// Copyright 2009-2021 Intel Corporation
// SPDX-License-Identifier: Apache-2.0

#pragma once

/*! \brief utility library containing sampling functions */

// convention is to return the sample (Vec3f) generated from given Vec2f 's'ample as last parameter
// sampling functions often come in pairs: sample and pdf (needed later for MIS)
// good reference is "Total Compendium" by Philip Dutre http://people.cs.kuleuven.be/~philip.dutre/GI/

#include "../math/vec.isph"
#include "../math/linearspace.isph"

struct Sample3f
{
  Vec3f v;
  float pdf;
};

inline uniform Sample3f make_Sample3f(const uniform Vec3f& v, const uniform float pdf) {
  uniform Sample3f s; s.v = v; s.pdf = pdf; return s;
}

#if defined(ISPC)
inline Sample3f make_Sample3f(const Vec3f& v, const float pdf) {
  Sample3f s; s.v = v; s.pdf = pdf; return s;
}
#endif

inline Vec3f cartesian(const float phi, const float sinTheta, const float cosTheta)
{
  const float sinPhi = sinf(phi);
  const float cosPhi = cosf(phi);
  //sincos(phi, &sinPhi, &cosPhi);
  return make_Vec3f(cosPhi * sinTheta,
                    sinPhi * sinTheta,
                    cosTheta);
}

inline Vec3f cartesian(const float phi, const float cosTheta)
{
  return cartesian(phi, cos2sin(cosTheta), cosTheta);
}


/// cosine-weighted sampling of hemisphere oriented along the +z-axis
////////////////////////////////////////////////////////////////////////////////

inline Vec3f cosineSampleHemisphere(const Vec2f s)
{
  const float phi = 2.0f * M_PI * s.x;
  const float cosTheta = sqrt(s.y);
  const float sinTheta = sqrt(1.0f - s.y);
  return cartesian(phi, sinTheta, cosTheta);
}

inline float cosineSampleHemispherePDF(const Vec3f &dir)
{
  return dir.z / M_PI;
}

inline float cosineSampleHemispherePDF(float cosTheta)
{
  return cosTheta / M_PI;
}

/*! Cosine weighted hemisphere sampling. Up direction is provided as argument. */
inline Sample3f cosineSampleHemisphere(const float  u, const float  v, const Vec3f& N)
{
  Vec3f localDir = cosineSampleHemisphere(make_Vec2f(u,v));
  Sample3f s;
  s.v = frame(N) * localDir;
  s.pdf = cosineSampleHemispherePDF(localDir);
  return s;
}

/// power cosine-weighted sampling of hemisphere oriented along the +z-axis
////////////////////////////////////////////////////////////////////////////////

inline Vec3f powerCosineSampleHemisphere(const float n, const Vec2f &s)
{
  const float phi = two_pi * s.x;
  const float cosTheta = pow(s.y, 1.0f / (n + 1.0f));
  return cartesian(phi, cosTheta);
}

inline float powerCosineSampleHemispherePDF(const float cosTheta, const float n) // TODO: order of arguments
{
  return (n + 1.0f) * (0.5f / M_PI) * pow(cosTheta, n);
}

inline float powerCosineSampleHemispherePDF(const Vec3f& dir, const float n) // TODO: order of arguments
{
  return (n + 1.0f) * (0.5f / M_PI) * pow(dir.z, n);
}

/// uniform sampling of cone of directions oriented along the +z-axis
////////////////////////////////////////////////////////////////////////////////

inline Vec3f uniformSampleCone(const float cosAngle, const Vec2f &s)
{
  const float phi = two_pi * s.x;
  const float cosTheta = 1.0f - s.y * (1.0f - cosAngle);
  return cartesian(phi, cosTheta);
}

inline float uniformSampleConePDF(const float cosAngle)
{
    return rcp(two_pi*(1.0f - cosAngle));
}

inline uniform float uniformSampleConePDF(const uniform float cosAngle)
{
    return rcp(two_pi*(1.0f - cosAngle));
}


/// uniform sampling of disk
////////////////////////////////////////////////////////////////////////////////

inline Vec3f uniformSampleDisk(const float radius, const Vec2f &s)
{
  const float r = sqrtf(s.x) * radius;
  const float phi = two_pi * s.y;
  const float sinPhi = sinf(phi);
  const float cosPhi = cosf(phi);
  //sincos(phi, &sinPhi, &cosPhi);
  return make_Vec3f(r * cosPhi, r * sinPhi, 0.f);
}

inline float uniformSampleDiskPDF(const float radius)
{
  return rcp(M_PI * sqr(radius));
}

inline uniform float uniformSampleDiskPDF(const uniform float radius)
{
  return rcp(M_PI * sqr(radius));
}


/// uniform sampling of triangle abc
////////////////////////////////////////////////////////////////////////////////

inline Vec3f uniformSampleTriangle(const Vec3f &a, const Vec3f &b, const Vec3f &c, const Vec2f &s)
{
  const float su = sqrtf(s.x);
  return c + (1.0f - su) * (a-c) + (s.y*su) * (b-c);
}

inline float uniformSampleTrianglePDF(const Vec3f &a, const Vec3f &b, const Vec3f &c)
{
  return 2.0f * rcp(abs(length(cross(a-c, b-c))));
}
