#include <iostream>

struct Vertex {
	float X, Y;
};

void testFirstPolygon();
void testSecondPolygon();
void calculateAndPrintArea(const Vertex *polygon, std::size_t vertexCount);

int main() {
	testFirstPolygon();
	testSecondPolygon();
}

void testFirstPolygon() {
	Vertex vertices[] = {
	  {-10, 0}, {10, 0}, {10, 10}, {-10, 10}, {-10, 0}, {10, 0}
	};

	std::size_t vertexCount = 4;

	calculateAndPrintArea(vertices, vertexCount);
}

void testSecondPolygon() {
	Vertex vertices[] = {
	  {90, 0}, {110, 0}, {110, 10}, {90, 10}, {90, 0}, {110, 0}
	};

	std::size_t vertexCount = 4;

	calculateAndPrintArea(vertices, vertexCount);
}

void calculateAndPrintArea(const Vertex *polygon, std::size_t vertexCount) {
	float area = 0;

	for(std::size_t index = 0; index < vertexCount; index += 2) {
		area += polygon[index + 1].X * (polygon[index + 2].Y - polygon[index].Y);
		area += polygon[index + 1].Y * (polygon[index].X - polygon[index + 2].X);
	}
	area /= 2;

	std::cout << "Polygon Area (method1): " << area << std::endl;

	// ------------------------------------------------------------------------

	area = 0;

	for(std::size_t index = 1; index < vertexCount; ++index) {
		area += polygon[index].X * (polygon[index + 1].Y - polygon[index - 1].Y);
	}
	area /= 2;

	std::cout << "Polygon Area (method2): " << area << std::endl;
}