chore: release v0.27.12

internal/geo/geo.go

@@ -11,6 +11,7 @@ package geo
 
 import (
 	"math"
+	"sort"
 	"strings"
 )
 
@@ -118,3 +119,65 @@ func NeighbourGrids(lat, lon float64, ring int) []string {
 	}
 	return out
 }
+
+// GridsWithin returns every Maidenhead square whose centre lies within km of
+// (lat, lon), nearest first, at most max of them.
+//
+// NeighbourGrids answers "the ring around here", which is the right shape for a
+// few hundred kilometres and the wrong one past that: a ring is a square, so
+// asking for 2000 km through it means 1369 squares, most of them further away
+// than the ones it left out. This measures instead, and the count then follows
+// the AREA asked for rather than the corner of a box.
+//
+// Nearest first because the caller has to be able to trim: these become one
+// broker subscription each, and when there are more than can be afforded, the
+// squares to keep are the close ones.
+func GridsWithin(lat, lon, km float64, max int) []string {
+	if km <= 0 || max <= 0 {
+		return nil
+	}
+	// One square is 1° of latitude and 2° of longitude. Sweep a box big enough
+	// to hold the circle — a degree of latitude is ~111 km everywhere, and a
+	// degree of longitude never MORE than that, so this cannot cut the circle.
+	steps := int(km/111.0) + 1
+	type cand struct {
+		grid string
+		d    float64
+	}
+	seen := map[string]bool{}
+	out := []cand{}
+	for dLat := -steps; dLat <= steps; dLat++ {
+		for dLon := -2 * steps; dLon <= 2*steps; dLon++ {
+			la := lat + float64(dLat)
+			lo := lon + float64(dLon)*2
+			if la > 90 || la < -90 {
+				continue
+			}
+			g := LatLonToGrid(la, lo)
+			if seen[g] {
+				continue
+			}
+			// Measured to the square's own centre, not to the sample point, so
+			// two samples landing in one square agree about how far it is.
+			cLat, cLon, ok := GridToLatLon(g)
+			if !ok {
+				continue
+			}
+			d := HaversineKm(lat, lon, cLat, cLon)
+			if d > km {
+				continue
+			}
+			seen[g] = true
+			out = append(out, cand{g, d})
+		}
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].d < out[j].d })
+	if len(out) > max {
+		out = out[:max]
+	}
+	grids := make([]string, len(out))
+	for i, c := range out {
+		grids[i] = c.grid
+	}
+	return grids
+}