add reverse

flat.go

@@ -120,6 +120,11 @@ func (g *geom1) NumCoords() int {
 	return len(g.flatCoords) / g.stride
 }
 
+// Reverse reverses the order of g's coordinates.
+func (g *geom1) Reverse() {
+	reverse1(g.flatCoords, 0, len(g.flatCoords), g.stride)
+}
+
 func (g *geom1) setCoords(coords1 []Coord) error {
 	var err error
 	g.flatCoords, err = deflate1(nil, coords1, g.stride)
@@ -152,6 +157,11 @@ func (g *geom2) Ends() []int {
 	return g.ends
 }
 
+// Reverse reverses the order of coordinates for each sub-structure in g.
+func (g *geom2) Reverse() {
+	reverse2(g.flatCoords, 0, g.ends, g.stride)
+}
+
 func (g *geom2) setCoords(coords2 [][]Coord) error {
 	var err error
 	g.flatCoords, g.ends, err = deflate2(nil, nil, coords2, g.stride)
@@ -200,6 +210,11 @@ func (g *geom3) Endss() [][]int {
 	return g.endss
 }
 
+// Reverse reverses the order of coordinates for each sub-sub-structure in g.
+func (g *geom3) Reverse() {
+	reverse3(g.flatCoords, 0, g.endss, g.stride)
+}
+
 func (g *geom3) setCoords(coords3 [][][]Coord) error {
 	var err error
 	g.flatCoords, g.endss, err = deflate3(nil, nil, coords3, g.stride)
@@ -386,3 +401,28 @@ func length3(flatCoords []float64, offset int, endss [][]int, stride int) float6
 	}
 	return length
 }
+
+func reverse1(flatCoords []float64, offset, end, stride int) {
+	for i, j := offset+stride, end; i <= j; i, j = i+stride, j-stride {
+		for k := 0; k < stride; k++ {
+			flatCoords[i-stride+k], flatCoords[j-stride+k] = flatCoords[j-stride+k], flatCoords[i-stride+k]
+		}
+	}
+}
+
+func reverse2(flatCoords []float64, offset int, ends []int, stride int) {
+	for _, end := range ends {
+		reverse1(flatCoords, offset, end, stride)
+		offset = end
+	}
+}
+
+func reverse3(flatCoords []float64, offset int, endss [][]int, stride int) {
+	for _, ends := range endss {
+		if len(ends) == 0 {
+			continue
+		}
+		reverse2(flatCoords, offset, ends, stride)
+		offset = ends[len(ends)-1]
+	}
+}

geom_test.go

@@ -572,3 +572,42 @@ func TestSetSRID(t *testing.T) {
 	_, err := SetSRID(nil, 4326)
 	assert.Error(t, err)
 }
+
+func TestReverse(t *testing.T) {
+	for _, tc := range []struct {
+		g interface {
+			Reverse()
+		}
+		want interface {
+			Reverse()
+		}
+	}{
+		{
+			g:    NewLinearRing(XYZM).MustSetCoords([]Coord{{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}}),
+			want: NewLinearRing(XYZM).MustSetCoords([]Coord{{9, 10, 11, 12}, {5, 6, 7, 8}, {1, 2, 3, 4}}),
+		},
+		{
+			g:    NewLineString(XYZM).MustSetCoords([]Coord{{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}}),
+			want: NewLineString(XYZM).MustSetCoords([]Coord{{9, 10, 11, 12}, {5, 6, 7, 8}, {1, 2, 3, 4}}),
+		},
+		{
+			g:    NewMultiLineString(XY).MustSetCoords([][]Coord{{}, {}, {{1, 2}, {3, 4}, {5, 6}}, {{7, 8}, {9, 10}, {11, 12}}, {}}),
+			want: NewMultiLineString(XY).MustSetCoords([][]Coord{{}, {}, {{5, 6}, {3, 4}, {1, 2}}, {{11, 12}, {9, 10}, {7, 8}}, {}}),
+		},
+		{
+			g:    NewMultiPoint(XY).MustSetCoords([]Coord{nil, {1, 2}, nil, {3, 4}, nil, {5, 6}, nil}),
+			want: NewMultiPoint(XY).MustSetCoords([]Coord{nil, {1, 2}, nil, {3, 4}, nil, {5, 6}, nil}),
+		},
+		{
+			g:    NewMultiPolygon(XY).MustSetCoords([][][]Coord{{{{1, 2}, {4, 5}, {6, 7}, {1, 2}}}, {}, {}, {{{1, 2}, {3, 4}, {5, 6}}, {{7, 8}, {9, 10}, {11, 12}}}, {}}),
+			want: NewMultiPolygon(XY).MustSetCoords([][][]Coord{{{{1, 2}, {6, 7}, {4, 5}, {1, 2}}}, {}, {}, {{{5, 6}, {3, 4}, {1, 2}}, {{11, 12}, {9, 10}, {7, 8}}}, {}}),
+		},
+		{
+			g:    NewPolygon(XY).MustSetCoords([][]Coord{{{1, 2}, {3, 4}, {5, 6}}, {{7, 8}, {9, 10}, {11, 12}}}),
+			want: NewPolygon(XY).MustSetCoords([][]Coord{{{5, 6}, {3, 4}, {1, 2}}, {{11, 12}, {9, 10}, {7, 8}}}),
+		},
+	} {
+		tc.g.Reverse()
+		assert.Equal(t, tc.want, tc.g)
+	}
+}