fix: calculate download BoundingBox based on zoom factor

instead of calling `setZoom()` before/after `MapView.boundingBox` which can get out of sync and cause inconsistencies in the tile count.
fixes #670

app/src/main/java/com/geeksville/mesh/ui/map/MapFragment.kt

@@ -52,6 +52,7 @@ import com.geeksville.mesh.util.EnableWakeLock
 import com.geeksville.mesh.util.SqlTileWriterExt
 import com.geeksville.mesh.util.requiredZoomLevel
 import com.geeksville.mesh.util.formatAgo
+import com.geeksville.mesh.util.zoomIn
 import com.geeksville.mesh.waypoint
 import com.google.accompanist.themeadapter.appcompat.AppCompatTheme
 import com.google.android.material.dialog.MaterialAlertDialogBuilder
@@ -461,21 +462,21 @@ fun MapView(model: UIViewModel = viewModel()) {
      */
     fun generateBoxOverlay(zoomLevel: Double) = map.apply {
         overlayManager = CustomOverlayManager(TilesOverlay(tileProvider, context))
+        val zoomFactor = 1.3 // zoom difference between view and download area polygon
+        controller.setZoom(zoomLevel - zoomFactor)
         setMultiTouchControls(false)
         // furthest back
         zoomLevelMax = zoomLevelHighest // FIXME zoomLevel
         // furthest in min should be > than max
-        zoomLevelMin = map.tileProvider.tileSource.maximumZoomLevel.toDouble()
-        controller.setZoom(zoomLevel)
-        downloadRegionBoundingBox = map.boundingBox
+        zoomLevelMin = tileProvider.tileSource.maximumZoomLevel.toDouble()
+        downloadRegionBoundingBox = boundingBox.zoomIn(zoomFactor)
         val polygon = Polygon().apply {
             points = Polygon.pointsAsRect(downloadRegionBoundingBox).map {
                 GeoPoint(it.latitude, it.longitude)
             }
         }
         overlayManager.add(polygon)
-        controller.setZoom(zoomLevel - 1.0)
-        val tileCount: Int = CacheManager(map).possibleTilesInArea(
+        val tileCount: Int = CacheManager(this).possibleTilesInArea(
             downloadRegionBoundingBox,
             zoomLevelMax.toInt(),
             zoomLevelMin.toInt()

app/src/main/java/com/geeksville/mesh/util/LocationUtils.kt

@@ -12,6 +12,7 @@ import kotlin.math.acos
 import kotlin.math.atan2
 import kotlin.math.cos
 import kotlin.math.log2
+import kotlin.math.pow
 import kotlin.math.sin
 
 /*******************************************************************************
@@ -251,3 +252,30 @@ fun BoundingBox.requiredZoomLevel(): Double {
     return maxOf(requiredLatZoom, requiredLonZoom)
 }
 
+/**
+ * Creates a new bounding box with adjusted dimensions based on the provided [zoomFactor].
+ * @return A new [BoundingBox] with added [zoomFactor]. Example:
+ * ```
+ * // Setting the zoom level directly using setZoom()
+ * map.setZoom(14.0)
+ * val boundingBoxZoom14 = map.boundingBox
+ *
+ * // Using zoomIn() results the equivalent BoundingBox with setZoom(15.0)
+ * val boundingBoxZoom15 = boundingBoxZoom14.zoomIn(1.0)
+ * ```
+ */
+fun BoundingBox.zoomIn(zoomFactor: Double): BoundingBox {
+    val center = GeoPoint((latNorth + latSouth) / 2, (lonWest + lonEast) / 2)
+    val latDiff = latNorth - latSouth
+    val lonDiff = lonEast - lonWest
+
+    val newLatDiff = latDiff / (2.0.pow(zoomFactor))
+    val newLonDiff = lonDiff / (2.0.pow(zoomFactor))
+
+    return BoundingBox(
+        center.latitude + newLatDiff / 2,
+        center.longitude + newLonDiff / 2,
+        center.latitude - newLatDiff / 2,
+        center.longitude - newLonDiff / 2
+    )
+}