using System;
using UnityEngine.Rendering;

namespace UnityEngine.Rendering.HighDefinition.LTC
{
    /// <summary>
    /// GGX implementation of the BRDF interface
    /// </summary>
    class BRDF_GGX : IBRDF
    {
        public double Eval(ref Vector3 _tsView, ref Vector3 _tsLight, float _alpha, out double _pdf)
        {
            if (_tsView.z <= 0)
            {
                _pdf = 0;
                return 0;
            }

            // masking
            double lambdaV = Lambda(_tsView.z, _alpha);

            // shadowing
            double G2 = 0;
            if (_tsLight.z > 0.0f)
            {
                double lambdaL = Lambda(_tsLight.z, _alpha);
                G2 = 1.0 / (1.0 + lambdaV + lambdaL);
            }

            // D
            Vector3 H = _tsView + _tsLight;
            float lengthH = H.magnitude;
            if (lengthH > 1e-8f)
                H = H / lengthH;
            else
                H = new Vector3(0, 0, 1);

            double slopex = H.x / H.z;
            double slopey = H.y / H.z;
            double D = 1.0 / (1.0 + (slopex * slopex + slopey * slopey) / _alpha / _alpha);
            D = D * D;
            D = D / (Math.PI * _alpha * _alpha * H.z * H.z * H.z * H.z);

            // Full specular mico-facet model is F * D * G / (4 * NdotL * NdotV) but since we're fitting with the NdotL included, it gets nicely canceled out!
            double res = D * G2 / 4.0 / _tsView.z;

            // pdf = D(H) * (N.H) / (4 * (L.H))
            _pdf = Math.Abs(D * H.z / 4.0 / Vector3.Dot(_tsView, H));

            return res;
        }

        public void GetSamplingDirection(ref Vector3 _tsView, float _alpha, float _U1, float _U2, ref Vector3 _direction)
        {
            float phi = 2.0f * Mathf.PI * _U1;
            float r = _alpha * Mathf.Sqrt(_U2 / (1.0f - _U2));
            Vector3 H = new Vector3(r * Mathf.Cos(phi), r * Mathf.Sin(phi), 1.0f).normalized;
            _direction = -_tsView + 2.0f * H * Vector3.Dot(H, _tsView);
        }

        double Lambda(float _cosTheta, float _alpha)
        {
            double a = 1.0f / _alpha / Math.Tan(Math.Acos(_cosTheta));
            double lambda = _cosTheta < 1.0 ? 0.5 * (-1.0 + Math.Sqrt(1.0 + 1.0 / (a * a))) : 0.0;
            return lambda;
        }

        public LTCLightingModel GetLightingModel()
        {
            return LTCLightingModel.GGX;
        }
    }
}