// Auto dark/bright from the sun: dark once it is far enough below the horizon, at the
// tablet's own rough location (Android's, not the Orin's: the tablet stands on its own).
// Google Maps on this tablet does the same (it reads no light sensor; checked). Needs no
// sensor; does not know about tunnels or car parks.

import 'dart:math' as math;

import 'package:flutter/services.dart';

/// The tablet's newest known location, roughly, from MainActivity; null without one
/// (or before location is granted: the first call asks).
Future<(double, double)?> androidLocation() async {
  try {
    final v = await const MethodChannel('yukart/location')
        .invokeListMethod<double>('last');
    return v == null || v.length < 2 ? null : (v[0], v[1]);
  } catch (_) {
    return null; // No Android side (tests, desktop).
  }
}

/// Dark while the sun is lower than this (degrees; sunset is -0.8, civil dusk -6).
const darkBelowDeg = -4.0;

/// The sun's elevation above the horizon, in degrees, at [lat], [lon] (degrees, east
/// positive) and [time]. The usual low-precision formulae (the Astronomical Almanac's,
/// as NOAA uses them): within about a degree, plenty for this.
double sunElevation(double lat, double lon, DateTime time) {
  double rad(double d) => d * math.pi / 180;
  double deg(double r) => r * 180 / math.pi;
  final d =
      time.toUtc().millisecondsSinceEpoch / 86400000 + 2440587.5 - 2451545.0;
  final g = rad(357.529 + 0.98560028 * d); // mean anomaly
  final q = 280.459 + 0.98564736 * d; // mean longitude
  final l = rad(q + 1.915 * math.sin(g) + 0.020 * math.sin(2 * g));
  final e = rad(23.439 - 0.00000036 * d); // obliquity
  final ra = math.atan2(math.cos(e) * math.sin(l), math.cos(l));
  final dec = math.asin(math.sin(e) * math.sin(l));
  final gmst = (18.697374558 + 24.06570982441908 * d) % 24;
  final ha = rad(gmst * 15 + lon) - ra;
  final phi = rad(lat);
  return deg(
    math.asin(
      math.sin(phi) * math.sin(dec) +
          math.cos(phi) * math.cos(dec) * math.cos(ha),
    ),
  );
}