// This code example is created for educational purpose
// by Thorsten Thormaehlen (contact: www.thormae.de).
// It is distributed without any warranty.

#include <QApplication>
#include <QOpenGLWidget>
#include <QOpenGLFunctions_2_0>
#include <QKeyEvent>
#include <math.h>

class Renderer : protected QOpenGLFunctions_2_0{

public:
  float t; //time
  const float d0; // initial distance

public:
  Renderer() : t(1.0), d0(3.0), width(0), height(0) {}

public:
  void display() {
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective (dollyZoomFovy(), (float)width/(float)height, 0.1, 50.0);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    // translate camera by 3 units
    glTranslatef(0.0f, 0.0f, -t*d0);

    // draw a cube in the local coordinate system
    drawCube();
    // draw random lines
    drawRandomLines();
  }

  void init() {
    initializeOpenGLFunctions();
    glEnable(GL_DEPTH_TEST);
    // create random values between -1.0 and 1.0
    for(unsigned r=0; r < 1000; r++) {
      int rval = rand();
      randVals.push_back(2.0f*float(rval)/float(RAND_MAX)-1.0f);
    }
  }

  void resize(int w, int h) {
    // ignore this for now
    glViewport(0, 0, w, h); 
    width = w;
    height = h;
  }

  void dispose() {

  }

  float dollyZoomFovy() {
    float fovyInit = 60.0f; // initial field of view
    float theta = fovyInit / 180.0f * M_PI; // degree to rad
    float f = 1.0f / tan(theta/2.0f);
    float fNew = f * (d0*t-1) / (d0-1);
    float thetaNew = atan(1.0f / fNew) * 2.0f;
    float val = 180.0f * thetaNew / M_PI; //rad to degree
    return val;
  }

  // implemention of gluPerspective to avoid GLU header
  void gluPerspective(double fovyInDegrees, double aspectRatio, double znear, double zfar) {
    double ymax = znear * tanf(fovyInDegrees * M_PI / 360.0);
    double xmax = ymax * aspectRatio;
    glFrustum(-xmax, xmax, -ymax, ymax, znear, zfar);
  }

private:
  int width;
  int height;
  std::vector<float> randVals;

private:
  void drawCube() {

    glColor3f(1.0f, 1.0f, 1.0f);
    glLineWidth(3.0f);
    glBegin(GL_LINE_LOOP);
    glVertex3f(-1.0f, 1.0f, 1.0f);
    glVertex3f( 1.0f, 1.0f, 1.0f);
    glVertex3f( 1.0f,-1.0f, 1.0f);
    glVertex3f(-1.0f,-1.0f, 1.0f);
    glEnd();
    glBegin(GL_LINE_LOOP);
    glVertex3f(-1.0f, 1.0f,-1.0f);
    glVertex3f( 1.0f, 1.0f,-1.0f);
    glVertex3f( 1.0f,-1.0f,-1.0f);
    glVertex3f(-1.0f,-1.0f,-1.0f);
    glEnd();

    glBegin(GL_LINE_LOOP);
    glVertex3f( 1.0f, 1.0f,-1.0f);
    glVertex3f( 1.0f, 1.0f, 1.0f);
    glVertex3f( 1.0f,-1.0f, 1.0f);
    glVertex3f( 1.0f,-1.0f,-1.0f);
    glEnd();

    glBegin(GL_LINE_LOOP);
    glVertex3f(-1.0f, 1.0f,-1.0f);
    glVertex3f(-1.0f, 1.0f, 1.0f);
    glVertex3f(-1.0f,-1.0f, 1.0f);
    glVertex3f(-1.0f,-1.0f,-1.0f);
    glEnd();
    glLineWidth(1.0);
  }

  void drawRandomLines() {
    if(randVals.size() % 5) return;
    unsigned i = 0;
    while(i < randVals.size()) {
      glColor3f(fabs(randVals[i++]), fabs(randVals[i++]), fabs(randVals[i++]));
      float x = randVals[i++];
      float y = randVals[i++];
      glBegin(GL_LINES);
      glVertex3f(x, y, -1.0f);
      glVertex3f(x, y,  1.0f);
      glEnd();
    }
  }
};

class MyWidget : public QOpenGLWidget {

private:
  Renderer *renderer;

public:
  MyWidget(QWidget *parent = NULL) : QOpenGLWidget(parent) {
    this->setWindowTitle("Use the 1 key to perform a dolly zoom");
    this->resize(320, 320);
    renderer = new Renderer();
  }

  ~MyWidget() {
    makeCurrent();
    renderer->dispose();
    doneCurrent();
    delete renderer;
  }

protected:
  void initializeGL() { renderer->init(); }
  void resizeGL(int w, int h){ renderer->resize(w, h); }
  void paintGL() { renderer->display(); }
  void keyPressEvent(QKeyEvent* event){
    bool redraw = false;
    float offset = 0.02f;
    if(event->modifiers() & Qt::AltModifier) offset = -offset;

    switch(event->key()) {
    case '1':
      renderer->t += offset;
      if(renderer->t <  1.0f) renderer->t = 4.0f;
      if(renderer->t >  4.0f) renderer->t = 1.0f;
      redraw = true;
      break;
    case '0':
      renderer->t = 1.0f;
      redraw = true;
      break;
    }
    if(redraw) {
      float distance = renderer->t*renderer->d0;
      float fovy = renderer->dollyZoomFovy();
      QString title = QString("Distance: %1 FieldofView: %2").arg(int(distance*10.0f)/10.0f, -4).arg(int(fovy*10.0f)/10.0f);
      this->setWindowTitle(title);
      this->update();
    }
  }
};

int main (int argc, char* argv[]) {
  // create a QApplication object that handles initialization,
  // finalization, and the main event loop
  QApplication appl(argc, argv);
  MyWidget widget;  // create a widget
  widget.show(); //show the widget and its children
  return appl.exec(); // execute the application
}