tiny glfw prog

#include "stdio.h"
#include "string.h"

#include "glfw3.h"

#include "windows.h"

GLFWwindow* window = 0;

//frame counter vars
static unsigned int frame_count = 0;
static int current_fps = 0;
static double start_time = 0;

//vars for sleep calculations
static double target_frame_rate = 60;
static double frame_start = 0;

void calc_frame_rate()
{
    frame_count++;

    double elapsed = (glfwGetTime() - start_time);

    if (elapsed > 1)
    {
        current_fps = frame_count;
        start_time = glfwGetTime();
        frame_count = 0;
    }
}

void calc_sleep()
{
    double wait_time = 1.0 / (target_frame_rate);
    double curr_frame_time = glfwGetTime() - frame_start;
    double dur = 1000.0 * ( wait_time - curr_frame_time ) + 0.5;
    int durDW = (int)dur;
    if( durDW > 0 ) // ensures that we don't have a dur > 0.0 which converts to a durDW of 0.
    {
        Sleep( (DWORD) durDW  );
    }

    double frame_end = glfwGetTime();
    frame_start = frame_end;
}

int main(int argc, char* args[])
{
    glfwInit();
    window = glfwCreateWindow(640, 480, "", 0, 0);
    glfwMakeContextCurrent(window);
    glfwSwapInterval(0);
    glfwWindowHint(GLFW_RESIZABLE, 0);

    while(!glfwWindowShouldClose(window))
    {
        calc_frame_rate();

            char str[64] = {};
            sprintf(str, "%i", current_fps);
        glfwSetWindowTitle(window, str);

            glfwSwapBuffers(window);

        calc_sleep();
        glfwPollEvents();
    }

    return 0;
}