import sys
import numpy as np

haversin = lambda alpha: np.sin(alpha / 2) ** 2


def gc_distance(airport_coords1, airport_coords2):
    """
    Return the great-circle distance between two coordinates on the Earth
    provided as two (latitude, longitude) tuples in radians.

    """

    # Earth's radius, km
    R = 6378.1
    (phi1, lambda1), (phi2, lambda2) = airport_coords1, airport_coords2
    d = (
        2
        * R
        * np.arcsin(
            np.sqrt(
                haversin(phi2 - phi1)
                + np.cos(phi1) * np.cos(phi2) * haversin(lambda2 - lambda1)
            )
        )
    )
    return d


iata1, iata2 = sys.argv[1:]

dt = np.dtype(
    [
        ("IATA", "S50"),
        ("Name", "S50"),
        ("Location", "S50"),
        ("Latitude", "f8"),
        ("Longitude", "f8"),
    ]
)

to_radians = lambda alpha: np.radians(float(alpha))
airports = np.loadtxt(
    "busiest_airports.txt",
    dtype=dt,
    delimiter="\t",
    converters={3: to_radians, 4: to_radians},
)


def get_airport(iata):
    """
    Return the airport with IATA code iata from the airports array or
    raise an Exception if it isn't recognised.

    """

    b_iata = bytes(iata, encoding="ascii")
    airport = airports[airports["IATA"] == b_iata]
    # check we retrieved an airport: NB we can't use assert airport, ...
    # because of a bug in numpy versions older than 1.8
    assert len(airport) == 1, "Airport not recognised: {:s}".format(iata)
    return airport


airport1 = get_airport(iata1)
airport2 = get_airport(iata2)
airport_coords1 = airport1[["Latitude", "Longitude"]][0]
airport_coords2 = airport2[["Latitude", "Longitude"]][0]
airport_name1 = airport1["Name"][0].decode()
airport_name2 = airport2["Name"][0].decode()
d = gc_distance(airport_coords1, airport_coords2)

print(
    f"Distance from {airport_name1:s} ({iata1:3s}) to {airport_name2:s}"
    f" ({iata2:3s}) is {int(d):d} km"
)