s2: Add RegionUnion.

Signed-off-by: David Symonds <dsymonds@golang.org>

Author: nes1983

README.md

@@ -129,6 +129,7 @@ Approximately ~40% complete.
 *   PointCompression
 *   Region
 *   RegionCoverer
+*   RegionUnion
 *   s2edge_clipping
 *   s2edge_crosser
 *   s2edge_crossings
@@ -193,7 +194,6 @@ started.
 *   PolygonMeasures
 *   RegionIntersection
 *   RegionTermIndexer
-*   RegionUnion
 *   ShapeIndexRegion - Allows ShapeIndexes to be used as Regions for things like
 
 ### Encode/Decode

s2/regionunion.go

@@ -0,0 +1,66 @@
+// Copyright 2020 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package s2
+
+// A RegionUnion represents a union of possibly overlapping regions.
+// It is convenient for computing a covering of a set of regions.
+type RegionUnion []Region
+
+// CapBound returns a bounding cap for this RegionUnion.
+func (ru RegionUnion) CapBound() Cap { return ru.RectBound().CapBound() }
+
+// RectBound returns a bounding latitude-longitude rectangle for this RegionUnion.
+func (ru RegionUnion) RectBound() Rect {
+	ret := EmptyRect()
+	for _, reg := range ru {
+		ret = ret.Union(reg.RectBound())
+	}
+	return ret
+}
+
+// ContainsCell reports whether the given Cell is contained by this RegionUnion.
+func (ru RegionUnion) ContainsCell(c Cell) bool {
+	for _, reg := range ru {
+		if reg.ContainsCell(c) {
+			return true
+		}
+	}
+	return false
+}
+
+// IntersectsCell reports whether this RegionUnion intersects the given cell.
+func (ru RegionUnion) IntersectsCell(c Cell) bool {
+	for _, reg := range ru {
+		if reg.IntersectsCell(c) {
+			return true
+		}
+	}
+	return false
+}
+
+// ContainsPoint reports whether this RegionUnion contains the Point.
+func (ru RegionUnion) ContainsPoint(p Point) bool {
+	for _, reg := range ru {
+		if reg.ContainsPoint(p) {
+			return true
+		}
+	}
+	return false
+}
+
+// CellUnionBound computes a covering of the RegionUnion.
+func (ru RegionUnion) CellUnionBound() []CellID {
+	return ru.CapBound().CellUnionBound()
+}

s2/regionunion_test.go

@@ -0,0 +1,111 @@
+// Copyright 2020 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package s2
+
+import (
+	"testing"
+)
+
+func TestEmptyRegionUnionHasEmptyCap(t *testing.T) {
+	var empty RegionUnion
+
+	got := empty.CapBound()
+
+	want := EmptyCap()
+	if !got.ApproxEqual(want) {
+		t.Errorf("empty region union cap = %v, want %v", got, want)
+	}
+}
+
+func TestEmptyRegionUnionHasEmptyBound(t *testing.T) {
+	var empty RegionUnion
+
+	got := empty.RectBound()
+
+	want := EmptyRect()
+	if !got.ApproxEqual(want) {
+		t.Errorf("empty region union cap = %v, want %v", got, want)
+	}
+}
+
+func TestRegionUnionOfTwoPointsHasCorrectBound(t *testing.T) {
+	got := twoPointsRegionUnion.RectBound()
+
+	want := makeRect("-35:-40,35:40")
+	if !got.ApproxEqual(want) {
+		t.Errorf("%v.RectBound() = %v, want %v", twoPointsRegionUnion, got, want)
+	}
+}
+
+var twoPointsRegionUnion = RegionUnion{
+	PointFromLatLng(LatLngFromDegrees(35, 40)),
+	PointFromLatLng(LatLngFromDegrees(-35, -40)),
+}
+
+func TestRegionUnionOfTwoPointsIntersectsFace0(t *testing.T) {
+	got := twoPointsRegionUnion.IntersectsCell(face0Cell)
+
+	if !got {
+		t.Errorf("%v.IntersectsCell(%v) = %v, want true", twoPointsRegionUnion, face0Cell, got)
+	}
+}
+
+func TestRegionUnionOfTwoPointsDoesNotContainFace0(t *testing.T) {
+	got := twoPointsRegionUnion.ContainsCell(face0Cell)
+
+	if got {
+		t.Errorf("%v.ContainsCell(%v) = %v, want false", twoPointsRegionUnion, face0Cell, got)
+	}
+}
+
+var face0Cell = CellFromCellID(CellIDFromFace(0))
+
+func TestRegionUnionOfTwoContainsPoint(t *testing.T) {
+	testCases := []struct {
+		ll   LatLng
+		want bool
+	}{
+		{LatLngFromDegrees(35, 40), true},
+		{LatLngFromDegrees(-35, -40), true},
+		{LatLngFromDegrees(0, 0), false},
+	}
+
+	for _, tc := range testCases {
+		got := twoPointsRegionUnion.ContainsPoint(PointFromLatLng(tc.ll))
+
+		if got != tc.want {
+			t.Errorf("%v.ContainsPoint(%v) = %t, want %t", twoPointsRegionUnion, tc.ll, got, tc.want)
+		}
+	}
+}
+
+func TestTwoPointsRegionCovering(t *testing.T) {
+	cov := NewRegionCoverer()
+	cov.MaxCells = 1
+
+	got := cov.Covering(twoPointsRegionUnion)
+
+	const wantLen = 1
+	if l := len(got); l != wantLen {
+		t.Fatalf("covering = %v, len %v, want %v", got, l, wantLen)
+	}
+	want := CellIDFromFace(0)
+	if g := got[0]; g != want {
+		t.Errorf("covering[0] = %v, want %v", g, want)
+	}
+}
+
+// Make sure RegionUnion implements Region.
+var _ Region = RegionUnion{}