map

frontend/src/lib/maidenhead.ts

@@ -49,6 +49,19 @@ export function gridToLatLon(grid: string): { lat: number; lon: number } | null
   return { lat, lon };
 }
 
+// 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):
+  { south: number; west: number; north: number; east: number } | null {
+  const c = gridToLatLon(grid);
+  if (!c) return null;
+  const g = grid.trim();
+  let dLon = 1, dLat = 0.5; // 4-char square: 2°×1°
+  if (g.length >= 6) { dLon = 1 / 24; dLat = 0.5 / 24; }
+  if (g.length >= 8) { dLon = 1 / 24 / 10; dLat = 0.5 / 24 / 10; }
+  return { south: c.lat - dLat, north: c.lat + dLat, west: c.lon - dLon, east: c.lon + dLon };
+}
+
 // PathInfo describes both short and long great-circle path between two
 // points. Bearing in degrees from true north (0–360). Distance in km.
 export interface PathInfo {
@@ -88,5 +101,41 @@ export function pathBetween(fromGrid: string, toGrid: string): PathInfo | null {
   };
 }
 
+// greatCirclePoints returns n+1 [lat, lon] points along the short great-circle
+// path between two lat/lon points (spherical slerp). Longitudes are unwrapped
+// to stay continuous (no ±180 jump) so a map polyline draws as one smooth arc.
+export function greatCirclePoints(
+  lat1: number, lon1: number, lat2: number, lon2: number, n = 96,
+): [number, number][] {
+  const φ1 = toRad(lat1), λ1 = toRad(lon1);
+  const φ2 = toRad(lat2), λ2 = toRad(lon2);
+  // Angular distance between the two points.
+  const sinΔφ = Math.sin((φ2 - φ1) / 2);
+  const sinΔλ = Math.sin((λ2 - λ1) / 2);
+  const h = sinΔφ * sinΔφ + Math.cos(φ1) * Math.cos(φ2) * sinΔλ * sinΔλ;
+  const d = 2 * Math.asin(Math.min(1, Math.sqrt(h)));
+  const out: [number, number][] = [];
+  if (d === 0) return [[lat1, lon1]];
+  let prevLon = NaN;
+  for (let i = 0; i <= n; i++) {
+    const f = i / n;
+    const A = Math.sin((1 - f) * d) / Math.sin(d);
+    const B = Math.sin(f * d) / Math.sin(d);
+    const x = A * Math.cos(φ1) * Math.cos(λ1) + B * Math.cos(φ2) * Math.cos(λ2);
+    const y = A * Math.cos(φ1) * Math.sin(λ1) + B * Math.cos(φ2) * Math.sin(λ2);
+    const z = A * Math.sin(φ1) + B * Math.sin(φ2);
+    const lat = toDeg(Math.atan2(z, Math.sqrt(x * x + y * y)));
+    let lon = toDeg(Math.atan2(y, x));
+    // Unwrap longitude so the polyline never snaps across the whole map.
+    if (!Number.isNaN(prevLon)) {
+      while (lon - prevLon > 180) lon -= 360;
+      while (lon - prevLon < -180) lon += 360;
+    }
+    prevLon = lon;
+    out.push([lat, lon]);
+  }
+  return out;
+}
+
 function toRad(d: number): number { return (d * Math.PI) / 180; }
 function toDeg(r: number): number { return (r * 180) / Math.PI; }