From ea388bdc14ac8477ac20105ac03fea1e67b3bbe5 Mon Sep 17 00:00:00 2001
From: Tom Payne <twpayne@gmail.com>
Date: Sat, 17 Aug 2024 19:26:55 +0200
Subject: [PATCH] Add xy.RegularPolygon

---
 xy/regular_polygon.go      | 22 ++++++++++++++++++++++
 xy/regular_polygon_test.go | 31 +++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+)
 create mode 100644 xy/regular_polygon.go
 create mode 100644 xy/regular_polygon_test.go

diff --git a/xy/regular_polygon.go b/xy/regular_polygon.go
new file mode 100644
index 00000000..55f6b738
--- /dev/null
+++ b/xy/regular_polygon.go
@@ -0,0 +1,22 @@
+package xy
+
+import (
+	"math"
+
+	"github.com/twpayne/go-geom"
+)
+
+// RegularPolygon returns an n-sided regular polygon centered at center with radius r.
+func RegularPolygon(n int, center *geom.Point, r float64) *geom.Polygon {
+	pointFlatCoords := center.FlatCoords()
+	stride := center.Stride()
+	flatCoords := make([]float64, (n+1)*stride)
+	for i := 0; i < n; i++ {
+		theta := 2 * math.Pi * float64(i) / float64(n)
+		flatCoords[i*stride] = pointFlatCoords[0] + r*math.Cos(theta)
+		flatCoords[i*stride+1] = pointFlatCoords[1] + r*math.Sin(theta)
+		copy(flatCoords[i*stride+2:(i+1)*stride], pointFlatCoords[2:stride])
+	}
+	copy(flatCoords[n*stride:(n+1)*stride], flatCoords[:stride])
+	return geom.NewPolygonFlat(center.Layout(), flatCoords, []int{(n + 1) * stride})
+}
diff --git a/xy/regular_polygon_test.go b/xy/regular_polygon_test.go
new file mode 100644
index 00000000..db78d6de
--- /dev/null
+++ b/xy/regular_polygon_test.go
@@ -0,0 +1,31 @@
+package xy_test
+
+import (
+	"testing"
+
+	"github.com/alecthomas/assert/v2"
+
+	"github.com/twpayne/go-geom"
+	"github.com/twpayne/go-geom/xy"
+)
+
+func TestRegularPolygon(t *testing.T) {
+	for _, tc := range []struct {
+		n              int
+		center         *geom.Point
+		r              float64
+		expectedCoords [][]geom.Coord
+	}{
+		{
+			n:              4,
+			center:         geom.NewPoint(geom.XY).MustSetCoords([]float64{1, 2}),
+			r:              1,
+			expectedCoords: [][]geom.Coord{{{2, 2}, {1, 3}, {0, 2}, {1, 1}, {2, 2}}},
+		},
+	} {
+		t.Run("", func(t *testing.T) {
+			actual := xy.RegularPolygon(tc.n, tc.center, tc.r)
+			assert.Equal(t, tc.expectedCoords, actual.Coords())
+		})
+	}
+}