// Created by Igor Markov, March 2001
// This program illustrates the direction-test
// (left-of aka counter-clock-wise)

#include <iostream>

struct Point { double x, y; };

bool counterClockWise(Point p0, Point p1, Point p)
// p is the most recent point, p1 is the next, p0 is the earliest point
// This tests tell whether p is to the left of the directed segment p0->p1
//  equivalent: vector (p-p0) turns counter-clockwise from vector (p1-p0)
// (the case of all three points being on the same line is not tested for).
//
// This can be tested by computing the vector-product of the two vectors:
//   if it points "up", then the answer is yes, otherwise the answer is no.
// Technically, we just need to compute the magnitude of the vector-product
//    (p-p0) x (p1-p0) which is the determinant of this matrix
//    ( p1.x-p0.x  p1.y-p0.y )
//    ( p.x-p0.x   p.y-p0.y  )
//  then we compare the result to zero
{ return   (p1.x-p0.x)*(p.y-p0.y)-(p.x-p0.x)*(p1.y-p0.y) < 0; }

int main()
{
  Point ar[4]={ {3,0}, {4,4}, {3,3}, {-1,4} };
  cout <<  clockWise( ar[0], ar[1], ar[2] ) << endl;
  cout <<  clockWise( ar[1], ar[2], ar[3] ) << endl;
  return 0;
}