ConvexHull4.cs

      public static IntVector2[] ConvexHull3(IntVector2 a, IntVector2 b, IntVector2 c) {
         var abc = Clockness(a, b, c);
         if (abc == Clk.Neither) {
            var (s, t) = FindCollinearBounds(a, b, c);
            return s == t ? new[] { s } : new[] { s, t };
         }
         if (abc == Clk.Clockwise) {
            return new[] { c, b, a };
         }
         return new[] { a, b, c };
      }

      public static (IntVector2, IntVector2) FindCollinearBounds(IntVector2 a, IntVector2 b, IntVector2 c) {
         var ab = a.To(b).SquaredNorm2();
         var ac = a.To(c).SquaredNorm2();
         var bc = b.To(c).SquaredNorm2();
         if (ab > ac) {
            return ab > bc ? (a, b) : (b, c);
         } else {
            return ac > bc ? (a, c) : (b, c);
         }
      }

      // See https://stackoverflow.com/questions/2122305/convex-hull-of-4-points
      public static IntVector2[] ConvexHull4(IntVector2 a, IntVector2 b, IntVector2 c, IntVector2 d) {
         var abc = Clockness(a, b, c);

         if (abc == Clk.Neither) {
            var (s, t) = FindCollinearBounds(a, b, c);
            return ConvexHull3(s, t, d);
         }

         // make abc ccw
         if (abc == Clk.Clockwise) (a, c) = (c, a);

         var abd = Clockness(a, b, d);
         var bcd = Clockness(b, c, d);
         var cad = Clockness(c, a, d);

         if (abd == Clk.Neither) {
            var (s, t) = FindCollinearBounds(a, b, d);
            return ConvexHull3(s, t, c);
         }

         if (bcd == Clk.Neither) {
            var (s, t) = FindCollinearBounds(b, c, d);
            return ConvexHull3(s, t, a);
         }

         if (cad == Clk.Neither) {
            var (s, t) = FindCollinearBounds(c, a, d);
            return ConvexHull3(s, t, b);
         }

         if (abd == Clk.CounterClockwise) {
            if (bcd == Clk.CounterClockwise && cad == Clk.CounterClockwise) return new[] { a, b, c };
            if (bcd == Clk.CounterClockwise && cad == Clk.Clockwise) return new[] { a, b, c, d };
            if (bcd == Clk.Clockwise && cad == Clk.CounterClockwise) return new[] { a, b, d, c };
            if (bcd == Clk.Clockwise && cad == Clk.Clockwise) return new[] { a, b, d };
            throw new InvalidStateException();
         } else {
            if (bcd == Clk.CounterClockwise && cad == Clk.CounterClockwise) return new[] { a, d, b, c };
            if (bcd == Clk.CounterClockwise && cad == Clk.Clockwise) return new[] { d, b, c };
            if (bcd == Clk.Clockwise && cad == Clk.CounterClockwise) return new[] { a, d, c };
            // 4th state impossible
            throw new InvalidStateException();
         }
      }