From 0118cac57afef559d9570341f335f57160a936bd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Gustav=20N=C3=A4slund?= <gustav@gntech.se>
Date: Wed, 18 Apr 2018 23:20:33 +0200
Subject: [PATCH] Add sagittaArc function and test

---
 cadquery/cq.py        | 39 +++++++++++++++++++++++++++++++++++++--
 tests/TestCadQuery.py |  4 ++++
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/cadquery/cq.py b/cadquery/cq.py
index 943f1fc..38d9b41 100644
--- a/cadquery/cq.py
+++ b/cadquery/cq.py
@@ -1331,6 +1331,41 @@ def threePointArc(self, point1, point2, forConstruction=False):
 
         return self.newObject([arc])
 
+    def sagittaArc(self, endPoint, sag, forConstruction=False):
+        """
+        Draw an arc from the current point to endPoint with an arc defined by the sag (sagitta).
+
+        :param endPoint: end point for the arc
+        :type endPoint: 2-tuple, in workplane coordinates
+        :param sag: the sagitta of the arc
+        :type sag: float, perpendicular distance from arc center to arc baseline.
+        :return: a workplane with the current point at the end of the arc
+
+        The sagitta is the distance from the center of the arc to the arc base.
+        Given that a closed contour is drawn clockwise;
+        A positive sagitta means convex arc and negative sagitta means concave arc.
+        See "https://en.wikipedia.org/wiki/Sagitta_(geometry)" for more information.
+        """
+
+        startPoint = self._findFromPoint(False)
+        endPoint = self.plane.toWorldCoords(endPoint)
+        midPoint = endPoint.add(startPoint).multiply(0.5)
+
+        sagVector = endPoint.sub(startPoint).normalized().multiply(abs(sag))
+        if(sag > 0):
+            sagVector.x, sagVector.y = -sagVector.y, sagVector.x # Rotate sagVector +90 deg
+        else:
+            sagVector.x, sagVector.y = sagVector.y, -sagVector.x # Rotate sagVector -90 deg
+
+        sagPoint = midPoint.add(sagVector)
+
+        arc = Edge.makeThreePointArc(startPoint, sagPoint, endPoint)
+
+        if not forConstruction:
+            self._addPendingEdge(arc)
+
+        return self.newObject([arc])
+
     def rotateAndCopy(self, matrix):
         """
         Makes a copy of all edges on the stack, rotates them according to the
@@ -2080,7 +2115,7 @@ def sweep(self, path, sweepAlongWires=False, makeSolid=True, isFrenet=False, com
         :param path: A wire along which the pending wires will be swept
         :param boolean sweepAlongWires:
             False to create mutliple swept from wires on the chain along path
-            True to create only one solid swept along path with shape following the list of wires on the chain 
+            True to create only one solid swept along path with shape following the list of wires on the chain
         :param boolean combine: True to combine the resulting solid with parent solids if found.
         :param boolean clean: call :py:meth:`clean` afterwards to have a clean shape
         :return: a CQ object with the resulting solid selected.
@@ -2407,7 +2442,7 @@ def _sweep(self, path, sweepAlongWires=False, makeSolid=True, isFrenet=False):
         :param path: A wire along which the pending wires will be swept
         :param boolean sweepAlongWires:
             False to create mutliple swept from wires on the chain along path
-            True to create only one solid swept along path with shape following the list of wires on the chain 
+            True to create only one solid swept along path with shape following the list of wires on the chain
         :return:a FreeCAD solid, suitable for boolean operations
         """
 
diff --git a/tests/TestCadQuery.py b/tests/TestCadQuery.py
index db329dc..5b16a7f 100644
--- a/tests/TestCadQuery.py
+++ b/tests/TestCadQuery.py
@@ -867,6 +867,10 @@ def test2DDrawing(self):
                           r.vertices(selectors.NearestToPointSelector((0.0, 0.0, 0.0)))\
                           .first().val().Y))
 
+        # Test the sagittaArc functions
+        s = Workplane(Plane.YZ())
+        r = s.sagittaArc((10, 8), 1).close()
+
     def testLargestDimension(self):
         """
         Tests the largestDimension function when no solids are on the stack and when there are