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/*
 *   NAME
 *     point_on_segment
 *
 *   PURPOSE
 *     Determine whether a point is on a line segment.
 *
 *   NOTES
 *     
 *
 *   HISTORY
 *      wiener - May 8, 1995: Created.
 */

#include <euclid/copyright.h>
#include <euclid/geometry.h>

/*
 * Return 1 if P is on the line segment formed by LP1 and LP2 and 0
 * otherwise.  Note that if P is on the line formed by LP1 and LP2 but
 * not on the line segment, this function returns 0.
 */
int EG_point_on_segment(Point_d *P, Point_d *LP1, Point_d *LP2)
{
  double dx1;
  double dy1;
  double dx2;
  double dy2;

  /* calculate vector components */
  dx1 = P->x - LP1->x;
  dy1 = P->y - LP1->y;

  dx2 = LP2->x - LP1->x;
  dy2 = LP2->y - LP1->y;

  /*
   * calculate cross product to determine whether the point P is even on
   * the line containing LP1 and LP2 (i.e., cross product should be zero
   */
  if (ABS(dx1 * dy2 - dy1 * dx2) < MACH_EPS)
    {
      /* check that the point is on the same side of LP1 as LP2 */
      if (dx1 * dx2 + dy1 * dy2 >= -MACH_EPS)
        {
          if (-MACH_EPS <= dx2 * dx2 + dy2 * dy2 - (dx1 * dx1 + dy1 * dy1))
            return(1);
          else
            return(0);
        }
      else
        return(0);
    }
  else
    return(0);
}