fix(live): restore the moving navigation pin, aimed at the signal gradient

app/src/main/java/com/wytehat/btlogger/LiveRangeFinderActivity.java

@@ -61,6 +61,8 @@ public class LiveRangeFinderActivity extends Activity implements LocationListene
     private float heading;
     /** Magnetic-to-true north correction for the current position. */
     private float declination;
+    /** Last bearing the signal gradient produced; NaN until one is solved. */
+    private float lastGradientBearing = Float.NaN;
     /** Smoothed compass azimuth, kept in sin/cos form to survive the 0/360 wrap. */
     private float headingSin, headingCos;
     private boolean headingPrimed;
@@ -516,6 +518,8 @@ public class LiveRangeFinderActivity extends Activity implements LocationListene
         }
         SpatialGradientEngine.Estimate estimate = spatial.consensusEstimate();
         if (estimate != null) {
+            if (!Float.isNaN(estimate.gradientBearing))
+                lastGradientBearing = estimate.gradientBearing;
             estimateText.setText(String.format(Locale.US,
                 "%s • %d stations • ±%.1f m • %.0f°",
                 estimate.preliminary ? "Estimate" : "Solved",
@@ -527,6 +531,8 @@ public class LiveRangeFinderActivity extends Activity implements LocationListene
         } else {
             SpatialGradientEngine.Estimate hint = spatial.preliminaryEstimate();
             if (hint != null) {
+                if (!Float.isNaN(hint.gradientBearing))
+                    lastGradientBearing = hint.gradientBearing;
                 js(String.format(Locale.US, "updateEstimate(%.7f,%.7f,%.1f,%.1f,true)",
                     hint.latitude, hint.longitude, hint.confidenceMeters, hint.gradientBearing));
                 js(String.format(Locale.US, "updateGradient(%.1f,'%s')", hint.gradientBearing, state));
@@ -545,16 +551,23 @@ public class LiveRangeFinderActivity extends Activity implements LocationListene
      * is what this used to do - just parks the pin wherever the user points,
      * so the arrow always appears to aim at it and it is useless for walking.
      */
+    /**
+     * Projects the item pin the estimated distance ahead so there is always
+     * something to walk toward, and it moves as the range estimate changes.
+     *
+     * It aims at the last bearing the signal gradient produced, so it holds
+     * a real direction instead of swinging around with the compass; only
+     * before any gradient exists does it fall back to the phone's heading.
+     */
     private void showFallbackPin(Location phone, float direction, double distance) {
-        if (target != null && target.hasLocation) {
+        float bearing = Float.isNaN(lastGradientBearing) ? direction : lastGradientBearing;
+        double projected = Math.max(1.5, Math.min(20.0, distance));
+        double radians = Math.toRadians(bearing);
+        double latitude = phone.getLatitude() + Math.cos(radians) * projected / 111319.49;
+        double cosine = Math.max(0.2, Math.cos(Math.toRadians(phone.getLatitude())));
+        double longitude = phone.getLongitude() + Math.sin(radians) * projected / (111319.49 * cosine);
         js(String.format(Locale.US, "updateEstimate(%.7f,%.7f,%.1f,%.1f,true)",
-                target.latitude, target.longitude,
+            latitude, longitude, Math.max(1.5, distance), bearing));
-                Math.max(1.5, target.accuracy), direction));
-        } else {
-            js(String.format(Locale.US, "updateEstimate(%.7f,%.7f,%.1f,%.1f,true)",
-                phone.getLatitude(), phone.getLongitude(),
-                Math.max(1.5, distance), direction));
-        }
     }
 
     private String sector(float bearing) {