// compile using
// g++ -o pftest pftest.cpp  -std=c++11 -lSDL2 -lepoxy
//
#include <iostream>

#include <SDL2/SDL.h>
#include <epoxy/gl.h>

namespace {
GLAPIENTRY void debugHandler(GLenum source, GLenum type, GLuint id,
                             GLenum severity, GLsizei length,
                             GLchar const *message, void const *userParam) {
  std::cout << "GL message: " << message << '\n';
}

float const vertices[] = {-1.0, -1.0, -1.0, 1.0, 1.0, -1.0, 1.0, 1.0};

unsigned short const indices[] = {0, 1, 2, 1, 2, 3};

char const* const vsSource = R"(
#version 330
#extension GL_ARB_shading_language_420pack : require

layout(binding = 0) uniform Block {
    float t;
};
layout(location = 0) in vec2 inPos;

void main() {
    float s = sin(t);
    float c = cos(t);
    gl_Position = vec4(inPos.x * c + inPos.y * s, inPos.y * c - inPos.x * s, 0, 2);
}
)";

char const* const fsSource = R"(
#version 330
#extension GL_ARB_shading_language_420pack : require
    
out vec4 color;

void main() {
    color = vec4(1.0);
}
)";

GLuint createShader(GLenum type, char const* source) {
    auto shader = glCreateShader(type);
    glShaderSource(shader, 1, &source, 0);
    glCompileShader(shader);
    return shader;
}

}

int main() {
  SDL_Init(SDL_INIT_VIDEO | SDL_INIT_EVENTS);

  SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
  SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
  SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
  SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, SDL_GL_CONTEXT_DEBUG_FLAG);
  SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);


  SDL_Window *window =
      SDL_CreateWindow("pageflip", 0, 0, 1920, 1080, SDL_WINDOW_OPENGL);
  SDL_GLContext context = SDL_GL_CreateContext(window);

  glDebugMessageCallbackARB(debugHandler, nullptr);

  glClearColor(0.f, 0.f, 0.f, 0.f);

  auto vs = createShader(GL_VERTEX_SHADER, vsSource);
  auto fs = createShader(GL_FRAGMENT_SHADER, fsSource);
  auto prog = glCreateProgram();
  glAttachShader(prog, vs);
  glAttachShader(prog, fs);
  glLinkProgram(prog);
  glUseProgram(prog);
  
  GLuint vao;
  glGenVertexArrays(1, &vao);
  glBindVertexArray(vao);
  
  GLuint vbo, ibo;
  glGenBuffers(1, &vbo);
  glBindBuffer(GL_ARRAY_BUFFER, vbo);
  glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
  glVertexAttribPointer(0, 2, GL_FLOAT, false, 8, nullptr);
  glEnableVertexAttribArray(0);
  
  glGenBuffers(1, &ibo);
  glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
  glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);
  
  bool shouldExit = false;
  
  float t = 0;
  for (;;) {
    SDL_Event event;
    while (SDL_PollEvent(&event)) {
      if (event.type == SDL_QUIT ||
          (event.type == SDL_KEYDOWN && event.key.keysym.sym == SDLK_ESCAPE))
        shouldExit = true;
      if(event.type == SDL_KEYDOWN && event.key.keysym.sym == SDLK_SPACE)
          t += 1.0;
    }
    if (shouldExit)
      break;

    glClear(GL_COLOR_BUFFER_BIT);
    
    GLuint ubo;
    glGenBuffers(1, &ubo);
    glBindBuffer(GL_UNIFORM_BUFFER, ubo);
    glBufferData(GL_UNIFORM_BUFFER, sizeof(t), &t, GL_STATIC_DRAW);
    
    glBindBufferBase(GL_UNIFORM_BUFFER, 0, ubo);

    glDrawElementsInstanced(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, nullptr, 1000);

    SDL_GL_SwapWindow(window);
  }

  // no cleanup: not needed
}