//
// Miminal C++ SDL2 OpenGL 4.5 example
//
// by Poddubny Michael
//
// Download OpenGL header:
//    wget https://www.khronos.org/registry/OpenGL/api/GL/glcorearb.h
//
// Compile:
//    g++ -std=c++11 sdl2_opengl_4_5_triangle.cpp -I. -lSDL2 -o sdl2_opengl_4_5_triangle

#include <SDL2/SDL.h>
#include <iostream>
#include <glcorearb.h>

extern int main(int argc, char *argv[]) {
    using namespace std;

    // initialize SDL2
    if (SDL_Init(SDL_INIT_VIDEO) < 0) {
        cerr << "Error: " << SDL_GetError() << '\n';
        return EXIT_FAILURE;
    }

    atexit(SDL_Quit);

    // configs
    constexpr int gl_major_version = 4;
    constexpr int gl_minor_version = 5;
    constexpr bool gl_debug = true;
    constexpr const char * window_title = "OpenGL 4.5 + SDL2 Basic window";
    constexpr int window_width = 1024;
    constexpr int window_height = 768;
    constexpr bool fullscreen = false;
    constexpr bool vsync = false;

    // window without borders and show immediately
    const auto window_flags = SDL_WINDOW_BORDERLESS | SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN | (fullscreen ? SDL_WINDOW_FULLSCREEN : 0);

    // opengl forawrd debug context flags
    const auto context_flags = SDL_GL_CONTEXT_FORWARD_COMPATIBLE_FLAG | (gl_debug ? SDL_GL_CONTEXT_DEBUG_FLAG : 0);

    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, gl_major_version);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, gl_minor_version);

    SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, context_flags);

    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);

    // create centered window
    auto window = SDL_CreateWindow(window_title, SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, window_width, window_height, window_flags);
    if (!window) {
        cerr << "Error: " << SDL_GetError() << '\n';
        return EXIT_FAILURE;
    }

    // create opengl context
    auto context = SDL_GL_CreateContext(window);
    if (!context) {
        cerr << "Error: " << SDL_GetError() << '\n';
        return EXIT_FAILURE;
    }

    if (vsync)
        SDL_GL_SetSwapInterval(1);

    // load opengl library
    if (SDL_GL_LoadLibrary(NULL) != 0) {
        cerr << "Error: " << SDL_GetError() << '\n';
        return EXIT_FAILURE;
    }

    // get functions
    PFNGLCLIPCONTROLPROC glClipControl = (PFNGLCLIPCONTROLPROC)SDL_GL_GetProcAddress("glClipControl");
    PFNGLVIEWPORTINDEXEDFPROC glViewportIndexedf = (PFNGLVIEWPORTINDEXEDFPROC)SDL_GL_GetProcAddress("glViewportIndexedf");
    PFNGLCLEARNAMEDFRAMEBUFFERFVPROC glClearNamedFramebufferfv = (PFNGLCLEARNAMEDFRAMEBUFFERFVPROC)SDL_GL_GetProcAddress("glClearNamedFramebufferfv");
    PFNGLCREATEBUFFERSPROC glCreateBuffers = (PFNGLCREATEBUFFERSPROC)SDL_GL_GetProcAddress("glCreateBuffers");
    PFNGLNAMEDBUFFERDATAPROC glNamedBufferData = (PFNGLNAMEDBUFFERDATAPROC)SDL_GL_GetProcAddress("glNamedBufferData");
    PFNGLCREATEVERTEXARRAYSPROC glCreateVertexArrays = (PFNGLCREATEVERTEXARRAYSPROC)SDL_GL_GetProcAddress("glCreateVertexArrays");
    PFNGLENABLEVERTEXARRAYATTRIBPROC glEnableVertexArrayAttrib = (PFNGLENABLEVERTEXARRAYATTRIBPROC)SDL_GL_GetProcAddress("glEnableVertexArrayAttrib");
    PFNGLVERTEXARRAYATTRIBFORMATPROC glVertexArrayAttribFormat = (PFNGLVERTEXARRAYATTRIBFORMATPROC)SDL_GL_GetProcAddress("glVertexArrayAttribFormat");
    PFNGLVERTEXARRAYVERTEXBUFFERPROC glVertexArrayVertexBuffer = (PFNGLVERTEXARRAYVERTEXBUFFERPROC)SDL_GL_GetProcAddress("glVertexArrayVertexBuffer");
    PFNGLBINDVERTEXARRAYPROC glBindVertexArray = (PFNGLBINDVERTEXARRAYPROC)SDL_GL_GetProcAddress("glBindVertexArray");
    PFNGLDRAWARRAYSPROC glDrawArrays = (PFNGLDRAWARRAYSPROC)SDL_GL_GetProcAddress("glDrawArrays");

    SDL_GL_UnloadLibrary();

    bool running = true;

    const float points[] = {  // vertices of triangle
        0.0f,  0.5f,  0.0f,
        0.5f, -0.5f,  0.0f,
        -0.5f, -0.5f,  0.0f
    };

    GLuint vbo;         // vertex buffer
    GLuint vao;         // vertex array

    // Create VBO
    glCreateBuffers(1, &vbo);
    // Allocate memory for data and send it
    glNamedBufferData(vbo, sizeof (points), points, GL_STATIC_DRAW);

    // Create VAO
    glCreateVertexArrays(1, &vao);

    // Enable vertex attribute (location = 0)
    glEnableVertexArrayAttrib(vao, 0);

    // Setup vertex array attributes
    glVertexArrayAttribFormat(vao, 0, 3, GL_FLOAT, GL_FALSE, 0);
    // Setup vertex buffer for this VAO
    glVertexArrayVertexBuffer(vao, 0, vbo, 0, sizeof(float) * 3);

    glBindVertexArray(vao);

    while (running) {
        SDL_Event ev;
        while (SDL_PollEvent(&ev)) {
            if (ev.key.keysym.sym == SDLK_ESCAPE)
                running = false; // Exit when esc
        }

        const float clear_color[4] = {0.4, 0.4, 0.4, 1};

        // Clip window
        glClipControl(GL_LOWER_LEFT, GL_NEGATIVE_ONE_TO_ONE);
        // Setup viewport 0
        glViewportIndexedf(0, 0, 0, (float)window_width , (float)window_height);
        // Clear color buffer of current framebuffer(0)
        glClearNamedFramebufferfv(0, GL_COLOR, 0, clear_color);

        // Draw triangle
        glDrawArrays(GL_TRIANGLES, 0, 3);

        SDL_GL_SwapWindow(window);
    }

    return 0;
}