From cc0fb2a718a3981332328b75ce2a86b4610b161d Mon Sep 17 00:00:00 2001
From: Artyom Degtyarev <artdegt98@mail.ru>
Date: Thu, 16 Feb 2023 18:47:42 +0300
Subject: [PATCH] non-existence of tangents throws exception

---
 .../kscience/kmath/trajectory/tangent.kt      | 31 +++++++++++--------
 .../kscience/kmath/trajectory/TangentTest.kt  | 25 +++++++++++++++
 2 files changed, 43 insertions(+), 13 deletions(-)

diff --git a/kmath-trajectory/src/commonMain/kotlin/space/kscience/kmath/trajectory/tangent.kt b/kmath-trajectory/src/commonMain/kotlin/space/kscience/kmath/trajectory/tangent.kt
index cf5bcd11d..945b140c2 100644
--- a/kmath-trajectory/src/commonMain/kotlin/space/kscience/kmath/trajectory/tangent.kt
+++ b/kmath-trajectory/src/commonMain/kotlin/space/kscience/kmath/trajectory/tangent.kt
@@ -38,20 +38,25 @@ public fun Circle2D.tangentsToCircle(
             } else {
                 r1.absoluteValue + r2.absoluteValue
             }
-            val l = (d * d - r * r).pow(0.5)
-            angle2 = if (r1.absoluteValue > r2.absoluteValue) {
-                angle1 + r1.sign * atan2(r.absoluteValue, l)
-            } else {
-                angle1 - r2.sign * atan2(r.absoluteValue, l)
-            }
-            val w = vector(-cos(angle2), sin(angle2))
-            put(
-                route,
-                LineSegment(
-                    center + w * r1,
-                    other.center + w * r2
+            if (d * d > r * r) {
+                val l = (d * d - r * r).pow(0.5)
+                angle2 = if (r1.absoluteValue > r2.absoluteValue) {
+                    angle1 + r1.sign * atan2(r.absoluteValue, l)
+                } else {
+                    angle1 - r2.sign * atan2(r.absoluteValue, l)
+                }
+                val w = vector(-cos(angle2), sin(angle2))
+                put(
+                    route,
+                    LineSegment(
+                        center + w * r1,
+                        other.center + w * r2
+                    )
                 )
-            )
+            }
+            else {
+                throw Exception("Circles should not be")
+            }
         }
     }
 }
\ No newline at end of file
diff --git a/kmath-trajectory/src/commonTest/kotlin/space/kscience/kmath/trajectory/TangentTest.kt b/kmath-trajectory/src/commonTest/kotlin/space/kscience/kmath/trajectory/TangentTest.kt
index 42442a55f..3e87b47de 100644
--- a/kmath-trajectory/src/commonTest/kotlin/space/kscience/kmath/trajectory/TangentTest.kt
+++ b/kmath-trajectory/src/commonTest/kotlin/space/kscience/kmath/trajectory/TangentTest.kt
@@ -13,6 +13,7 @@ import space.kscience.kmath.geometry.equalsLine
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertTrue
+import kotlin.test.assertFailsWith
 
 class TangentTest {
     @Test
@@ -53,4 +54,28 @@ class TangentTest {
             assertTrue(segments[i].equalsLine(Euclidean2DSpace, tangentMapValues[i]))
         }
     }
+
+    @Test
+    fun nonExistingTangents() {
+        assertFailsWith<Exception> {
+            val c1 = Circle2D(vector(0.0, 0.0), 10.0)
+            val c2 = Circle2D(vector(0.0, 0.0), 1.0)
+            val segments = c1.tangentsToCircle(c2)
+        }
+        assertFailsWith<Exception> {
+            val c1 = Circle2D(vector(0.0, 0.0), 1.0)
+            val c2 = Circle2D(vector(0.0, 0.0), 10.0)
+            val segments = c1.tangentsToCircle(c2)
+        }
+        assertFailsWith<Exception> {
+            val c1 = Circle2D(vector(0.0, 0.0), 1.0)
+            val c2 = Circle2D(vector(2.0, 0.0), 1.0)
+            val segments = c1.tangentsToCircle(c2)
+        }
+        assertFailsWith<Exception> {
+            val c1 = Circle2D(vector(0.0, 0.0), 1.0)
+            val c2 = Circle2D(vector(0.5, 0.0), 1.0)
+            val segments = c1.tangentsToCircle(c2)
+        }
+    }
 }
\ No newline at end of file