From 4a47bb0bebd3ac17ae6ff97e579d9456b6e4bf3b Mon Sep 17 00:00:00 2001
From: Bernhard <b_walter@arcor.de>
Date: Sun, 21 May 2023 17:16:24 +0200
Subject: [PATCH] introduce short_sagitta parameter to RadiusArc to select
 between both possible cases

---
 src/build123d/objects_curve.py |  8 +++++++-
 tests/test_build_line.py       | 27 +++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/src/build123d/objects_curve.py b/src/build123d/objects_curve.py
index 09f8373..8776758 100644
--- a/src/build123d/objects_curve.py
+++ b/src/build123d/objects_curve.py
@@ -630,6 +630,8 @@ class RadiusArc(BaseLineObject):
         start_point (VectorLike): start
         end_point (VectorLike): end
         radius (float): radius
+        short_sagitta (bool): If True selects the short sagitta, else the
+            long sagitta crossing the center. Defaults to True.
         mode (Mode, optional): combination mode. Defaults to Mode.ADD.
 
     Raises:
@@ -643,6 +645,7 @@ class RadiusArc(BaseLineObject):
         start_point: VectorLike,
         end_point: VectorLike,
         radius: float,
+        short_sagitta: bool = True,
         mode: Mode = Mode.ADD,
     ):
         context: BuildLine = BuildLine._get_context(self)
@@ -652,7 +655,10 @@ class RadiusArc(BaseLineObject):
         # Calculate the sagitta from the radius
         length = end.sub(start).length / 2.0
         try:
-            sagitta = abs(radius) - sqrt(radius**2 - length**2)
+            if short_sagitta:
+                sagitta = abs(radius) - sqrt(radius**2 - length**2)
+            else:
+                sagitta = -abs(radius) - sqrt(radius**2 - length**2)
         except ValueError as exception:
             raise ValueError(
                 "Arc radius is not large enough to reach the end point."
diff --git a/tests/test_build_line.py b/tests/test_build_line.py
index 1a9b198..1b9535c 100644
--- a/tests/test_build_line.py
+++ b/tests/test_build_line.py
@@ -166,6 +166,33 @@ class BuildLineTests(unittest.TestCase):
             Spline((0, 0), (1, 1), (2, 0))
         self.assertTupleAlmostEquals((test.edges()[0] @ 1).to_tuple(), (2, 0, 0), 5)
 
+    def test_radius_arc(self):
+        """Test center arc as arc and circle"""
+        with BuildSketch() as s:
+            c = Circle(10)
+
+        e = c.edges()[0]
+        r = e.radius
+        p1, p2 = e @ 0.3, e @ 0.9
+
+        with BuildLine() as l:
+            arc1 = RadiusArc(p1, p2, r)
+            self.assertAlmostEqual(arc1.length, 2 * r * pi * 0.4, 6)
+            self.assertAlmostEqual(arc1.bounding_box().max.X, c.bounding_box().max.X)
+
+            arc2 = RadiusArc(p1, p2, r, short_sagitta=False)
+            self.assertAlmostEqual(arc2.length, 2 * r * pi * 0.6, 6)
+            self.assertAlmostEqual(arc2.bounding_box().min.X, c.bounding_box().min.X)
+
+            arc3 = RadiusArc(p1, p2, -r)
+            self.assertAlmostEqual(arc3.length, 2 * r * pi * 0.4, 6)
+            self.assertGreater(arc3.bounding_box().min.X, c.bounding_box().min.X)
+            self.assertLess(arc3.bounding_box().min.X, c.bounding_box().max.X)
+
+            arc4 = RadiusArc(p1, p2, -r, short_sagitta=False)
+            self.assertAlmostEqual(arc4.length, 2 * r * pi * 0.6, 6)
+            self.assertGreater(arc4.bounding_box().max.X, c.bounding_box().max.X)
+
     def test_center_arc(self):
         """Test center arc as arc and circle"""
         with BuildLine() as arc: