/*
 * Ambient Occlusion and Direction.
 * 
 * Calculates AO and the average direction where the ambient light came from.
 *
 * Original AO code from https://github.com/sambler/osl-shaders/blob/master/ramps/BaAmbientOcclusion/BaAmbientOcclusion.osl
 *
 */

void rng_seed(output int rng, int seed)
{
  int chash = seed;
  if (chash == 0) chash = 1;
  rng = chash * 891694213;
}

float rng_uniform(output int rng)
{
  float res = rng / float(2137483647) * 0.5 + 0.5;
  rng *= 891694213;
  return res;
}

void to_unit_disk(float x, float y, output float x_out, output float y_out)
{
  float r, phi;
  float a = 2.0 * x - 1.0;
  float b = 2.0 * y - 1.0;

  if(a > -b) {
    if(a > b) {
      r = a;
      phi = M_PI_4 *(b/a);
    }
    else {
      r = b;
      phi = M_PI_4 *(2.0 - a/b);
    }
  }
  else {
    if(a < b) {
      r = -a;
      phi = M_PI_4 *(4.0 + b/a);
    }
    else {
      r = -b;
      if(b != 0.0) phi = M_PI_4 *(6.0 - a/b);
      else phi = 0.0;
    }
  }
  x_out = r * cos(phi);
  y_out = r * sin(phi);
}

void make_orthonormals(vector N, output vector a, output vector b)
{
  if(N[0] != N[1] || N[0] != N[2]) a = cross(vector(1, 1, 1), N);
  else a = cross(vector(-1, 1, 1), N);

  a = normalize(a);
  b = cross(N, a);
}

vector sample_cos_hemisphere(vector N, float randu, float randv)
{
  vector T, B;

  make_orthonormals(N, T, B);
  to_unit_disk(randu, randv, randu, randv);
  float costheta = sqrt(max(1.0 - randu * randu - randv * randv, 0.0));

  return randu * T + randv * B + costheta * N;
}

shader AmbientOcclusionAndDirection(
  normal Normal = N,
  float Distance = 1,
  int Samples = 8,
  output float AmbientAmount = 1,
  output normal NonOccludedDirection = Normal )
{
  int i, rng, hits = 0;
  float f, randu, randv, ray_t;
  vector ray_P, ray_R;

  f = fmod(cellnoise(P*123456.0), 1.0);
  rng_seed(rng, int(f * 2137483647));

  vector accumulatedNonOccludedNormals = vector(0,0,0);

  for(i = 0; i < Samples; i++) {
    randu = rng_uniform(rng);
    randv = rng_uniform(rng);

    ray_R = sample_cos_hemisphere(Normal, randu, randv);

    if(trace(P, ray_R, "maxdist", Distance))
    {
      hits++;
    }
    else
    {
      accumulatedNonOccludedNormals += ray_R;
    }
  }

  if(Samples - hits > 0)
  {
    // ambient light rays found.
    // calculate the amount of ambient light and the direction where most of it came from

    AmbientAmount = 1.0 - (((float)hits)/Samples);

    NonOccludedDirection = normalize(accumulatedNonOccludedNormals);
  }
  else
  {
    // every ray has hit some geometry. no ambient light detected.

    AmbientAmount = 0.0;

    // should the resulting direction be equal to Normal, or zero???
    // NonOccludedDirection = vector(0,0,0);
  }
}