Add GeoJSON point containment helper

2026-06-11 00:10:01 +00:00
parent b2c429983c
commit 42c646c328
3 changed files with 393 additions and 0 deletions
--- a/internal/geo/geojson.go
+++ b/internal/geo/geojson.go
@@ -0,0 +1,107 @@
+package geo
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+type geometry struct {
+	Type        string          `json:"type"`
+	Coordinates json.RawMessage `json:"coordinates"`
+}
+
+// ContainsPoint reports whether a GeoJSON Polygon or MultiPolygon contains p.
+// GeoJSON coordinate order is [longitude, latitude].
+func ContainsPoint(raw []byte, p Point) (bool, error) {
+	if len(raw) == 0 {
+		return false, fmt.Errorf("geojson geometry is empty")
+	}
+
+	var geom geometry
+	if err := json.Unmarshal(raw, &geom); err != nil {
+		return false, fmt.Errorf("decode geojson geometry: %w", err)
+	}
+
+	switch geom.Type {
+	case "Polygon":
+		polygon, err := decodePolygon(geom.Coordinates)
+		if err != nil {
+			return false, fmt.Errorf("decode polygon: %w", err)
+		}
+		return polygonContainsPoint(polygon, p), nil
+	case "MultiPolygon":
+		multiPolygon, err := decodeMultiPolygon(geom.Coordinates)
+		if err != nil {
+			return false, fmt.Errorf("decode multipolygon: %w", err)
+		}
+		for _, polygon := range multiPolygon {
+			if polygonContainsPoint(polygon, p) {
+				return true, nil
+			}
+		}
+		return false, nil
+	case "":
+		return false, fmt.Errorf("geojson geometry type is required")
+	default:
+		return false, fmt.Errorf("unsupported geojson geometry type %q", geom.Type)
+	}
+}
+
+func decodePolygon(raw json.RawMessage) (Polygon, error) {
+	var coords [][][]float64
+	if err := json.Unmarshal(raw, &coords); err != nil {
+		return nil, err
+	}
+	return polygonFromCoordinates(coords)
+}
+
+func decodeMultiPolygon(raw json.RawMessage) ([]Polygon, error) {
+	var coords [][][][]float64
+	if err := json.Unmarshal(raw, &coords); err != nil {
+		return nil, err
+	}
+	if len(coords) == 0 {
+		return nil, fmt.Errorf("multipolygon has no polygons")
+	}
+
+	out := make([]Polygon, 0, len(coords))
+	for i, polygonCoords := range coords {
+		polygon, err := polygonFromCoordinates(polygonCoords)
+		if err != nil {
+			return nil, fmt.Errorf("polygons[%d]: %w", i, err)
+		}
+		out = append(out, polygon)
+	}
+	return out, nil
+}
+
+func polygonFromCoordinates(coords [][][]float64) (Polygon, error) {
+	if len(coords) == 0 {
+		return nil, fmt.Errorf("polygon has no rings")
+	}
+
+	polygon := make(Polygon, 0, len(coords))
+	for i, ringCoords := range coords {
+		ring, err := ringFromCoordinates(ringCoords)
+		if err != nil {
+			return nil, fmt.Errorf("rings[%d]: %w", i, err)
+		}
+		polygon = append(polygon, ring)
+	}
+	return polygon, nil
+}
+
+func ringFromCoordinates(coords [][]float64) (Ring, error) {
+	if len(coords) == 0 {
+		return nil, fmt.Errorf("ring has no points")
+	}
+
+	ring := make(Ring, 0, len(coords))
+	for i, pair := range coords {
+		if len(pair) < 2 {
+			return nil, fmt.Errorf("points[%d] has %d values, need longitude and latitude", i, len(pair))
+		}
+		ring = append(ring, Point{Longitude: pair[0], Latitude: pair[1]})
+	}
+	return ring, nil
+}