Calculate the bounding box for any GeoJSON object

spec/bbox_spec.cr

@@ -0,0 +1,96 @@
+require "./spec_helper"
+
+describe GeoJSON do
+  pt = GeoJSON::Point.new([102.0, 0.5])
+
+  multi_pt = GeoJSON::MultiPoint.new([
+    [102.0, 0.5],
+    [103.0, 1.0],
+  ])
+
+  line = GeoJSON::LineString.new([
+    [102.0, -10.0],
+    [103.0, 1.0],
+    [104.0, 0.0],
+    [130.0, 4.0],
+  ])
+
+  poly = GeoJSON::Polygon.new([
+    [
+      [101.0, 0.0],
+      [101.0, 1.0],
+      [100.0, 1.0],
+      [100.0, 0.0],
+      [101.0, 0.0],
+    ],
+  ])
+
+  multi_line = GeoJSON::MultiLineString.new([
+    [
+      [100.0, 0.0],
+      [101.0, 1.0],
+    ],
+    [
+      [102.0, 2.0],
+      [103.0, 3.0],
+    ],
+  ])
+
+  multi_poly = GeoJSON::MultiPolygon.new([
+    [
+      [
+        [102.0, 2.0],
+        [103.0, 2.0],
+        [103.0, 3.0],
+        [102.0, 3.0],
+        [102.0, 2.0],
+      ],
+    ],
+    [
+      [
+        [100.0, 0.0],
+        [101.0, 0.0],
+        [101.0, 1.0],
+        [100.0, 1.0],
+        [100.0, 0.0],
+      ],
+      [
+        [100.2, 0.2],
+        [100.8, 0.2],
+        [100.8, 0.8],
+        [100.2, 0.8],
+        [100.2, 0.2],
+      ],
+    ],
+  ])
+
+  geometry_collection = GeoJSON::GeometryCollection.new([pt, line, poly, multi_line, multi_poly])
+
+  feature = GeoJSON::Feature.new(poly)
+
+  pt_feature = GeoJSON::Feature.new(pt)
+  line_feature = GeoJSON::Feature.new(line)
+  poly_feature = GeoJSON::Feature.new(poly)
+  multi_line_feature = GeoJSON::Feature.new(multi_line)
+  multi_poly_feature = GeoJSON::Feature.new(multi_poly)
+
+  feature_collection = GeoJSON::FeatureCollection.new([
+    pt_feature,
+    line_feature,
+    poly_feature,
+    multi_line_feature,
+    multi_poly_feature,
+  ])
+
+  context ".bbox" do
+    it { pt.bbox.should eq([102, 0.5, 102, 0.5]) }
+    it { multi_pt.bbox.should eq([102.0, 0.5, 103.0, 1.0]) }
+    it { line.bbox.should eq([102, -10, 130, 4]) }
+    it { poly.bbox.should eq([100, 0, 101, 1]) }
+    it { multi_line.bbox.should eq([100, 0, 103, 3]) }
+    it { multi_poly.bbox.should eq([100, 0, 103, 3]) }
+    it { geometry_collection.bbox.should eq([100, -10, 130, 4]) }
+    it { feature.bbox.should eq([100, 0, 101, 1]) }
+    it { feature_collection.bbox.should eq([100, -10, 130, 4]) }
+  end
+end

src/geojson/coordinates.cr

@@ -27,6 +27,8 @@ module GeoJSON
       end
     end
 
+    delegate "[]", to: coordinates
+
     # Gets this Coordinates' longitude in decimal degrees according to WGS84.
     def longitude
       coordinates[0]

src/geojson/feature.cr

@@ -22,5 +22,11 @@ module GeoJSON
     # *properties*, *id*, and bounding box *bbox*.
     def initialize(@geometry, @properties = nil, *, @id = nil, @bbox = nil)
     end
+
+    def bbox
+      return @bbox if @bbox
+
+      geometry.try(&.bbox)
+    end
   end
 end

src/geojson/feature_collection.cr

@@ -11,5 +11,22 @@ module GeoJSON
     # Creates a new `FeatureCollection` with the given *features*.
     def initialize(@features : Array(Feature), *, @bbox = nil)
     end
+
+    def bbox
+      return @bbox if @bbox
+
+      result = [Float64::INFINITY, Float64::INFINITY, -Float64::INFINITY, -Float64::INFINITY]
+
+      features.each do |feature|
+        feature.bbox.try do |feature_bbox|
+          result[0] = feature_bbox[0] if result[0] > feature_bbox[0]
+          result[1] = feature_bbox[1] if result[1] > feature_bbox[1]
+          result[2] = feature_bbox[2] if result[2] < feature_bbox[2]
+          result[3] = feature_bbox[3] if result[3] < feature_bbox[3]
+        end
+      end
+
+      result
+    end
   end
 end

src/geojson/geometry_collection.cr

@@ -54,5 +54,22 @@ module GeoJSON
 
       geometries
     end
+
+    def bbox
+      return @bbox if @bbox
+
+      result = [Float64::INFINITY, Float64::INFINITY, -Float64::INFINITY, -Float64::INFINITY]
+
+      geometries.each do |object|
+        object.bbox.try do |object_bbox|
+          result[0] = object_bbox[0] if result[0] > object_bbox[0]
+          result[1] = object_bbox[1] if result[1] > object_bbox[1]
+          result[2] = object_bbox[2] if result[2] < object_bbox[2]
+          result[3] = object_bbox[3] if result[3] < object_bbox[3]
+        end
+      end
+
+      result
+    end
   end
 end

src/geojson/line_string.cr

@@ -33,6 +33,21 @@ module GeoJSON
       raise_if_invalid
     end
 
+    def bbox
+      return @bbox if @bbox
+
+      result = [Float64::INFINITY, Float64::INFINITY, -Float64::INFINITY, -Float64::INFINITY]
+
+      @coordinates.each do |coord|
+        result[0] = coord[0] if result[0] > coord[0]
+        result[1] = coord[1] if result[1] > coord[1]
+        result[2] = coord[0] if result[2] < coord[0]
+        result[3] = coord[1] if result[3] < coord[1]
+      end
+
+      result
+    end
+
     private def raise_if_invalid
       if coordinates.size < 2
         raise GeoJSON::Exception.new("a line needs to have two or more coordinates to be valid")

src/geojson/multi_line_string.cr

@@ -34,5 +34,22 @@ module GeoJSON
     def <<(coordinate : Array(Array(Float64)))
       @coordinates << LineString.new(coordinate).coordinates
     end
+
+    def bbox
+      return @bbox if @bbox
+
+      result = [Float64::INFINITY, Float64::INFINITY, -Float64::INFINITY, -Float64::INFINITY]
+
+      @coordinates.each do |line|
+        line.each do |coord|
+          result[0] = coord[0] if result[0] > coord[0]
+          result[1] = coord[1] if result[1] > coord[1]
+          result[2] = coord[0] if result[2] < coord[0]
+          result[3] = coord[1] if result[3] < coord[1]
+        end
+      end
+
+      result
+    end
   end
 end

src/geojson/multi_point.cr

@@ -34,5 +34,20 @@ module GeoJSON
     def <<(coordinate : Array(Float64))
       @coordinates << Point.new(coordinate).coordinates
     end
+
+    def bbox
+      return @bbox if @bbox
+
+      result = [Float64::INFINITY, Float64::INFINITY, -Float64::INFINITY, -Float64::INFINITY]
+
+      @coordinates.each do |coord|
+        result[0] = coord[0] if result[0] > coord[0]
+        result[1] = coord[1] if result[1] > coord[1]
+        result[2] = coord[0] if result[2] < coord[0]
+        result[3] = coord[1] if result[3] < coord[1]
+      end
+
+      result
+    end
   end
 end

src/geojson/multi_polygon.cr

@@ -34,5 +34,24 @@ module GeoJSON
     def <<(coordinate : Array(Array(Array(Float64))))
       @coordinates << Polygon.new(coordinate).coordinates
     end
+
+    def bbox
+      return @bbox if @bbox
+
+      result = [Float64::INFINITY, Float64::INFINITY, -Float64::INFINITY, -Float64::INFINITY]
+
+      @coordinates.each do |polygon|
+        polygon.each do |ring|
+          ring.each do |coord|
+            result[0] = coord[0] if result[0] > coord[0]
+            result[1] = coord[1] if result[1] > coord[1]
+            result[2] = coord[0] if result[2] < coord[0]
+            result[3] = coord[1] if result[3] < coord[1]
+          end
+        end
+      end
+
+      result
+    end
   end
 end

src/geojson/object.cr

@@ -22,6 +22,9 @@ module GeoJSON
 
     abstract def type : String
 
+    # Calculates the bounding box for any GeoJSON object, including `FeatureCollection`.
+    abstract def bbox
+
     use_json_discriminator "type", {
       Point:           Point,
       MultiPoint:      MultiPoint,

src/geojson/point.cr

@@ -26,6 +26,12 @@ module GeoJSON
       @coordinates = GeoJSON::Coordinates.new(coordinates)
     end
 
+    def bbox
+      return @bbox if @bbox
+
+      [coordinates[0], coordinates[1], coordinates[0], coordinates[1]]
+    end
+
     # Gets this Point's longitude in decimal degrees according to WGS84.
     delegate longitude, to: coordinates
     # Gets this Point's latitude in decimal degrees according to WGS84.

src/geojson/polygon.cr

@@ -34,6 +34,23 @@ module GeoJSON
       raise_if_invalid
     end
 
+    def bbox
+      return @bbox if @bbox
+
+      result = [Float64::INFINITY, Float64::INFINITY, -Float64::INFINITY, -Float64::INFINITY]
+
+      @coordinates.each do |ring|
+        ring.each do |coord|
+          result[0] = coord[0] if result[0] > coord[0]
+          result[1] = coord[1] if result[1] > coord[1]
+          result[2] = coord[0] if result[2] < coord[0]
+          result[3] = coord[1] if result[3] < coord[1]
+        end
+      end
+
+      result
+    end
+
     private def raise_if_invalid
       if coordinates.empty?
         "a number was found where a coordinate array should have been found: this needs to be nested more deeply"