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frontend/src/lib/maidenhead.ts

@@ -49,6 +49,31 @@ export function gridToLatLon(grid: string): { lat: number; lon: number } | null
   return { lat, lon };
 }
 
+// latLonToGrid encodes a lat/lon to a Maidenhead locator (default 6 chars).
+// Inverse of gridToLatLon. Used to derive a default grid from a cty.dat entity
+// centroid when no provider grid is available (e.g. a portable/rare DX call) —
+// a centre-of-entity locator is more useful than an empty field.
+export function latLonToGrid(lat: number, lon: number, precision = 6): string {
+  let adjLon = Math.min(359.9999, Math.max(0, lon + 180));
+  let adjLat = Math.min(179.9999, Math.max(0, lat + 90));
+  const A = 'A'.charCodeAt(0);
+  const a = 'a'.charCodeAt(0);
+  const fLon = Math.floor(adjLon / 20);
+  const fLat = Math.floor(adjLat / 10);
+  adjLon -= fLon * 20;
+  adjLat -= fLat * 10;
+  const sLon = Math.floor(adjLon / 2);
+  const sLat = Math.floor(adjLat);
+  let grid = String.fromCharCode(A + fLon) + String.fromCharCode(A + fLat) + sLon + sLat;
+  if (precision >= 6) {
+    adjLon -= sLon * 2;
+    adjLat -= sLat;
+    grid += String.fromCharCode(a + Math.min(23, Math.floor(adjLon * 12)))
+          + String.fromCharCode(a + Math.min(23, Math.floor(adjLat * 24)));
+  }
+  return grid;
+}
+
 // gridSquareBounds returns the SW/NE corners of a Maidenhead square so a map
 // can draw its outline. Half-extents shrink with locator precision.
 export function gridSquareBounds(grid: string):