from math import pi, radians, cos, sin, asin, sqrt

# -------------------------------------------------------------------
# function: calculate great circle distance
# -------------------------------------------------------------------

def great_circle_distance(rad,lat1,lon1,lat2,lon2):

    # ---- convert degrees to radians

    lat1 = radians(lat1)
    lon1 = radians(lon1)
    lat2 = radians(lat2)
    lon2 = radians(lon2)

    # ---- Haversine formula

    dlat = lat2 - lat1
    dlon = lon2 - lon1

    a = sin(dlat/2)**2 + cos(lat1) * cos(lat2) * sin(dlon / 2)**2 

    c = 2 * asin(sqrt(a))

    return (c * rad)


# -------------------------------------------------------------------
# ---- main
# -------------------------------------------------------------------

if '__main__' == __name__:

    # ---- radius of earth

    radm = 3956.0              # miles
    radk = 6371.0              # kilometers

    # ---- test values from article (webpage)

    d = great_circle_distance(radk, 53.32055555555556,    # lat1
                                    -1.7297222222222221,  # lon1
                                    53.31861111111111,    # lat2
                                    -1.6997222222222223)  # lon2

    print(f'great circle distance: {d}')

    print()