#include <GL/glew.h>
#include <GL/freeglut.h>
#include <GL/gl.h>
#include <stdio.h>
#include <string.h>

typedef struct
{
  float x, y;
  GLubyte r, g, b, a;
} Vert;

static const GLchar
vert_shader_source[] =
  "attribute vec4 other_color;\n"
  "attribute vec4 pos;\n"
  "attribute vec4 color;\n"
  "\n"
  "void\n"
  "main ()\n"
  "{\n"
  "  gl_Position = pos;\n"
  "  gl_FrontColor = color + other_color;\n"
  "}\n";

static const GLchar
frag_shader_source[] =
  "\n"
  "void\n"
  "main ()\n"
  "{\n"
  "  gl_FragColor = gl_Color;\n"
  "}\n";

static void
display_cb (void)
{
  static const Vert verts[] =
    {
      { -0.5, -0.5, 255, 0, 0, 255 },
      { 0.5, -0.5, 0, 255, 0, 255 },
      { -0.5, 0.5, 255, 0, 0, 255 },
      { 0.5, 0.5, 0, 255, 0, 255 }
    };
  const GLchar *source;
  GLint status;
  GLuint shader;
  GLuint glsl_program;
  char buf[1024];
  GLint pos_location;
  GLint color_location;
  GLint other_color_location;

  glClear (GL_COLOR_BUFFER_BIT);

  glsl_program = glCreateProgram ();

  shader = glCreateShader (GL_VERTEX_SHADER);
  source = vert_shader_source;
  glShaderSource (shader, 1, &source, 0);
  glCompileShader (shader);
  glGetShaderInfoLog (shader, sizeof (buf), NULL, buf);
  if (*buf)
    printf ("vertex shader info: %s\n", buf);
  glGetShaderiv (shader, GL_COMPILE_STATUS, &status);
  if (!status)
    printf ("vertex shader compile failed\n");
  glAttachShader (glsl_program, shader);

  shader = glCreateShader (GL_FRAGMENT_SHADER);
  source = frag_shader_source;
  glShaderSource (shader, 1, &source, 0);
  glCompileShader (shader);
  glGetShaderInfoLog (shader, sizeof (buf), NULL, buf);
  if (*buf)
    printf ("fragment shader info: %s\n", buf);
  glGetShaderiv (shader, GL_COMPILE_STATUS, &status);
  if (!status)
    printf ("fragment shader compile failed\n");
  glAttachShader (glsl_program, shader);

  glLinkProgram (glsl_program);
  glUseProgram (glsl_program);

  pos_location = glGetAttribLocation (glsl_program, "pos");
  printf ("glGetAttribLocation(\"pos\") = %i\n", pos_location);

  color_location = glGetAttribLocation (glsl_program, "color");
  printf ("glGetAttribLocation(\"color\") = %i\n", color_location);

  other_color_location = glGetAttribLocation (glsl_program, "other_color");
  printf ("glGetAttribLocation(\"other_color\") = %i\n", other_color_location);

  glEnableVertexAttribArray (pos_location);
  glVertexAttribPointer (pos_location,
                         2,
                         GL_FLOAT,
                         GL_FALSE,
                         sizeof (Vert),
                         &verts[0].x);

  glEnableVertexAttribArray (color_location);
  glVertexAttribPointer (color_location,
                         4,
                         GL_UNSIGNED_BYTE,
                         GL_TRUE,
                         sizeof (Vert),
                         &verts[0].r);

  glVertexAttrib4f (other_color_location, 0.0f, 0.0f, 1.0f, 0.0f);

  glDrawArrays (GL_TRIANGLE_STRIP, 0, 4);

  glDisableVertexAttribArray (pos_location);
  glDisableVertexAttribArray (color_location);

  glDeleteShader (shader);
  glUseProgram (0);
  glDeleteProgram (glsl_program);

  glutSwapBuffers ();
}

int
main (int argc, char **argv)
{
  glutInit (&argc, argv);
  glutInitDisplayMode (GLUT_RGB | GLUT_DOUBLE);
  glutInitWindowSize (512, 512);
  glutCreateWindow ("Look! A triangle!");
  glutDisplayFunc (display_cb);
  glewInit ();

  glutMainLoop ();

  return 0;
}