//Initialisation and opengl version setup
bool initOpenGL() {
    // init glfw and glew
    // setup callback functions for handling keyboard and mouse
    // create window and setup viewport

    if(all_ok)
    return true;
    else
    return false;
}

int main(){
    if (!initOpenGL())
        {
            // An error occured
            std::cerr << "initialization failed" << std::endl;
            return -1;
        }

    // load and compile shader logic

    ShaderProgram myShader;
    myShader.loadShaders("shaders/shader_name.vert", "shaders/shader_name.frag");

    //Load Mesh
    Mesh myMesh;

    /*
    For simple demo here, I am loading wavefront file but in the problem the vertices and the surfaces changes continuously, that means the vbos and vaos would be changing in the game loop .
    */

    myMesh.loadOBJ("models/some_mesh.obj");

    //load texture for texturing the wavefront object (again for demonstration purpose)

    Texture2D myTexture;

    texture.loadTexture("textures/some_image.png", true);

    /* Setup light pos and other stuff like model  world position */
    [..]

    while (!glfwWindowShouldClose(gWindow)){
        //Setup model, view, projection matrices
        [..]

        //setup uniforms in shaders like:
        myShader.use();
        myShader.setUniform("model", glm::mat4(1.0));
        myShader.setUniform("view", view);
        myShader.setUniform("projection", projection);
        myShader.setUniform("viewPos", viewPos);
        [...]

        // DRAW mesh
        myTexture.bind(0);
        myMesh.draw(); // Renders the OBJ mesh
        myTexture.unbind(0);
    }

}

bool Mesh::loadOBJ(const std::string &filename){
    //parse obj file

    // iterate over each face to construct one flat array *mVertices* containing the position, normal and texCoord info like:
    mVertices = [[vec3(x,y,z), vec3(nx,ny,nx), vec2(tx,ty)], [...], [...], ...]

    /* mVertices is an array of 'Vertex'. where :

        struct Vertex{
        glm::vec3 position;
        glm::vec3 normal;
        glm::vec2 texCoords;
        };

    */
    //initialize VBOs and VAOs
    glGenVertexArrays(1, &mVAO);
    glGenBuffers(1, &mVBO);

    glBindVertexArray(mVAO);
    glBindBuffer(GL_ARRAY_BUFFER, mVBO);
    glBufferData(GL_ARRAY_BUFFER, mVertices.size() * sizeof(Vertex), &mVertices[0], GL_STATIC_DRAW);

    // Vertex Positions
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid *)0);
    glEnableVertexAttribArray(0);

    // Normals attribute
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid *)(3 * sizeof(GLfloat)));
    glEnableVertexAttribArray(1);

    // Vertex Texture Coords
    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid *)(6 * sizeof(GLfloat)));
    glEnableVertexAttribArray(2);

    // unbind to make sure other code does not change it somewhere else
    glBindVertexArray(0);

}

glBindVertexArray(mVAO);
glDrawArrays(GL_TRIANGLES, 0, mVertices.size());
glBindVertexArray(0);

bool Texture2D::loadTexture(const string &fileName, bool generateMipMaps)
{
    int width, height, components;

    // Use stbi image library to load our image
    unsigned char *imageData = stbi_load(fileName.c_str(), &width, &height, &components, STBI_rgb_alpha);

    if (imageData == NULL)
    {
        std::cerr << "Error loading texture '" << fileName << "'" << std::endl;
        return false;
    }

    // Invert image
    int widthInBytes = width * 4;
    unsigned char *top = NULL;
    unsigned char *bottom = NULL;
    unsigned char temp = 0;
    int halfHeight = height / 2;
    for (int row = 0; row < halfHeight; row++)
    {
        top = imageData + row * widthInBytes;
        bottom = imageData + (height - row - 1) * widthInBytes;
        for (int col = 0; col < widthInBytes; col++)
        {
            temp = *top;
            *top = *bottom;
            *bottom = temp;
            top++;
            bottom++;
        }
    }

    glGenTextures(1, &mTexture);
    glBindTexture(GL_TEXTURE_2D, mTexture);


    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageData);

    if (generateMipMaps)
        glGenerateMipmap(GL_TEXTURE_2D);

    stbi_image_free(imageData);
    glBindTexture(GL_TEXTURE_2D, 0); // unbind texture when done so we don't accidentally mess up our mTexture

    return true;
}

glActiveTexture(GL_TEXTURE0 + texUnit);
glBindTexture(GL_TEXTURE_2D, mTexture);