Add incidentEdgeTracker and indexCellData types and tests

s2/incident_edge_tracker.go

@@ -0,0 +1,181 @@
+// Copyright 2025 The S2 Geometry Project Authors. 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
+
+// incidentEdgeKey is a tuple of (shape id, vertex) that compares by shape id.
+type incidentEdgeKey struct {
+	shapeID int32
+	vertex  Point
+}
+
+// We need a strict ordering to be a valid key for an ordered container, but
+// we don't actually care about the ordering of the vertices (as long as
+// they're grouped by shape id). Vertices are 3D points so they don't have a
+// natural ordering, so we'll just compare them lexicographically.
+func (i incidentEdgeKey) Cmp(o incidentEdgeKey) int {
+	if i.shapeID < o.shapeID {
+		return -1
+	}
+	if i.shapeID > o.shapeID {
+		return 1
+	}
+
+	return i.vertex.Cmp(o.vertex.Vector)
+}
+
+// incidentVertexEdge is a tuple of vertex and edgeID for processing incident edges.
+type incidentVertexEdge struct {
+	vertex Point
+	edgeID int32
+}
+
+// incidentEdgeTracker is a used for detecting and tracking shape edges that
+// are incident on the same vertex. Edges of multiple shapes may be tracked,
+// but lookup is by shape id and vertex: there is no facility to get all
+// edges of all shapes at a vertex. Edge vertices must compare exactly equal
+// to be considered the same vertex, no tolerance is applied as this isn't
+// intended for actions like snapping shapes together, which Builder does better
+// and more robustly.
+//
+// To use, instantiate and then add edges with one or more sequences of calls;
+// each sequence begins with startShape(), followed by repeated addEdge() calls
+// to add edges for that shape, and ends with finishShape(). Those sequences do
+// not need to visit shapes or edges in order. Then, read the edgeMap to get
+// the resulting map from incidentEdgeKeys (which are shapeID, vertex pairs) to
+// a set of edgeIDs of the shape that are incident to that vertex.
+//
+// This works on a block of edges at a time, meaning that to detect incident
+// edges on a particular vertex, we must have at least three edges incident
+// at that vertex when finishShape() is called. We don't maintain partial
+// information between calls. However, subject to this constraint, a single
+// shape's edges may be defined with multiple sequences of startShape(),
+// addEdge()... , finishShape() calls.
+//
+// The reason for this is simple: most vertices don't have more than two incident
+// edges (the incoming and outgoing edge). If we had to maintain information
+// between calls, we'd end up with a map that contains every vertex, to no
+// benefit. Instead, when finishShape() is called, we discard vertices that
+// contain two or fewer incident edges.
+//
+// In principle this isn't a real limitation because generally we process a
+// ShapeIndex one cell at a time, and, if a vertex has multiple edges, we'll see
+// all the edges in the same cell as the vertex, and, in general, it's possible
+// to aggregate edges before calling.
+//
+// The tracker maintains incident edges until it's reset. If you call it with
+// each cell from an ShapeIndex, then at the end you will have all the
+// incident edge information for the whole index. If only a subset is needed,
+// call reset() when you're done edding edges.
+type incidentEdgeTracker struct {
+	currentShapeID int32
+
+	// nursery is used to store points being processed for the map.
+	nursery []incidentVertexEdge
+
+	// edgeMap tracks edgeIDs for each incidentEdgeKey since we can, and do,
+	// encounter the same edges multiple times. This map gives us easy
+	// deduplication of edges as they're inserted.
+	edgeMap map[incidentEdgeKey]map[int32]bool
+}
+
+// newIncidentEdgeTracker returns a new incidentEdgeTracker.
+func newIncidentEdgeTracker() *incidentEdgeTracker {
+	return &incidentEdgeTracker{
+		currentShapeID: -1,
+		nursery:        []incidentVertexEdge{},
+		edgeMap:        make(map[incidentEdgeKey]map[int32]bool),
+	}
+}
+
+// startShape is used to start a new shape.
+func (t *incidentEdgeTracker) startShape(id int32) {
+	t.currentShapeID = id
+	t.nursery = t.nursery[:0]
+}
+
+// addEdge adds the given edge for the current shape to the nursery. If the
+// edge is not degenerate, add its second endpoint as well.
+func (t *incidentEdgeTracker) addEdge(edgeID int32, e Edge) {
+	if t.currentShapeID < 0 {
+		return
+	}
+
+	// Add non-degenerate edges to the nursery.
+	t.nursery = append(t.nursery, incidentVertexEdge{vertex: e.V0, edgeID: edgeID})
+	if !e.IsDegenerate() {
+		t.nursery = append(t.nursery, incidentVertexEdge{vertex: e.V1, edgeID: edgeID})
+	}
+}
+
+// finishShape is called once finished adding edges so they can be processed.
+//
+// After calling, only vertices with 2 or more incident edges will appear in
+// the edge map.
+func (t *incidentEdgeTracker) finishShape() {
+	// We want to keep any vertices with more than two incident edges. We could
+	// sort the array by vertex and remove any with fewer, but that would require
+	// shifting the array and could turn quadratic quickly.
+	//
+	// Instead we'll scan forward from each vertex, swapping entries with the same
+	// vertex into a contiguous range. Once we've done all the swapping we can
+	// just make sure that we have at least three edges in the range.
+	nurserySize := len(t.nursery)
+	for start := 0; start < nurserySize; {
+		end := start + 1
+
+		// Scan to the end of the array, swap entries so that entries with
+		// the same vertex as the start are adjacent.
+		next := start
+		currVertex := t.nursery[start].vertex
+		for next+1 < nurserySize {
+			next++
+			if t.nursery[next].vertex == currVertex {
+				t.nursery[next], t.nursery[end] = t.nursery[end], t.nursery[next]
+				end++
+			}
+		}
+
+		// Most vertices will have two incident edges (the incoming edge and the
+		// outgoing edge), which aren't interesting, skip them.
+		numEdges := end - start
+		if numEdges <= 2 {
+			start = end
+			continue
+		}
+
+		key := incidentEdgeKey{
+			shapeID: t.currentShapeID,
+			vertex:  t.nursery[start].vertex,
+		}
+
+		// If we don't have this key yet, create it manually.
+		if _, ok := t.edgeMap[key]; !ok {
+			t.edgeMap[key] = map[int32]bool{}
+		}
+
+		for ; start != end; start++ {
+			t.edgeMap[key][t.nursery[start].edgeID] = true
+		}
+	}
+
+	// We are finished with this shape, clear the nursery.
+	t.nursery = t.nursery[:0]
+}
+
+// reset removes all incident edges from the tracker.
+func (t *incidentEdgeTracker) reset() {
+	t.nursery = t.nursery[:0]
+	t.edgeMap = make(map[incidentEdgeKey]map[int32]bool)
+}

s2/incident_edge_tracker_test.go

@@ -0,0 +1,70 @@
+// Copyright 2025 The S2 Geometry Project Authors. 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 TestIncidentEdgeTrackerBasic(t *testing.T) {
+	tests := []struct {
+		index string
+		want  int
+	}{
+		// These shapeindex strings came from validation query's test
+		// corpus to determine which ones ended up actually getting
+		// tracked edges.
+		{
+			// Has 0 tracked edges
+			index: "## 0:0, 1:1",
+			want:  0,
+		},
+		{
+			// Has 1 tracked edges
+			index: "## 2:0, 0:-2, -2:0, 0:2; 2:0, 0:-1, -1:0, 0:1",
+			want:  1,
+		},
+		{
+			// Has 2 tracked edges
+			index: "## 2:0, 0:-2, -2:0, 0:2; 2:0, 0:-1, -2:0, 0:1",
+			want:  2,
+		},
+	}
+
+	for _, test := range tests {
+		index := makeShapeIndex(test.index)
+		index.Build()
+
+		iter := index.Iterator()
+		celldata := newIndexCellData()
+		celldata.loadCell(index, iter.CellID(), iter.IndexCell())
+
+		tracker := newIncidentEdgeTracker()
+
+		for _, clipped := range celldata.indexCell.shapes {
+			shapeID := clipped.shapeID
+			tracker.startShape(shapeID)
+			for _, e := range celldata.shapeEdges(shapeID) {
+				tracker.addEdge(e.ID, e.Edge)
+			}
+			tracker.finishShape()
+		}
+
+		if got := len(tracker.edgeMap); got != test.want {
+			t.Errorf("incidentEdgeTracker should have %d edges, got %d",
+				test.want, got)
+		}
+	}
+}