/*
 * Adapted from Open Shading Language with this license:
 *
 * Copyright (c) 2009-2010 Sony Pictures Imageworks Inc., et al.
 * All Rights Reserved.
 *
 * Modifications Copyright 2011, Blender Foundation.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 * * Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * * Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 * * Neither the name of Sony Pictures Imageworks nor the names of its
 *   contributors may be used to endorse or promote products derived from
 *   this software without specific prior written permission.
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#ifndef __BSDF_UTIL_H__
#define __BSDF_UTIL_H__

CCL_NAMESPACE_BEGIN

ccl_device float fresnel_dielectric(float eta,
                                    const float3 N,
                                    const float3 I,
                                    float3 *R,
                                    float3 *T,
#ifdef __RAY_DIFFERENTIALS__
                                    const float3 dIdx,
                                    const float3 dIdy,
                                    float3 *dRdx,
                                    float3 *dRdy,
                                    float3 *dTdx,
                                    float3 *dTdy,
#endif
                                    bool *is_inside)
{
  float cos = dot(N, I), neta;
  float3 Nn;

  // check which side of the surface we are on
  if (cos > 0) {
    // we are on the outside of the surface, going in
    neta = 1 / eta;
    Nn = N;
    *is_inside = false;
  }
  else {
    // we are inside the surface
    cos = -cos;
    neta = eta;
    Nn = -N;
    *is_inside = true;
  }

  // compute reflection
  *R = (2 * cos) * Nn - I;
#ifdef __RAY_DIFFERENTIALS__
  *dRdx = (2 * dot(Nn, dIdx)) * Nn - dIdx;
  *dRdy = (2 * dot(Nn, dIdy)) * Nn - dIdy;
#endif

  float arg = 1 - (neta * neta * (1 - (cos * cos)));
  if (arg < 0) {
    *T = make_float3(0.0f, 0.0f, 0.0f);
#ifdef __RAY_DIFFERENTIALS__
    *dTdx = make_float3(0.0f, 0.0f, 0.0f);
    *dTdy = make_float3(0.0f, 0.0f, 0.0f);
#endif
    return 1;  // total internal reflection
  }
  else {
    float dnp = max(sqrtf(arg), 1e-7f);
    float nK = (neta * cos) - dnp;
    *T = -(neta * I) + (nK * Nn);
#ifdef __RAY_DIFFERENTIALS__
    *dTdx = -(neta * dIdx) + ((neta - neta * neta * cos / dnp) * dot(dIdx, Nn)) * Nn;
    *dTdy = -(neta * dIdy) + ((neta - neta * neta * cos / dnp) * dot(dIdy, Nn)) * Nn;
#endif
    // compute Fresnel terms
    float cosTheta1 = cos;  // N.R
    float cosTheta2 = -dot(Nn, *T);
    float pPara = (cosTheta1 - eta * cosTheta2) / (cosTheta1 + eta * cosTheta2);
    float pPerp = (eta * cosTheta1 - cosTheta2) / (eta * cosTheta1 + cosTheta2);
    return 0.5f * (pPara * pPara + pPerp * pPerp);
  }
}

ccl_device float fresnel_dielectric_cos(float cosi, float eta)
{
  // compute fresnel reflectance without explicitly computing
  // the refracted direction
  float c = fabsf(cosi);
  float g = eta * eta - 1 + c * c;
  if (g > 0) {
    g = sqrtf(g);
    float A = (g - c) / (g + c);
    float B = (c * (g + c) - 1) / (c * (g - c) + 1);
    return 0.5f * A * A * (1 + B * B);
  }
  return 1.0f;  // TIR(no refracted component)
}

ccl_device float3 fresnel_conductor(float cosi, const float3 eta, const float3 k)
{
  float3 cosi2 = make_float3(cosi * cosi, cosi * cosi, cosi * cosi);
  float3 one = make_float3(1.0f, 1.0f, 1.0f);
  float3 tmp_f = eta * eta + k * k;
  float3 tmp = tmp_f * cosi2;
  float3 Rparl2 = (tmp - (2.0f * eta * cosi) + one) / (tmp + (2.0f * eta * cosi) + one);
  float3 Rperp2 = (tmp_f - (2.0f * eta * cosi) + cosi2) / (tmp_f + (2.0f * eta * cosi) + cosi2);
  return (Rparl2 + Rperp2) * 0.5f;
}

ccl_device float schlick_fresnel(float u)
{
  float m = clamp(1.0f - u, 0.0f, 1.0f);
  float m2 = m * m;
  return m2 * m2 * m;  // pow(m, 5)
}

/* Calculate the fresnel color which is a blend between white and the F0 color (cspec0) */
ccl_device_forceinline float3
interpolate_fresnel_color(float3 L, float3 H, float ior, float F0, float3 cspec0)
{
  /* Calculate the fresnel interpolation factor
   * The value from fresnel_dielectric_cos(...) has to be normalized because
   * the cspec0 keeps the F0 color
   */
  float F0_norm = 1.0f / (1.0f - F0);
  float FH = (fresnel_dielectric_cos(dot(L, H), ior) - F0) * F0_norm;

  /* Blend between white and a specular color with respect to the fresnel */
  return cspec0 * (1.0f - FH) + make_float3(1.0f, 1.0f, 1.0f) * FH;
}

CCL_NAMESPACE_END

#endif /* __BSDF_UTIL_H__ */
