#include <stdlib.h>
#include <stdio.h>
#include <GL/glew.h>
#include <GLFW/glfw3.h>

// Open an OpenGL window
static GLFWwindow* window;

/****Step 1: define vertices in (x, y, z) form****/
static const GLfloat coordinates[] = {
    -1.0f, -1.0f, 0.0f,
    1.0f, -1.0f, 0.0f,
    0.0f, 1.0f, 0.0f
};
/************************/

/**Step 2: send this triangle vertices to OpenGL through a buffer**/
static GLuint vertexBuffer; // identify vertex buffer

static void Render(void){
    /************************/
    glEnableVertexAttribArray(0);
    glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
    glVertexAttribPointer(0, 3 /*size*/, GL_FLOAT /*type*/, GL_FALSE, 0, (void*)0);
    glDrawArrays(GL_TRIANGLES, 0, 3);
    //glDisableVertexAttribArray(0);
    /************************/

    // Swap front and back rendering buffers
    glfwSwapBuffers(window);
    //Poll for and process events
    glfwPollEvents();
}

static void error_callback(int error, const char *description)
{
    puts(description);
}

int main( void ) {
    /*Initializing steps here*/
    GLenum glewinit_res;

    glfwSetErrorCallback(error_callback);

    if (!glfwInit()) {
        fprintf(stderr, "Error on GLWF init\n");
        return EXIT_FAILURE;
    }

    // Create a windowed mode window and its OpenGL context
    window = glfwCreateWindow(700, 500, "Hello World", NULL, NULL);
    if (window == NULL) {
        fprintf(stderr, "Error in glfwCreateWindow\n");
        return EXIT_FAILURE;
    }

    // Make the window's context current
    glfwMakeContextCurrent(window);

    glewinit_res = glewInit();
    if (glewinit_res != GLEW_OK) {
        fprintf(stderr, "Glew init error: %s\n",
                glewGetErrorString(glewinit_res));
        return EXIT_FAILURE;
    }

    /**Step 2: send this triangle vertices to OpenGL through a buffer**/
    glGenBuffers(1, &vertexBuffer); // generating 1 buffer, put resulting identifier in this buffer
    glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
    glBufferData(GL_ARRAY_BUFFER, sizeof(coordinates), coordinates, GL_STATIC_DRAW);
    /************************/

    // Main loop
    while( glfwWindowShouldClose(window) == 0) {
        // OpenGL rendering goes here...
        Render();
    }
    // Close window and terminate GLFW
    glfwDestroyWindow(window);
    glfwTerminate();

    // Exit program
    exit( EXIT_SUCCESS );
}