"""Great-circle geometry helpers for projecting estimated flight paths."""
from __future__ import annotations

import math

EARTH_RADIUS_KM = 6371.0088
KNOTS_TO_KMH = 1.852


def destination_point(lat: float, lon: float, bearing_deg: float, distance_km: float):
    """Great-circle destination given a start point, bearing and distance."""
    ang = distance_km / EARTH_RADIUS_KM
    brg = math.radians(bearing_deg)
    lat1 = math.radians(lat)
    lon1 = math.radians(lon)

    lat2 = math.asin(
        math.sin(lat1) * math.cos(ang) + math.cos(lat1) * math.sin(ang) * math.cos(brg)
    )
    lon2 = lon1 + math.atan2(
        math.sin(brg) * math.sin(ang) * math.cos(lat1),
        math.cos(ang) - math.sin(lat1) * math.sin(lat2),
    )
    return math.degrees(lat2), (math.degrees(lon2) + 540) % 360 - 180


def haversine_km(lat1, lon1, lat2, lon2):
    p1, p2 = math.radians(lat1), math.radians(lat2)
    dphi = math.radians(lat2 - lat1)
    dlmb = math.radians(lon2 - lon1)
    a = math.sin(dphi / 2) ** 2 + math.cos(p1) * math.cos(p2) * math.sin(dlmb / 2) ** 2
    return 2 * EARTH_RADIUS_KM * math.asin(math.sqrt(a))


def initial_bearing(lat1, lon1, lat2, lon2):
    p1, p2 = math.radians(lat1), math.radians(lat2)
    dl = math.radians(lon2 - lon1)
    y = math.sin(dl) * math.cos(p2)
    x = math.cos(p1) * math.sin(p2) - math.sin(p1) * math.cos(p2) * math.cos(dl)
    return (math.degrees(math.atan2(y, x)) + 360) % 360


def estimate_endpoint(lat, lon, track_deg, gspeed_knots, seconds_remaining,
                      dest_lat=None, dest_lon=None):
    """Return (end_lat, end_lon, distance_km, used_dest).

    If we know the destination airport coords, the arc goes there. Otherwise we
    dead-reckon forward along the current track for the remaining ETA.
    """
    if dest_lat is not None and dest_lon is not None:
        d = haversine_km(lat, lon, dest_lat, dest_lon)
        return dest_lat, dest_lon, d, True

    if not gspeed_knots or not seconds_remaining or seconds_remaining <= 0:
        # Fall back to a short forward stub so the arc is still visible.
        seconds_remaining = 1800
        gspeed_knots = gspeed_knots or 450
    dist_km = gspeed_knots * KNOTS_TO_KMH * (seconds_remaining / 3600.0)
    elat, elon = destination_point(lat, lon, track_deg or 0, dist_km)
    return elat, elon, dist_km, False