//drawPolygon
//showing the function of drawPolygon()
//A square turns round counter clockwise.
// T. Kosaka CS TNCT 2001

#include <stdio.h>
#include <math.h>
#include <GL/glut.h>

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

void setColor(float red,float green,float blue)
{
    glColor3f(red, green, blue);    
}

void drawDot(int x,int y)
{
    glBegin(GL_POINTS);
        glVertex2i(x, y);
    glEnd();
}

void drawLine(int x1, int y1, int x2, int y2)
{
    int dx,dy,dx2,dy2,x,y,inc,c,tmp;
    dx=0<=x2-x1?x2-x1:x1-x2;
    dy=0<=y2-y1?y2-y1:y1-y2;
    dx2=dx<<1;dy2=dy<<1;
    if (dy<=dx) {
        if (x2<x1) {
            tmp=x1;x1=x2;x2=tmp;
            tmp=y1;y1=y2;y2=tmp;
        }
        inc=0<=y2-y1?1:-1;
        x=x1;y=y1;c=-dx;
        drawDot(x,y);
        for (x++;x<=x2;x++) {
            c+=dy2;
            if (0<=c) {c-=dx2;y+=inc;}
            drawDot(x,y);
        }
    } else {
        if (y2<y1) {
            tmp=x1;x1=x2;x2=tmp;
            tmp=y1;y1=y2;y2=tmp;
        }
        inc=0<=x2-x1?1:-1;
        x=x1;y=y1;c=-dy;
        drawDot(x,y);
        for (y++;y<=y2;y++) {
            c+=dx2;
            if (0<=c) {c-=dy2;x+=inc;}
            drawDot(x,y);
        }
    }
}

void drawLine(point_t p1,point_t p2)
{
    drawLine((int)p1.x,(int)p1.y,(int)p2.x,(int)p2.y);
}

//n: number of points
void drawPolyline(point_t pnt[],int n)
{
    int i;
    for (i=0;i<n-1;i++) {
        drawLine(pnt[i],pnt[i+1]);
    }
}

//n: number of vertices
void drawPolygon(point_t pnt[],int n)
{
    int i;
    for (i=0;i<n-1;i++) {
        drawLine(pnt[i],pnt[i+1]);
    }
    drawLine(pnt[n-1],pnt[0]);
}

void userdraw(void);

void display(void)
{
    glClear( GL_COLOR_BUFFER_BIT);
    userdraw();
    glutSwapBuffers();
}

void userdraw(void)
{
    static int tick=0;
    point_t pnt[4];
    pnt[0].x=200.*cos(-(float)tick*.005)+320;
    pnt[0].y=200.*sin(-(float)tick*.005)+240;
    pnt[1].x=200.*cos(-(float)tick*.005+3.1415926535*.5)+320;
    pnt[1].y=200.*sin(-(float)tick*.005+3.1415926535*.5)+240;
    pnt[2].x=200.*cos(-(float)tick*.005+3.1415926535)+320;
    pnt[2].y=200.*sin(-(float)tick*.005+3.1415926535)+240;
    pnt[3].x=200.*cos(-(float)tick*.005+3.1415926535*1.5)+320;
    pnt[3].y=200.*sin(-(float)tick*.005+3.1415926535*1.5)+240;
    setColor(1,1,1);
    drawPolygon(pnt,4);
    tick++;
}

int main(int argc, char **argv)
{
    glutInit(&argc,argv);
    glutInitDisplayMode ( GLUT_DOUBLE | GLUT_RGB );
    glutInitWindowPosition(100,100);
    glutInitWindowSize(640,480);
    glutCreateWindow ("polygon");
    glClearColor(0.0, 0.0, 0.0, 0.0);
    gluOrtho2D(0., 640., 0.0, 480.0);
      // Define the dimensions of the Orthographic Viewing Volume
    glutIdleFunc(display);  // idle event call back
    glutDisplayFunc(display);
    glutMainLoop();
    return 0;
}