static bool BoundingBoxInitialized = false;

static twTriple BBCenter;       // the center point of the bounding box.
static twTriple BBMin;          // the min corner
static twTriple BBMax;          // the max corner
static GLfloat  OuterRadius;    // radius of the bounding sphere
static GLfloat  InnerRadius;    // radius of the inner sphere

GLfloat min(GLfloat x, GLfloat y) {
    return x < y ? x : y;
}

/* computes near and far using bounding box coordinates */
void twBoundingBox(GLfloat xmin, GLfloat xmax,
                   GLfloat ymin, GLfloat ymax,
                   GLfloat zmin, GLfloat zmax) {  
    if( xmin > xmax ) printf("xmax (%f) must be greater than or equal to xmin (%f)\n",xmax,xmin);
    if( ymin > ymax ) printf("ymax (%f) must be greater than or equal to ymin (%f)\n",ymax,ymin);
    if( zmin > zmax ) printf("zmax (%f) must be greater than or equal to zmin (%f)\n",zmax,zmin);
    // store the box
    twTripleInit(BBMin,xmin,ymin,zmin);
    twTripleInit(BBMax,xmax,ymax,zmax);
    twTripleInit(BBCenter,(xmax+xmin)*0.5,(ymax+ymin)*0.5,(zmax+zmin)*0.5);
    OuterRadius = twPointDistance(BBMin,BBMax)*0.5;
    InnerRadius = min(min(xmax-xmin,ymax-ymin),zmax-zmin)*0.5;
    BoundingBoxInitialized = true;
    if(TW_BOUNDING_BOX & twMessageKinds) {
        twTriplePrint("BB Center",BBCenter);
        twTriplePrint("BB Min",BBMin);
        twTriplePrint("BB Max",BBMax);
        printf("Outer radius = %f, inner = %f\n",OuterRadius,InnerRadius);
    }
}

/* Takes the student's array va with n points and computes the bounding
   box, then calls twBoundingBox. */
void twVertexArray(twTriple va[], int n) {
  int i;
  GLfloat minx = va[0][0];
  GLfloat maxx = va[0][0];
  GLfloat miny = va[0][1];
  GLfloat maxy = va[0][1];
  GLfloat minz = va[0][2];
  GLfloat maxz = va[0][2];
 
  for(i=1; i<n; i++) { 
    if (va[i][0] < minx) minx = va[i][0];
    if (va[i][0] > maxx) maxx = va[i][0];
    if (va[i][1] < miny) miny = va[i][1];
    if (va[i][1] > maxy) maxy = va[i][1];
    if (va[i][2] < minz) minz = va[i][2];
    if (va[i][2] > maxz) maxz = va[i][2];
  }
  twBoundingBox(minx, maxx, miny, maxy, minz, maxz);
}


void twDrawBoundingBox() {
    twError();
    glPushAttrib(GL_ALL_ATTRIB_BITS);
    glDisable(GL_LIGHTING);
    glLineWidth(1);
    glBegin(GL_LINES);
    glColor3f(1,0,0);           // red for lines parallel to x
    glVertex3f(BBMin[0],BBMin[1],BBMin[2]);
    glVertex3f(BBMax[0],BBMin[1],BBMin[2]);

    glVertex3f(BBMin[0],BBMax[1],BBMin[2]);
    glVertex3f(BBMax[0],BBMax[1],BBMin[2]);

    glVertex3f(BBMin[0],BBMin[1],BBMax[2]);
    glVertex3f(BBMax[0],BBMin[1],BBMax[2]);

    glVertex3f(BBMin[0],BBMax[1],BBMax[2]);
    glVertex3f(BBMax[0],BBMax[1],BBMax[2]);

    glColor3f(0,1,0);           // green for lines parallel to y
    glVertex3f(BBMin[0],BBMin[1],BBMin[2]);
    glVertex3f(BBMin[0],BBMax[1],BBMin[2]);

    glVertex3f(BBMax[0],BBMin[1],BBMin[2]);
    glVertex3f(BBMax[0],BBMax[1],BBMin[2]);

    glVertex3f(BBMin[0],BBMin[1],BBMax[2]);
    glVertex3f(BBMin[0],BBMax[1],BBMax[2]);

    glVertex3f(BBMax[0],BBMin[1],BBMax[2]);
    glVertex3f(BBMax[0],BBMax[1],BBMax[2]);

    glColor3f(0,0,1);           // blue for lines parallel to z
    glVertex3f(BBMin[0],BBMin[1],BBMin[2]);
    glVertex3f(BBMin[0],BBMin[1],BBMax[2]);

    glVertex3f(BBMax[0],BBMin[1],BBMin[2]);
    glVertex3f(BBMax[0],BBMin[1],BBMax[2]);

    glVertex3f(BBMin[0],BBMax[1],BBMin[2]);
    glVertex3f(BBMin[0],BBMax[1],BBMax[2]);

    glVertex3f(BBMax[0],BBMax[1],BBMin[2]);
    glVertex3f(BBMax[0],BBMax[1],BBMax[2]);

    glEnd();
    glPopAttrib();
    twError();
}