using System;
using System.Collections.Generic;
using System.IO;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;

namespace Kav
{
    public class Renderer
    {
        private const int MAX_INSTANCE_VERTEX_COUNT = 1000;
        private const int MAX_SHADOW_CASCADES = 4;
        private int ShadowMapSize { get; }

        private GraphicsDevice GraphicsDevice { get; }

        private VertexBuffer FullscreenTriangle { get; }
        private int NumShadowCascades { get; }

        private RenderTarget2D ColorRenderTarget { get; }
        private RenderTarget2D[] ShadowRenderTargets { get; }

        private DeferredPBREffect DeferredPBREffect { get; }
        /* FIXME: these next two dont actually have anything to do with PBR */
        private DeferredPBR_GBufferEffect Deferred_GBufferEffect { get; }
        private DeferredPBR_AmbientLightEffect DeferredAmbientLightEffect { get; }
        private DeferredPBR_PointLightEffect DeferredPointLightEffect { get; }
        private DeferredPBR_DirectionalLightEffect DeferredDirectionalLightEffect { get; }
        private Deferred_ToonEffect Deferred_ToonEffect { get; }
        private SimpleDepthEffect SimpleDepthEffect { get; }
        private LinearDepthEffect LinearDepthEffect { get; }
        private Effect ToneMapEffect { get; }
        private SkyboxEffect SkyboxEffect { get; }
        private DiffuseLitSpriteEffect DiffuseLitSpriteEffect { get; }

        private RenderTargetCube PointShadowCubeMap { get; }

        private Kav.Model UnitCube { get; }

        private SpriteBatch SpriteBatch { get; }

        private DynamicVertexBuffer GBufferInstanceVertexBuffer { get; }
        private readonly GBufferInstanceVertex[] GBufferInstanceVertices = new GBufferInstanceVertex[MAX_INSTANCE_VERTEX_COUNT];

        public Renderer(
            GraphicsDevice graphicsDevice,
            int renderDimensionsX,
            int renderDimensionsY,
            int numShadowCascades,
            int shadowMapSize
        ) {
            GraphicsDevice = graphicsDevice;

            ShadowMapSize = shadowMapSize;

            NumShadowCascades = (int)MathHelper.Clamp(numShadowCascades, 1, MAX_SHADOW_CASCADES);
            ShadowRenderTargets = new RenderTarget2D[numShadowCascades];

            for (var i = 0; i < numShadowCascades; i++)
            {
                ShadowRenderTargets[i] = new RenderTarget2D(
                    GraphicsDevice,
                    ShadowMapSize,
                    ShadowMapSize,
                    false,
                    SurfaceFormat.Single,
                    DepthFormat.Depth24
                );
            }

            ColorRenderTarget = new RenderTarget2D(
                graphicsDevice,
                renderDimensionsX,
                renderDimensionsY,
                false,
                SurfaceFormat.Color,
                DepthFormat.Depth24,
                0,
                RenderTargetUsage.PreserveContents
            );

            PointShadowCubeMap = new RenderTargetCube(
                GraphicsDevice,
                shadowMapSize,
                false,
                SurfaceFormat.Single,
                DepthFormat.Depth24
            );

            SimpleDepthEffect = new SimpleDepthEffect(GraphicsDevice);
            LinearDepthEffect = new LinearDepthEffect(GraphicsDevice);
            DeferredPBREffect = new DeferredPBREffect(GraphicsDevice);

            Deferred_GBufferEffect = new DeferredPBR_GBufferEffect(GraphicsDevice);
            DeferredAmbientLightEffect = new DeferredPBR_AmbientLightEffect(GraphicsDevice);
            DeferredPointLightEffect = new DeferredPBR_PointLightEffect(GraphicsDevice);
            DeferredDirectionalLightEffect = new DeferredPBR_DirectionalLightEffect(GraphicsDevice);
            DeferredDirectionalLightEffect.ShadowMapSize = ShadowMapSize;
            ToneMapEffect = new Effect(graphicsDevice, Resources.ToneMapEffect);
            Deferred_ToonEffect = new Deferred_ToonEffect(GraphicsDevice);
            SkyboxEffect = new SkyboxEffect(GraphicsDevice);
            DiffuseLitSpriteEffect = new DiffuseLitSpriteEffect(GraphicsDevice);

            FullscreenTriangle = new VertexBuffer(GraphicsDevice, typeof(VertexPositionTexture), 3, BufferUsage.WriteOnly);
            FullscreenTriangle.SetData(new VertexPositionTexture[3] {
                new VertexPositionTexture(new Vector3(-1, -3, 0), new Vector2(0, 2)),
                new VertexPositionTexture(new Vector3(-1, 1, 0), new Vector2(0, 0)),
                new VertexPositionTexture(new Vector3(3, 1, 0), new Vector2(2, 0))
            });

            UnitCube = Kav.ModelLoader.Load(
                GraphicsDevice,
                Smuggler.Importer.ImportGLB(GraphicsDevice, new MemoryStream(Resources.UnitCubeModel))
            );

            SpriteBatch = new SpriteBatch(graphicsDevice);

            GBufferInstanceVertexBuffer = new DynamicVertexBuffer(
                GraphicsDevice,
                VertexDeclarations.GBufferInstanceDeclaration,
                MAX_INSTANCE_VERTEX_COUNT,
                BufferUsage.WriteOnly
            );
        }

        // TODO: we could make this a lot more efficient probably
        // draws mesh sprites with a forward rendered diffuse lighting technique
        public void MeshSpriteRender(
            RenderTarget2D renderTarget,
            PerspectiveCamera camera,
            IEnumerable<(Model, Matrix)> modelTransforms,
            IEnumerable<(MeshSprite, Matrix)> meshSpriteTransforms,
            AmbientLight ambientLight,
            IEnumerable<PointLight> pointLights,
            DirectionalLight? directionalLight
        ) {
            GraphicsDevice.SetRenderTarget(ColorRenderTarget);
            GraphicsDevice.Clear(ClearOptions.Target | ClearOptions.DepthBuffer, Color.Black, 1f, 0);
            GraphicsDevice.DepthStencilState = DepthStencilState.Default;

            DepthRender(ColorRenderTarget, camera, modelTransforms);
            GraphicsDevice.Clear(ClearOptions.Target, new Color(0, 0, 0, 0), 1f, 0);

            GraphicsDevice.DepthStencilState = DepthStencilState.DepthRead;
            GraphicsDevice.RasterizerState = RasterizerState.CullNone;
            GraphicsDevice.SamplerStates[0] = SamplerState.PointClamp;
            GraphicsDevice.SamplerStates[1] = SamplerState.PointClamp;
            GraphicsDevice.BlendState = BlendState.AlphaBlend;

            DiffuseLitSpriteEffect.View = camera.View;
            DiffuseLitSpriteEffect.Projection = camera.Projection;

            DiffuseLitSpriteEffect.AmbientColor =
                ambientLight.Color.ToVector3();

            if (directionalLight.HasValue)
            {
                DiffuseLitSpriteEffect.DirectionalLightDirection =
                    directionalLight.Value.Direction;
                DiffuseLitSpriteEffect.DirectionalLightColor =
                    directionalLight.Value.Color.ToVector3() * directionalLight.Value.Intensity;
            }
            else
            {
                DiffuseLitSpriteEffect.DirectionalLightColor = Vector3.Zero;
            }

            var i = 0;
            foreach (var pointLight in pointLights)
            {
                if (i > DiffuseLitSpriteEffect.MaxPointLights) { break; }
                DiffuseLitSpriteEffect.PointLights[i] = pointLight;
                i += 1;
            }

            var boundingFrustum = new BoundingFrustum(camera.View * camera.Projection);

            foreach (var (sprite, transform) in FrustumCull(boundingFrustum, meshSpriteTransforms))
            {
                DiffuseLitSpriteEffect.NormalMapEnabled = sprite.Normal != null;

                if (sprite.BillboardConstraint == SpriteBillboardConstraint.None)
                {
                    DiffuseLitSpriteEffect.World = transform;
                }
                else if (sprite.BillboardConstraint == SpriteBillboardConstraint.Horizontal)
                {
                    DiffuseLitSpriteEffect.World = Matrix.CreateConstrainedBillboard(
                        transform.Translation,
                        camera.Position,
                        Vector3.Up,
                        camera.Forward,
                        camera.Position - transform.Translation
                    );
                }
                else
                {
                    DiffuseLitSpriteEffect.World = Matrix.CreateConstrainedBillboard(
                        transform.Translation,
                        camera.Position,
                        Vector3.Up,
                        null,
                        null
                    );
                }

                GraphicsDevice.Textures[0] = sprite.Texture;
                GraphicsDevice.Textures[1] = sprite.Normal;

                GraphicsDevice.SetVertexBuffer(sprite.VertexBuffer);
                GraphicsDevice.Indices = sprite.IndexBuffer;

                foreach (var pass in DiffuseLitSpriteEffect.CurrentTechnique.Passes)
                {
                    pass.Apply();

                    GraphicsDevice.DrawIndexedPrimitives(
                        PrimitiveType.TriangleList,
                        0,
                        0,
                        sprite.VertexBuffer.VertexCount,
                        0,
                        2
                    );
                }
            }

            GraphicsDevice.SetRenderTarget(renderTarget);
            GraphicsDevice.Clear(new Color(0, 0, 0, 0));
            SpriteBatch.Begin(SpriteSortMode.Deferred, BlendState.AlphaBlend, null, null, null, null);
            SpriteBatch.Draw(ColorRenderTarget, Vector2.Zero, Color.White);
            SpriteBatch.End();
        }

        public void DepthRender(
            RenderTarget2D renderTarget,
            PerspectiveCamera camera,
            IEnumerable<(Model, Matrix)> modelTransforms
        ) {
            GraphicsDevice.SetRenderTarget(renderTarget);
            GraphicsDevice.DepthStencilState = DepthStencilState.Default;

            var boundingFrustum = new BoundingFrustum(camera.View * camera.Projection);

            foreach (var (model, transform) in FrustumCull(boundingFrustum, modelTransforms))
            {
                foreach (var modelMesh in model.Meshes)
                {
                    foreach (var meshPart in modelMesh.MeshParts)
                    {
                        SimpleDepthEffect.Model = transform;
                        SimpleDepthEffect.View = camera.View;
                        SimpleDepthEffect.Projection = camera.Projection;

                        GraphicsDevice.SetVertexBuffer(meshPart.VertexBuffer);
                        GraphicsDevice.Indices = meshPart.IndexBuffer;

                        foreach (var pass in SimpleDepthEffect.CurrentTechnique.Passes)
                        {
                            pass.Apply();

                            GraphicsDevice.DrawIndexedPrimitives(
                                PrimitiveType.TriangleList,
                                0,
                                0,
                                meshPart.VertexBuffer.VertexCount,
                                0,
                                meshPart.Triangles.Length
                            );
                        }
                    }
                }
            }
        }

        public void SkyboxRender(
            RenderTarget2D renderTarget,
            PerspectiveCamera camera,
            TextureCube skybox
        ) {
            GraphicsDevice.SetRenderTarget(renderTarget);
            GraphicsDevice.RasterizerState.CullMode = CullMode.CullClockwiseFace;
            SkyboxEffect.Skybox = skybox;

            var view = camera.View;
            view.Translation = Vector3.Zero;
            SkyboxEffect.View = view;

            SkyboxEffect.Projection = camera.Projection;

            GraphicsDevice.SetVertexBuffer(UnitCube.Meshes[0].MeshParts[0].VertexBuffer);
            GraphicsDevice.Indices = UnitCube.Meshes[0].MeshParts[0].IndexBuffer;

            foreach (var pass in SkyboxEffect.CurrentTechnique.Passes)
            {
                pass.Apply();

                GraphicsDevice.DrawIndexedPrimitives(
                    PrimitiveType.TriangleList,
                    0,
                    0,
                    UnitCube.Meshes[0].MeshParts[0].VertexBuffer.VertexCount,
                    0,
                    UnitCube.Meshes[0].MeshParts[0].Triangles.Length
                );
            }
            GraphicsDevice.RasterizerState.CullMode = CullMode.CullCounterClockwiseFace;
        }

        /// <summary>
        /// GBuffer binding must have 4 render targets.
        /// </summary>
        public void InstancedGBufferRender<T>(
            RenderTargetBinding[] gBuffer,
            PerspectiveCamera camera,
            T drawable,
            int numInstances,
            IEnumerable<Matrix> transforms
        ) where T : IIndexDrawable, IGBufferDrawable {
            GraphicsDevice.SetRenderTargets(gBuffer);
            GraphicsDevice.DepthStencilState = DepthStencilState.Default;
            GraphicsDevice.BlendState = BlendState.Opaque;

            Deferred_GBufferEffect.HardwareInstancingEnabled = true;

            Deferred_GBufferEffect.Albedo = drawable.Albedo;
            Deferred_GBufferEffect.Metallic = drawable.Metallic;
            Deferred_GBufferEffect.Roughness = drawable.Roughness;

            Deferred_GBufferEffect.AlbedoTexture = drawable.AlbedoTexture;
            Deferred_GBufferEffect.NormalTexture = drawable.NormalTexture;
            Deferred_GBufferEffect.MetallicRoughnessTexture = drawable.MetallicRoughnessTexture;

            Deferred_GBufferEffect.View = camera.View;
            Deferred_GBufferEffect.Projection = camera.Projection;

            int i = 0;
            foreach (var transform in transforms)
            {
                if (i >= numInstances) { break; }
                GBufferInstanceVertices[i].World = transform;
                i += 1;
            }

            GBufferInstanceVertexBuffer.SetData(
                GBufferInstanceVertices,
                0,
                numInstances,
                SetDataOptions.Discard
            );

            GraphicsDevice.SetVertexBuffers(
                drawable.VertexBuffer,
                new VertexBufferBinding(GBufferInstanceVertexBuffer, 0, 1)
            );
            GraphicsDevice.Indices = drawable.IndexBuffer;

            foreach (var pass in Deferred_GBufferEffect.CurrentTechnique.Passes)
            {
                pass.Apply();
                GraphicsDevice.DrawInstancedPrimitives(
                    PrimitiveType.TriangleList,
                    0,
                    0,
                    drawable.VertexBuffer.VertexCount,
                    0,
                    drawable.IndexBuffer.IndexCount / 3,
                    numInstances
                );
            }
        }

        public void GBufferRender(
            RenderTargetBinding[] gBuffer,
            PerspectiveCamera camera,
            IEnumerable<(Model, Matrix)> modelTransforms
        ) {
            GraphicsDevice.SetRenderTargets(gBuffer);
            GraphicsDevice.DepthStencilState = DepthStencilState.Default;
            GraphicsDevice.BlendState = BlendState.Opaque;

            var boundingFrustum = new BoundingFrustum(camera.View * camera.Projection);

            foreach (var (model, transform) in FrustumCull(boundingFrustum, modelTransforms))
            {
                foreach (var modelMesh in model.Meshes)
                {
                    foreach (var meshPart in modelMesh.MeshParts)
                    {
                        Deferred_GBufferEffect.World = transform;
                        Deferred_GBufferEffect.View = camera.View;
                        Deferred_GBufferEffect.Projection = camera.Projection;

                        Deferred_GBufferEffect.HardwareInstancingEnabled = false;

                        Deferred_GBufferEffect.Albedo = meshPart.Albedo;
                        Deferred_GBufferEffect.Metallic = meshPart.Metallic;
                        Deferred_GBufferEffect.Roughness = meshPart.Roughness;

                        Deferred_GBufferEffect.AlbedoTexture = meshPart.AlbedoTexture;
                        Deferred_GBufferEffect.NormalTexture = meshPart.NormalTexture;
                        Deferred_GBufferEffect.MetallicRoughnessTexture = meshPart.MetallicRoughnessTexture;

                        GraphicsDevice.SetVertexBuffer(meshPart.VertexBuffer);
                        GraphicsDevice.Indices = meshPart.IndexBuffer;

                        foreach (var pass in Deferred_GBufferEffect.CurrentTechnique.Passes)
                        {
                            pass.Apply();

                            GraphicsDevice.DrawIndexedPrimitives(
                                PrimitiveType.TriangleList,
                                0,
                                0,
                                meshPart.VertexBuffer.VertexCount,
                                0,
                                meshPart.Triangles.Length
                            );
                        }
                    }
                }
            }
        }

        public void AmbientLightRender(
            RenderTarget2D renderTarget,
            Texture2D gPosition,
            Texture2D gAlbedo,
            AmbientLight ambientLight
        ) {
            GraphicsDevice.SetRenderTarget(renderTarget);
            GraphicsDevice.BlendState = BlendState.Opaque;
            GraphicsDevice.DepthStencilState = DepthStencilState.DepthRead;

            DeferredAmbientLightEffect.GPosition = gPosition;
            DeferredAmbientLightEffect.GAlbedo = gAlbedo;
            DeferredAmbientLightEffect.AmbientColor = ambientLight.Color.ToVector3();

            foreach (var pass in DeferredAmbientLightEffect.CurrentTechnique.Passes)
            {
                pass.Apply();
                GraphicsDevice.SetVertexBuffer(FullscreenTriangle);
                GraphicsDevice.DrawPrimitives(PrimitiveType.TriangleList, 0, 1);
            }
        }

        public void PointLightRender(
            RenderTarget2D renderTarget,
            Texture2D gPosition,
            Texture2D gAlbedo,
            Texture2D gNormal,
            Texture2D gMetallicRoughness,
            PerspectiveCamera camera,
            IEnumerable<(Model, Matrix)> modelTransforms,
            PointLight pointLight
        ) {
            RenderPointShadows(camera, modelTransforms, pointLight);

            GraphicsDevice.SetRenderTarget(renderTarget);
            GraphicsDevice.DepthStencilState = DepthStencilState.DepthRead;
            GraphicsDevice.BlendState = BlendState.Additive;

            DeferredPointLightEffect.GPosition = gPosition;
            DeferredPointLightEffect.GAlbedo = gAlbedo;
            DeferredPointLightEffect.GNormal = gNormal;
            DeferredPointLightEffect.GMetallicRoughness = gMetallicRoughness;
            DeferredPointLightEffect.ShadowMap = PointShadowCubeMap;

            DeferredPointLightEffect.EyePosition = camera.Position;

            DeferredPointLightEffect.PointLightPosition = pointLight.Position;
            DeferredPointLightEffect.PointLightColor =
                pointLight.Color.ToVector3() * pointLight.Intensity;

            DeferredPointLightEffect.FarPlane = 25f; // FIXME: magic value

            foreach (var pass in DeferredPointLightEffect.CurrentTechnique.Passes)
            {
                pass.Apply();
                GraphicsDevice.SetVertexBuffer(FullscreenTriangle);
                GraphicsDevice.DrawPrimitives(PrimitiveType.TriangleList, 0, 1);
            }
        }

        public void DirectionalLightRender(
            RenderTarget2D renderTarget,
            Texture2D gPosition,
            Texture2D gAlbedo,
            Texture2D gNormal,
            Texture2D gMetallicRoughness,
            PerspectiveCamera camera,
            IEnumerable<(Model, Matrix)> modelTransforms,
            DirectionalLight directionalLight,
            int numShadowCascades
        ) {
            RenderDirectionalShadows(camera, modelTransforms, directionalLight, DeferredDirectionalLightEffect);

            GraphicsDevice.SetRenderTarget(renderTarget);
            GraphicsDevice.DepthStencilState = DepthStencilState.DepthRead;
            GraphicsDevice.BlendState = BlendState.Additive;

            DeferredDirectionalLightEffect.GPosition = gPosition;
            DeferredDirectionalLightEffect.GAlbedo = gAlbedo;
            DeferredDirectionalLightEffect.GNormal = gNormal;
            DeferredDirectionalLightEffect.GMetallicRoughness = gMetallicRoughness;

            DeferredDirectionalLightEffect.ShadowMapOne = ShadowRenderTargets[0];
            if (numShadowCascades > 1)
            {
                DeferredDirectionalLightEffect.ShadowMapTwo = ShadowRenderTargets[1];
            }
            if (numShadowCascades > 2)
            {
                DeferredDirectionalLightEffect.ShadowMapThree = ShadowRenderTargets[2];
            }
            if (numShadowCascades > 3)
            {
                DeferredDirectionalLightEffect.ShadowMapFour = ShadowRenderTargets[3];
            }

            DeferredDirectionalLightEffect.DirectionalLightDirection = directionalLight.Direction;
            DeferredDirectionalLightEffect.DirectionalLightColor =
                directionalLight.Color.ToVector3() * directionalLight.Intensity;

            DeferredDirectionalLightEffect.ViewMatrix = camera.View;
            DeferredDirectionalLightEffect.EyePosition = camera.Position;

            foreach (EffectPass pass in DeferredDirectionalLightEffect.CurrentTechnique.Passes)
            {
                pass.Apply();
                GraphicsDevice.SetVertexBuffer(FullscreenTriangle);
                GraphicsDevice.DrawPrimitives(PrimitiveType.TriangleList, 0, 1);
            }
        }

        public void DirectionalLightToonRender(
            RenderTarget2D renderTarget,
            Texture2D gPosition,
            Texture2D gAlbedo,
            Texture2D gNormal,
            Texture2D gMetallicRoughness,
            PerspectiveCamera camera,
            IEnumerable<(Model, Matrix)> modelTransforms,
            DirectionalLight directionalLight,
            int numShadowCascades,
            bool ditheredShadows
        ) {
            RenderDirectionalShadows(camera, modelTransforms, directionalLight, Deferred_ToonEffect);

            GraphicsDevice.SetRenderTarget(renderTarget);
            GraphicsDevice.DepthStencilState = DepthStencilState.DepthRead;
            GraphicsDevice.BlendState = BlendState.Additive;

            Deferred_ToonEffect.GPosition = gPosition;
            Deferred_ToonEffect.GAlbedo = gAlbedo;
            Deferred_ToonEffect.GNormal = gNormal;
            Deferred_ToonEffect.GMetallicRoughness = gMetallicRoughness;

            Deferred_ToonEffect.DitheredShadows = ditheredShadows;

            Deferred_ToonEffect.EyePosition = camera.Position;

            Deferred_ToonEffect.DirectionalLightDirection = directionalLight.Direction;
            Deferred_ToonEffect.DirectionalLightColor =
                directionalLight.Color.ToVector3() * directionalLight.Intensity;

            Deferred_ToonEffect.ShadowMapOne = ShadowRenderTargets[0];
            if (numShadowCascades > 1)
            {
                Deferred_ToonEffect.ShadowMapTwo = ShadowRenderTargets[1];
            }
            if (numShadowCascades > 2)
            {
                Deferred_ToonEffect.ShadowMapThree = ShadowRenderTargets[2];
            }
            if (numShadowCascades > 3)
            {
                Deferred_ToonEffect.ShadowMapFour = ShadowRenderTargets[3];
            }

            Deferred_ToonEffect.ViewMatrix = camera.View;

            foreach (EffectPass pass in Deferred_ToonEffect.CurrentTechnique.Passes)
            {
                pass.Apply();
                GraphicsDevice.SetVertexBuffer(FullscreenTriangle);
                GraphicsDevice.DrawPrimitives(PrimitiveType.TriangleList, 0, 1);
            }
        }

        private void RenderDirectionalShadows(
            PerspectiveCamera camera,
            IEnumerable<(Model, Matrix)> modelTransforms,
            DirectionalLight directionalLight,
            ShadowCascadeEffect effect
        ) {
            // render the individual shadow cascades
            var previousFarPlane = camera.NearPlane;
            for (var i = 0; i < NumShadowCascades; i++)
            {
                var farPlane = camera.FarPlane / (MathHelper.Max((NumShadowCascades - i - 1) * 2f, 1f));

                // divide the view frustum
                var shadowCamera = new PerspectiveCamera(
                    camera.Position,
                    camera.Forward,
                    camera.Up,
                    camera.FieldOfView,
                    camera.AspectRatio,
                    previousFarPlane,
                    farPlane
                );

                // TODO: This is tightly coupled to the effect and it sucks
                RenderDirectionalShadowMap(shadowCamera, modelTransforms, directionalLight, effect, i);

                effect.CascadeFarPlanes[i] = farPlane;
                previousFarPlane = farPlane;
            }
        }

        private void RenderDirectionalShadowMap(
            PerspectiveCamera camera,
            IEnumerable<(Model, Matrix)> modelTransforms,
            DirectionalLight directionalLight,
            ShadowCascadeEffect effect,
            int shadowCascadeIndex
        ) {
            GraphicsDevice.SetRenderTarget(ShadowRenderTargets[shadowCascadeIndex]);
            GraphicsDevice.Clear(Color.White);
            GraphicsDevice.DepthStencilState = DepthStencilState.Default;
            GraphicsDevice.BlendState = BlendState.Opaque;

            var cameraBoundingFrustum = new BoundingFrustum(camera.View * camera.Projection);
            Vector3[] frustumCorners = cameraBoundingFrustum.GetCorners();

            Vector3 frustumCenter = Vector3.Zero;
            for (var i = 0; i < frustumCorners.Length; i++)
            {
                frustumCenter += frustumCorners[i];
            }
            frustumCenter /= 8f;

            var lightView = Matrix.CreateLookAt(frustumCenter + directionalLight.Direction, frustumCenter, Vector3.Backward);

            for (var i = 0; i < frustumCorners.Length; i++)
            {
                frustumCorners[i] = Vector3.Transform(frustumCorners[i], lightView);
            }

            BoundingBox lightBox = BoundingBox.CreateFromPoints(frustumCorners);

            SimpleDepthEffect.View = lightView;
            SimpleDepthEffect.Projection = Matrix.CreateOrthographicOffCenter(
                lightBox.Min.X,
                lightBox.Max.X,
                lightBox.Min.Y,
                lightBox.Max.Y,
                -lightBox.Max.Z - 10f, // TODO: near clip plane needs scene AABB info to get rid of this magic value
                -lightBox.Min.Z
            );

            var lightSpaceMatrix = SimpleDepthEffect.View * SimpleDepthEffect.Projection;

            if (shadowCascadeIndex == 0)
            {
                effect.LightSpaceMatrixOne = lightSpaceMatrix;
            }
            else if (shadowCascadeIndex == 1)
            {
                effect.LightSpaceMatrixTwo = lightSpaceMatrix;
            }
            else if (shadowCascadeIndex == 2)
            {
                effect.LightSpaceMatrixThree = lightSpaceMatrix;
            }
            else if (shadowCascadeIndex == 3)
            {
                effect.LightSpaceMatrixFour = lightSpaceMatrix;
            }

            var boundingFrustum = new BoundingFrustum(lightSpaceMatrix);

            foreach (var (model, transform) in FrustumCull(boundingFrustum, modelTransforms))
            {
                foreach (var modelMesh in model.Meshes)
                {
                    foreach (var meshPart in modelMesh.MeshParts)
                    {
                        GraphicsDevice.SetVertexBuffer(meshPart.VertexBuffer);
                        GraphicsDevice.Indices = meshPart.IndexBuffer;

                        SimpleDepthEffect.Model = transform;

                        foreach (var pass in SimpleDepthEffect.CurrentTechnique.Passes)
                        {
                            pass.Apply();

                            GraphicsDevice.DrawIndexedPrimitives(
                                PrimitiveType.TriangleList,
                                0,
                                0,
                                meshPart.VertexBuffer.VertexCount,
                                0,
                                meshPart.Triangles.Length
                            );
                        }
                    }
                }
            }
        }

        private void RenderPointShadows(
            PerspectiveCamera camera,
            IEnumerable<(Model, Matrix)> modelTransforms,
            PointLight pointLight
        ) {
            GraphicsDevice.DepthStencilState = DepthStencilState.Default;
            GraphicsDevice.BlendState = BlendState.Opaque;

            LinearDepthEffect.Projection = Matrix.CreatePerspectiveFieldOfView(
                MathHelper.PiOver2,
                1,
                0.1f,
                25f // FIXME: magic value
            );
            LinearDepthEffect.FarPlane = 25f;
            LinearDepthEffect.LightPosition = pointLight.Position;

            foreach (CubeMapFace face in Enum.GetValues(typeof(CubeMapFace)))
            {
                GraphicsDevice.SetRenderTarget(PointShadowCubeMap, face);

                Vector3 targetDirection;
                Vector3 targetUpDirection;
                switch(face)
                {
                    case CubeMapFace.PositiveX:
                        targetDirection = Vector3.Right;
                        targetUpDirection = Vector3.Up;
                        break;

                    case CubeMapFace.NegativeX:
                        targetDirection = Vector3.Left;
                        targetUpDirection = Vector3.Up;
                        break;

                    case CubeMapFace.PositiveY:
                        targetDirection = Vector3.Up;
                        targetUpDirection = Vector3.Forward;
                        break;

                    case CubeMapFace.NegativeY:
                        targetDirection = Vector3.Down;
                        targetUpDirection = Vector3.Backward;
                        break;

                    case CubeMapFace.PositiveZ:
                        targetDirection = Vector3.Backward;
                        targetUpDirection = Vector3.Up;
                        break;

                    case CubeMapFace.NegativeZ:
                        targetDirection = Vector3.Forward;
                        targetUpDirection = Vector3.Up;
                        break;

                    default:
                        targetDirection = Vector3.Right;
                        targetUpDirection = Vector3.Up;
                        break;
                }

                LinearDepthEffect.View = Matrix.CreateLookAt(
                    pointLight.Position,
                    pointLight.Position + targetDirection,
                    targetUpDirection
                );

                var boundingFrustum = new BoundingFrustum(LinearDepthEffect.View * LinearDepthEffect.Projection);

                foreach (var (model, transform) in FrustumCull(boundingFrustum, modelTransforms))
                {
                    foreach (var modelMesh in model.Meshes)
                    {
                        foreach (var meshPart in modelMesh.MeshParts)
                        {
                            GraphicsDevice.SetVertexBuffer(meshPart.VertexBuffer);
                            GraphicsDevice.Indices = meshPart.IndexBuffer;

                            LinearDepthEffect.Model = transform;

                            foreach (var pass in LinearDepthEffect.CurrentTechnique.Passes)
                            {
                                pass.Apply();

                                GraphicsDevice.DrawIndexedPrimitives(
                                    PrimitiveType.TriangleList,
                                    0,
                                    0,
                                    meshPart.VertexBuffer.VertexCount,
                                    0,
                                    meshPart.Triangles.Length
                                );
                            }
                        }
                    }
                }
            }
        }

        private static IEnumerable<(T, Matrix)> FrustumCull<T>(
            BoundingFrustum boundingFrustum,
            IEnumerable<(T, Matrix)> cullableTransforms
        ) where T : ICullable {
            foreach (var (cullable, transform) in cullableTransforms)
            {
                var boundingBox = TransformedBoundingBox(cullable.BoundingBox, transform);
                var containment = boundingFrustum.Contains(boundingBox);
                if (containment != ContainmentType.Disjoint)
                {
                    yield return (cullable, transform);
                }
            }
        }

        private static BoundingBox TransformedBoundingBox(BoundingBox boundingBox, Matrix matrix)
        {
            var center = (boundingBox.Min + boundingBox.Max) / 2f;
            var extent = (boundingBox.Max - boundingBox.Min) / 2f;

            var newCenter = Vector3.Transform(center, matrix);
            var newExtent = Vector3.TransformNormal(extent, AbsoluteMatrix(matrix));

            return new BoundingBox(newCenter - newExtent, newCenter + newExtent);
        }

        private static Matrix AbsoluteMatrix(Matrix matrix)
        {
            return new Matrix(
                Math.Abs(matrix.M11), Math.Abs(matrix.M12), Math.Abs(matrix.M13), Math.Abs(matrix.M14),
                Math.Abs(matrix.M21), Math.Abs(matrix.M22), Math.Abs(matrix.M23), Math.Abs(matrix.M24),
                Math.Abs(matrix.M31), Math.Abs(matrix.M32), Math.Abs(matrix.M33), Math.Abs(matrix.M34),
                Math.Abs(matrix.M41), Math.Abs(matrix.M42), Math.Abs(matrix.M43), Math.Abs(matrix.M44)
            );
        }
    }
}