/**
 * From the OpenGL Programming wikibook: http://en.wikibooks.org/wiki/OpenGL_Programming
 * This file is in the public domain.
 * Contributors: Sylvain Beucler
 */
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
/* Use glew.h instead of gl.h to get all the GL prototypes declared */
#include <GL/glew.h>
/* Using the GLUT library for the base windowing setup */
#include <GL/freeglut.h>
/* GLM */
// #define GLM_MESSAGES
#define GLM_FORCE_RADIANS

// Vertex shader
const GLchar* vertexShaderSrc = R"glsl(
    #version 150 core

    in float inValue;
    out float outValue;

    void main()
    {
        outValue = sqrt(inValue);
    }
)glsl";
int screen_width=800, screen_height=600;

int main(int argc, char* argv[]) {
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_RGBA|GLUT_ALPHA|GLUT_DOUBLE|GLUT_DEPTH);
  glutInitWindowSize(screen_width, screen_height);
  glutCreateWindow("My Rotating Cube");

  GLenum glew_status = glewInit();
  if (glew_status != GLEW_OK) {
    fprintf(stderr, "Error: %s\n", glewGetErrorString(glew_status));
    return 1;
  }

  if (!GLEW_VERSION_2_0) {
    fprintf(stderr, "Error: your graphic card does not support OpenGL 2.0\n");
    return 1;
  }

    // Initialize GLEW
    glewExperimental = GL_TRUE;
    glewInit();

    // Compile shader
    GLuint shader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(shader, 1, &vertexShaderSrc, 0);
    glCompileShader(shader);

    // Create program and specify transform feedback variables
    GLuint program = glCreateProgram();
    glAttachShader(program, shader);

    const GLchar* feedbackVaryings[] = { "outValue" };
    glTransformFeedbackVaryings(program, 1, feedbackVaryings, GL_INTERLEAVED_ATTRIBS);

    glLinkProgram(program);
    glUseProgram(program);

    // Create VAO
    GLuint vao;
    glGenVertexArrays(1, &vao);
    glBindVertexArray(vao);

    // Create input VBO and vertex format
    GLfloat data[] = { 1.0f, 2.0f, 3.0f, 4.0f, 5.0f };
    
    GLuint vbo;
    glGenBuffers(1, &vbo);
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glBufferData(GL_ARRAY_BUFFER, sizeof(data), data, GL_STATIC_DRAW);

    GLint inputAttrib = glGetAttribLocation(program, "inValue");
    glEnableVertexAttribArray(inputAttrib);
    glVertexAttribPointer(inputAttrib, 1, GL_FLOAT, GL_FALSE, 0, 0);

    // Create transform feedback buffer
    GLuint tbo;
    glGenBuffers(1, &tbo);
    glBindBuffer(GL_ARRAY_BUFFER, tbo);
    glBufferData(GL_ARRAY_BUFFER, sizeof(data), 0, GL_STATIC_READ);

    // Perform feedback transform
    glEnable(GL_RASTERIZER_DISCARD);
    glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, tbo);
    glBeginTransformFeedback(GL_POINTS);
        glDrawArrays(GL_POINTS, 0, 5);
    glEndTransformFeedback();
    glDisable(GL_RASTERIZER_DISCARD);
    glFlush();

    // Fetch and print results
    GLfloat feedback[5];
    glGetBufferSubData(GL_TRANSFORM_FEEDBACK_BUFFER, 0, sizeof(feedback), feedback);

    printf("%f %f %f %f %f\n", feedback[0], feedback[1], feedback[2], feedback[3], feedback[4]);

    // Now do it again!

    glBindBuffer(GL_ARRAY_BUFFER, tbo);
//    glBufferData(GL_ARRAY_BUFFER, sizeof(data), feedback, GL_STATIC_DRAW);  This makes the copy - don't do this!
    glVertexAttribPointer(inputAttrib, 1, GL_FLOAT, GL_FALSE, 0, 0);

    // Perform feedback transform
    glEnable(GL_RASTERIZER_DISCARD);
    glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, vbo);
    glBeginTransformFeedback(GL_POINTS);
        glDrawArrays(GL_POINTS, 0, 5);
    glEndTransformFeedback();
    glDisable(GL_RASTERIZER_DISCARD);
    glFlush();

    // Fetch and print results
    glGetBufferSubData(GL_TRANSFORM_FEEDBACK_BUFFER, 0, sizeof(feedback), feedback);
    printf("%f %f %f %f %f\n", feedback[0], feedback[1], feedback[2], feedback[3], feedback[4]);

    glDeleteProgram(program);
    glDeleteShader(shader);
    glDeleteBuffers(1, &tbo);
    glDeleteBuffers(1, &vbo);

    glDeleteVertexArrays(1, &vao);

  return 0;
}
// Tutorial from here:  
// https://open.gl/feedback
// Compile with:  g++ -std=c++14 c8.cpp -lGL -lGLEW -lsfml-system -lsfml-graphics -lsfml-window

// Link statically with GLEW
//#define GLEW_STATIC