Remove typecasting in IsCollinear (#825)

* Remove typecasting in `IsCollinear`

* Update the C# and Delphi versions as well

* In the C++ version, multiply unsigned numbers (so that the result just wraps, and there is no undefined behavior)

* Add test case for `IsCollinear`

CPP/CMakeLists.txt

@@ -228,6 +228,7 @@ endif()
 
   set(ClipperTests_SRC
     Tests/TestExportHeaders.cpp
+    Tests/TestIsCollinear.cpp
     Tests/TestLines.cpp
     Tests/TestOffsets.cpp
     Tests/TestOffsetOrientation.cpp

CPP/Clipper2Lib/include/clipper2/clipper.core.h

@@ -663,10 +663,10 @@ namespace Clipper2Lib
   inline bool IsCollinear(const Point<T>& pt1,
     const Point<T>& sharedPt, const Point<T>& pt2) // #777
   {
-    const double a = static_cast<double>(sharedPt.x - pt1.x);
-    const double b = static_cast<double>(pt2.y - sharedPt.y);
-    const double c = static_cast<double>(sharedPt.y - pt1.y);
-    const double d = static_cast<double>(pt2.x - sharedPt.x);
+    const auto a = static_cast<uintmax_t>(sharedPt.x - pt1.x);
+    const auto b = static_cast<uintmax_t>(pt2.y - sharedPt.y);
+    const auto c = static_cast<uintmax_t>(sharedPt.y - pt1.y);
+    const auto d = static_cast<uintmax_t>(pt2.x - sharedPt.x);
     return a * b == c * d;
   }
 

CPP/Tests/TestIsCollinear.cpp

@@ -0,0 +1,13 @@
+#include <gtest/gtest.h>
+#include "clipper2/clipper.core.h"
+
+TEST(Clipper2Tests, TestIsCollinear) {
+  // a large integer not representable by double
+  const int64_t i = 9007199254740993;
+
+  const Clipper2Lib::Point64 pt1(0, 0);
+  const Clipper2Lib::Point64 sharedPt(i, i * 10);
+  const Clipper2Lib::Point64 pt2(i * 10, i * 100);
+
+  EXPECT_TRUE(IsCollinear(pt1, sharedPt, pt2));
+}

CSharp/Clipper2Lib/Clipper.Core.cs

@@ -604,9 +604,8 @@ internal static double CrossProduct(Point64 pt1, Point64 pt2, Point64 pt3)
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     internal static bool IsCollinear(Point64 pt1, Point64 pt2, Point64 pt3)
     {
-      // typecast to double to avoid potential int overflow
-      return (double) (pt2.X - pt1.X) * (double) (pt3.Y - pt2.Y) ==
-        (double) (pt2.Y - pt1.Y) * (double) (pt3.X - pt2.X);
+      return (pt2.X - pt1.X) * (pt3.Y - pt2.Y) ==
+             (pt2.Y - pt1.Y) * (pt3.X - pt2.X);
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]

Delphi/Clipper2Lib/Clipper.Core.pas

@@ -1866,7 +1866,7 @@ function IsPositive(const path: TPathD): Boolean;
 
 function IsCollinear(const pt1, pt2, pt3: TPoint64): Boolean;
 var
-  a,b,c,d: double; // avoids potential int overflow
+  a,b,c,d: Int64;
 begin
   a := (pt2.X - pt1.X);
   b := (pt2.Y - pt1.Y);