Rotate a line

#include <windows.h>
#include <gl/Gl.h>
#include <gl/glut.h>
#include <cmath>
 
int screenheight = 600;
int screenwidth = 800;
bool flag = true;

double angle = 30;                //The angle for the rotation (in degrees)

typedef struct{
    float x;
    float y;
}Point2D;

Point2D p1,p2;

void DrawLineSegment(Point2D pt1, Point2D pt2){
       glPointSize(1.0);
       glBegin(GL_LINES);
       glVertex2i(pt1.x, pt1.y);
       glVertex2i(pt2.x, pt2.y);
       glEnd();

       glPointSize(6.0);
       glBegin(GL_POINTS);
       glVertex2i(pt2.x, pt2.y);
       glEnd();
       glFlush();

}

Point2D translate(Point2D p, float tx, float ty){
       p.x =p.x; //.....wite the equations for translation 
       p.y = p.y; //.....wite the equations for translation
       return p;
}

Point2D  rotate(Point2D p, float ang){
    ang = ang * 3.14 / 180.0;                                 //angle in radians
    Point2D ptemp;

    ptemp.x = p.x * cos(ang) - p.y * sin(ang);
    ptemp.y = p.x * sin(ang) + p.y * cos(ang);

 return ptemp;
}

void myMouse(int button, int state, int x, int y) {
       if(state == GLUT_DOWN) {
              if(button == GLUT_LEFT_BUTTON) {
                     glClear(GL_COLOR_BUFFER_BIT);
                     Point2D p1;
                     p1.x = 0;
                     p1.y = 0;

                     p2.x = x;
                     p2.y = screenheight - y;

                     DrawLineSegment(p1, p2);
              }
              else if (button == GLUT_RIGHT_BUTTON) {
                     Point2D Rotated_p2 = rotate(p2,angle);
                     DrawLineSegment(p1,Rotated_p2);
              }
       }
}

void myDisplay(){
       glClearColor(1.0f, 1.0f, 1.0f, 0.0f);
       glClear(GL_COLOR_BUFFER_BIT);
       glColor3f(0.0f, 0.0f, 0.0f);
}
 
int main( int argc, char ** argv ) {
       glutInit( &argc, argv );
       glutInitWindowPosition( 0, 0 );
       glutInitWindowSize( 800, 600 );
       glutCreateWindow( "My Drawing Screen" );
    glMatrixMode( GL_PROJECTION );
       glLoadIdentity();
       gluOrtho2D( 0, 800, 0, 600 );
       glViewport(0, 0, 800, 600);

       glutMouseFunc( myMouse );
       glutDisplayFunc( myDisplay );

       glutMainLoop();
       return( 0 );
}