using Unity.Burst;
using Unity.Collections;
using Unity.Jobs;
using Unity.Mathematics;
using UnityEngine;
using UnityEngine.Rendering;

namespace DanieleMarotta.RiversongCodeShowcase
{
    [BurstCompile]
    public struct GenerateTerrainChunkJob : IJobParallelFor
    {
        public int2 WorldSize;

        [ReadOnly]
        public NativeArray<int2> ChunkCoordsArray;

        public int ChunkSize;

        [ReadOnly]
        public NativeArray<int> Heightmap;

        [NativeDisableParallelForRestriction]
        public Mesh.MeshDataArray MeshDataArray;

        public void Execute(int index)
        {
            var coords = ChunkCoordsArray[index];

            var vertexAttributes = new NativeArray<VertexAttributeDescriptor>(3, Allocator.Temp);
            vertexAttributes[0] = new VertexAttributeDescriptor(VertexAttribute.Position);
            vertexAttributes[1] = new VertexAttributeDescriptor(VertexAttribute.Normal);
            vertexAttributes[2] = new VertexAttributeDescriptor(VertexAttribute.TexCoord0, dimension: 2);

            var tileCount = ChunkSize * ChunkSize;
            var vertexTemp = new NativeList<Vertex>(20 * tileCount, Allocator.Temp);
            var indexTemp = new NativeList<int>(30 * tileCount, Allocator.Temp);

            var tileMin = coords * ChunkSize;
            var tileMax = math.min(tileMin + ChunkSize, WorldSize);
            var tile = int2.zero;

            for (tile.y = tileMin.y; tile.y < tileMax.y; tile.y++)
            for (tile.x = tileMin.x; tile.x < tileMax.x; tile.x++)
            {
                var h = Heightmap[math.mad(tile.y, WorldSize.x, tile.x)];
                if (h == 0) continue;

                var v0 = new Vertex(new float3(tile.x, h, tile.y), math.up(), float2.zero);
                var v1 = new Vertex(new float3(tile.x, h, tile.y + 1), math.up(), float2.zero);
                var v2 = new Vertex(new float3(tile.x + 1, h, tile.y + 1), math.up(), float2.zero);
                var v3 = new Vertex(new float3(tile.x + 1, h, tile.y), math.up(), float2.zero);
                WriteQuad(vertexTemp, indexTemp, v0, v1, v2, v3, false);
            }

            var cliffSubMeshIndexStart = indexTemp.Length;

            for (tile.y = tileMin.y; tile.y < tileMax.y && tile.y < WorldSize.y - 1; tile.y++)
            for (tile.x = tileMin.x; tile.x < tileMax.x && tile.x < WorldSize.x - 1; tile.x++)
            {
                var h = Heightmap[math.mad(tile.y, WorldSize.x, tile.x)];

                var hx = Heightmap[math.mad(tile.y, WorldSize.x, tile.x + 1)];
                if (h != hx)
                {
                    var hMin = math.min(h, hx);
                    var hMax = math.max(h, hx);

                    for (var y = hMin; y < hMax; y++)
                    {
                        var bottomUV = math.unlerp(hMin, hMax, y);
                        var topUV = math.unlerp(hMin, hMax, y + 1);

                        var normal = h > hx ? math.right() : math.left();
                        var v0 = new Vertex(new float3(tile.x + 1, y, tile.y), normal, bottomUV);
                        var v1 = new Vertex(new float3(tile.x + 1, y + 1, tile.y), normal, topUV);
                        var v2 = new Vertex(new float3(tile.x + 1, y + 1, tile.y + 1), normal, topUV);
                        var v3 = new Vertex(new float3(tile.x + 1, y, tile.y + 1), normal, bottomUV);
                        WriteQuad(vertexTemp, indexTemp, v0, v1, v2, v3, h < hx);
                    }
                }

                var hz = Heightmap[math.mad(tile.y + 1, WorldSize.x, tile.x)];
                if (h != hz)
                {
                    var hMin = math.min(h, hz);
                    var hMax = math.max(h, hz);

                    for (var y = hMin; y < hMax; y++)
                    {
                        var bottomUV = math.unlerp(hMin, hMax, y);
                        var topUV = math.unlerp(hMin, hMax, y + 1);

                        var normal = h > hz ? math.forward() : math.back();
                        var v0 = new Vertex(new float3(tile.x + 1, y, tile.y + 1), normal, bottomUV);
                        var v1 = new Vertex(new float3(tile.x + 1, y + 1, tile.y + 1), normal, topUV);
                        var v2 = new Vertex(new float3(tile.x, y + 1, tile.y + 1), normal, topUV);
                        var v3 = new Vertex(new float3(tile.x, y, tile.y + 1), normal, bottomUV);
                        WriteQuad(vertexTemp, indexTemp, v0, v1, v2, v3, h < hz);
                    }
                }
            }

            var mesh = MeshDataArray[index];

            mesh.SetVertexBufferParams(vertexTemp.Length, vertexAttributes);

            var chunkVertexData = mesh.GetVertexData<Vertex>();
            chunkVertexData.CopyFrom(vertexTemp.AsArray());

            mesh.SetIndexBufferParams(indexTemp.Length, IndexFormat.UInt32);
            var chunkIndexData = mesh.GetIndexData<int>();
            chunkIndexData.CopyFrom(indexTemp.AsArray());

            mesh.subMeshCount = indexTemp.Length > cliffSubMeshIndexStart ? 2 : 1;
            mesh.SetSubMesh(0, new SubMeshDescriptor(0, cliffSubMeshIndexStart));
            if (mesh.subMeshCount > 1) mesh.SetSubMesh(1, new SubMeshDescriptor(cliffSubMeshIndexStart, chunkIndexData.Length - cliffSubMeshIndexStart));
        }

        private void WriteQuad(NativeList<Vertex> vertices, NativeList<int> indices, in Vertex v0, in Vertex v1, in Vertex v2, in Vertex v3, bool flip)
        {
            var v = vertices.Length;

            vertices.AddNoResize(v0);
            vertices.AddNoResize(v1);
            vertices.AddNoResize(v2);
            vertices.AddNoResize(v3);

            var i0 = v + (flip ? 2 : 0);
            var i1 = v + 1;
            var i2 = v + (flip ? 0 : 2);
            var i3 = v + 3;

            indices.AddNoResize(i0);
            indices.AddNoResize(i1);
            indices.AddNoResize(i2);
            indices.AddNoResize(i2);
            indices.AddNoResize(i3);
            indices.AddNoResize(i0);
        }

        private struct Vertex
        {
            public float3 Position;

            public float3 Normal;

            public float2 TexCoord0;

            public Vertex(float3 position, float3 normal, float2 texCoord0)
            {
                Position = position;
                Normal = normal;
                TexCoord0 = texCoord0;
            }
        }
    }
}