From 0e66c22c462520e5af75a43248b142f7337f5951 Mon Sep 17 00:00:00 2001
From: Roel Arents <roel.arents@kadaster.nl>
Date: Mon, 11 Dec 2023 16:57:34 +0100
Subject: [PATCH] fix filtering out small rings after deduping

---
 snap/snap.go      | 30 +++++++++++++++++-------------
 snap/snap_test.go | 14 +++++++++++++-
 2 files changed, 30 insertions(+), 14 deletions(-)

diff --git a/snap/snap.go b/snap/snap.go
index c567b7b..89cf373 100644
--- a/snap/snap.go
+++ b/snap/snap.go
@@ -147,25 +147,30 @@ func cleanupNewRing(newRing [][2]float64, isOuter bool) [][2]float64 {
 	// LinearRings(): "The last point in the linear ring will not match the first point."
 	if newRingLen > 1 && newRing[0] == newRing[newRingLen-1] {
 		newRing = newRing[:newRingLen-1]
+		newRingLen--
 	}
-	switch newRingLen {
-	case 0:
+
+	// filter out too small rings
+	if newRingLen == 0 || newRingLen < 3 && !(isOuter && keepPointsAndLines) {
 		return nil
-	case 1, 2:
-		if isOuter && keepPointsAndLines {
-			// keep outer ring as point or line
-			return newRing
-		}
-	default:
+	}
+
+	if newRingLen > 2 {
 		// deduplicate points in the ring, check winding order, then add to the polygon
-		deduplicatedRing := kmpDeduplicate(newRing)
+		newRing = kmpDeduplicate(newRing)
 		// inner rings (ringIdx != 0) should be clockwise
 		shouldBeClockwise := !isOuter
 		// winding order is reversed if incorrect
-		ensureCorrectWindingOrder(deduplicatedRing, shouldBeClockwise)
-		return deduplicatedRing
+		ensureCorrectWindingOrder(newRing, shouldBeClockwise)
+		newRingLen = len(newRing)
 	}
-	return nil
+
+	// filter out too small rings again after deduping
+	if newRingLen == 0 || newRingLen < 3 && !(isOuter && keepPointsAndLines) {
+		return nil
+	}
+
+	return newRing
 }
 
 func floatPolygonsToGeomPolygons(floaters map[Level][][][2]float64) map[Level]geom.Polygon {
@@ -207,7 +212,6 @@ func isClockwise(points [3][2]float64, shouldBeClockwise bool) bool {
 }
 
 // deduplication using an implementation of the Knuth-Morris-Pratt algorithm
-// function also determines the rightmost lowest point of the ring, which is used to ensure correct winding order
 //
 //nolint:cyclop,funlen
 func kmpDeduplicate(newRing [][2]float64) [][2]float64 {
diff --git a/snap/snap_test.go b/snap/snap_test.go
index 461330e..acef7b9 100644
--- a/snap/snap_test.go
+++ b/snap/snap_test.go
@@ -220,7 +220,19 @@ func Test_snapPolygon(t *testing.T) {
 				{90673.689, 530324.552},
 				{90664.068, 530379.532},
 			}},
-			want: nil,
+			want: map[tms20.TMID]geom.Polygon{0: nil},
+		},
+		{
+			name:  "ring length < 3 _after_ deduping, also should be filtered out",
+			tms:   loadEmbeddedTileMatrixSet(t, "NetherlandsRDNewQuad"),
+			tmIDs: []tms20.TMID{0},
+			polygon: geom.Polygon{{
+				{211124.566, 574932.941},
+				{211142.954, 574988.796},
+				{211059.858, 574971.321},
+				{211163.163, 574994.581},
+			}},
+			want: map[tms20.TMID]geom.Polygon{0: nil},
 		},
 	}
 	for _, tt := range tests {