Auto dark/bright follows the sun at the tablet's own location

Without a light sensor an app may read (the Redmi Pad SE has none), auto is dark while
the sun is more than 4° below the horizon (lib/theme/sun.dart), at Android's coarse
location, polled and kept across restarts. The tablet's own, not the Orin's GNSS: the
tablet has its own GNSS and cellular. Android's dark theme is the last fallback.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/README.md

@@ -42,7 +42,7 @@ pirate-station-aide's (main views, then the rarely used ones grouped as Tools).
   `onFill(colour)` (text on a fill). No widget names a colour. Themes are JSON in `assets/themes/`, `rei` by
   default; smart-serow's four-key files still load (their `highlight` is `alarm`).
 - **Dark or bright.** `Themes.mode` is `auto` (a light sensor where the device allows one, `theme/light.dart`;
-  else Android's dark theme, schedule included), `dark` or
+  else the sun at the tablet's own location, `theme/sun.dart`; else Android's dark theme), `dark` or
   `bright`, chosen in Settings and kept across restarts. The status bar's button holds the other variant;
   a long press there is auto again.
 - **Cosmetic fonts are opt-in.** A widget that wants the HUD face names `fontFamily: hudFont` in its own

lib/main.dart

@@ -1,16 +1,18 @@
 // Yukart display: the tablet app. Starts the link to the Orin and hands the screen to
 // the shell (lib/shell.dart). Code layout: lib/README.md.
 
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 
 import 'link/boundary.dart';
 import 'link/net.dart';
-import 'link/reading.dart';
 import 'pages/link_test.dart';
 import 'shell.dart';
 import 'shell_scope.dart';
 import 'theme/light.dart';
 import 'theme/palette.dart';
+import 'theme/sun.dart';
 
 /// Override for development, e.g. `--dart-define=YUKART_GW=ws://halogen.local:7700/`.
 const defaultUrl = String.fromEnvironment(
@@ -25,14 +27,15 @@ Future<void> main() async {
   final usb = UsbLink();
   await usb.bindFor(Uri.parse(defaultUrl));
   final boundary = Boundary(defaultUrl)..start();
-  // The sun, for auto dark/bright: the car's position while GNSS has one.
-  boundary.addListener(() {
-    final lat = Reading.of(boundary, 'gnss_lat');
-    final lon = Reading.of(boundary, 'gnss_lon');
-    if (!lat.dim && !lon.dim && lat.number != null && lon.number != null) {
-      themes.setPosition(lat.number!, lon.number!);
-    }
-  });
+  // The sun, for auto dark/bright: the tablet's own location, every minute until there
+  // is one (the first ask brings up the permission), then every ten.
+  Future<void> locate() async {
+    final p = await androidLocation();
+    if (p != null) themes.setPosition(p.$1, p.$2);
+    Timer(Duration(minutes: p == null ? 1 : 10), locate);
+  }
+
+  locate();
   final linkTest = LinkTest(usb);
   // Development hook: `am start ... --es linktest [dl:|bi:]<label>[@<host>]` runs the test
   // (`dl:` the download series, `bi:` burst latency against idle gaps), on the gateway's host unless one is given.

lib/pages/settings.dart

@@ -65,9 +65,9 @@ class SettingsPage extends StatelessWidget {
                       ? 'now ${lux.toStringAsFixed(lux < 10 ? 1 : 0)} lux; dark at '
                             '${LightSwitch.darkAt.round()} or less, bright from ${LightSwitch.brightAt.round()}'
                       : sun != null
-                      ? 'now ${sun.round()}° above the horizon at the last GNSS position; '
-                            'dark below ${darkBelowDeg.round()}°'
-                      : 'no GNSS position yet';
+                      ? 'the sun is ${sun.abs().round()}° ${sun < 0 ? 'below' : 'above'} the horizon here; '
+                            'dark from ${(-darkBelowDeg).round()}° below'
+                      : 'no location yet';
                   return Text(
                     'Auto follows ${themes.autoSource} ($now). '
                     'The status bar button holds one; a long press there is auto again.',

lib/theme/palette.dart

@@ -127,7 +127,7 @@ enum VariantMode { auto, dark, bright }
 
 /// A theme: both variants, and which one is showing. Listen to it to follow a switch.
 /// In auto it follows a light sensor ([followLight]) where the device lets an app read
-/// one; else the sun at the car's last position ([setPosition]); else Android's dark
+/// one; else the sun at the tablet's last location ([setPosition]); else Android's dark
 /// theme (by hand or on its schedule). The mode and the position are kept across restarts.
 class Themes extends ChangeNotifier with WidgetsBindingObserver {
   Themes(
@@ -210,7 +210,7 @@ class Themes extends ChangeNotifier with WidgetsBindingObserver {
   bool? _sunDark;
   late final Timer _sunTimer;
 
-  /// The car's position, for the sun. Kept (to 0.1°, about 10 km: plenty for the sun)
+  /// The tablet's location, for the sun. Kept (to 0.1°, about 10 km: plenty for the sun)
   /// so auto knows the sun before the first fix.
   void setPosition(double lat, double lon) {
     double r(double x) => (x * 10).round() / 10;

lib/theme/sun.dart

@@ -0,0 +1,48 @@
+// 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),
+    ),
+  );
+}