Fix area calculation for polygons with holes

twpayne · Aug 18, 2024 · 3ad9aea · 3ad9aea
1 parent 8e46749
commit 3ad9aea
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Showing 3 changed files with 45 additions and 12 deletions.
diff --git a/flat.go b/flat.go
@@ -265,13 +265,8 @@ func doubleArea1(flatCoords []float64, offset, end, stride int) float64 {
 
 func doubleArea2(flatCoords []float64, offset int, ends []int, stride int) float64 {
 	var doubleArea float64
-	for i, end := range ends {
-		da := doubleArea1(flatCoords, offset, end, stride)
-		if i == 0 {
-			doubleArea = da
-		} else {
-			doubleArea -= da
-		}
+	for _, end := range ends {
+		doubleArea += doubleArea1(flatCoords, offset, end, stride)
 		offset = end
 	}
 	return doubleArea

diff --git a/geom_test.go b/geom_test.go
@@ -38,7 +38,7 @@ func aliases(x, y []float64) bool {
 }
 
 func TestArea(t *testing.T) {
-	for _, tc := range []struct {
+	for i, tc := range []struct {
 		g interface {
 			Area() float64
 		}
@@ -115,7 +115,7 @@ func TestArea(t *testing.T) {
 		{
 			g: NewPolygon(XY).MustSetCoords([][]Coord{
 				{{-3, -2}, {-1, 4}, {6, 1}, {3, 10}, {-4, 9}, {-3, -2}},
-				{{0, 6}, {2, 6}, {2, 8}, {0, 8}, {0, 6}},
+				{{0, 6}, {0, 8}, {2, 8}, {2, 6}, {0, 6}},
 			}),
 			want: 56,
 		},
@@ -151,7 +151,7 @@ func TestArea(t *testing.T) {
 			g: NewMultiPolygon(XY).MustSetCoords([][][]Coord{
 				{
 					{{-3, -2}, {-1, 4}, {6, 1}, {3, 10}, {-4, 9}, {-3, -2}},
-					{{0, 6}, {2, 6}, {2, 8}, {0, 8}, {0, 6}},
+					{{0, 6}, {0, 8}, {2, 8}, {2, 6}, {0, 6}},
 				},
 			}),
 			want: 56,
@@ -163,13 +163,28 @@ func TestArea(t *testing.T) {
 				},
 				{
 					{{-3, -2}, {-1, 4}, {6, 1}, {3, 10}, {-4, 9}, {-3, -2}},
-					{{0, 6}, {2, 6}, {2, 8}, {0, 8}, {0, 6}},
+					{{0, 6}, {0, 8}, {2, 8}, {2, 6}, {0, 6}},
 				},
 			}),
 			want: 57,
 		},
+		{
+			g: NewPolygon(XY).MustSetCoords([][]Coord{
+				{{0, 0}, {1, 0}, {1, 1}, {0, 1}, {0, 0}},
+			}),
+			want: 1,
+		},
+		{
+			g: NewPolygon(XY).MustSetCoords([][]Coord{
+				{{0, 0}, {3, 0}, {3, 3}, {0, 3}, {0, 0}},
+				{{1, 1}, {1, 2}, {2, 2}, {2, 1}, {1, 1}},
+			}),
+			want: 8,
+		},
 	} {
-		assert.Equal(t, tc.want, tc.g.Area())
+		t.Run(strconv.Itoa(i), func(t *testing.T) {
+			assert.Equal(t, tc.want, tc.g.Area())
+		})
 	}
 }
 

diff --git a/polygon_test.go b/polygon_test.go
@@ -116,3 +116,26 @@ func TestPolygonSetSRID(t *testing.T) {
 	assert.Equal(t, 4326, NewPolygon(NoLayout).SetSRID(4326).SRID())
 	assert.Equal(t, 4326, Must(SetSRID(NewPolygon(NoLayout), 4326)).SRID())
 }
+
+func TestPolygonArea(t *testing.T) {
+	for i, tc := range []struct {
+		polygon  *Polygon
+		expected float64
+	}{
+		{
+			polygon:  NewPolygon(XY).MustSetCoords([][]Coord{{{0, 0}, {1, 0}, {1, 1}, {0, 1}, {0, 0}}}),
+			expected: 1,
+		},
+		{
+			polygon: NewPolygon(XY).MustSetCoords([][]Coord{
+				{{0, 0}, {3, 0}, {3, 3}, {0, 3}, {0, 0}},
+				{{1, 1}, {1, 2}, {2, 2}, {2, 1}, {1, 1}},
+			}),
+			expected: 8,
+		},
+	} {
+		t.Run(strconv.Itoa(i), func(t *testing.T) {
+			assert.Equal(t, tc.expected, tc.polygon.Area())
+		})
+	}
+}