using UnityEngine;
using UnityEngine.Rendering;

public class GPUGrassRenderer : MonoBehaviour
{
    public Mesh grassMesh;            // одна модель травинки (один submesh)
    public Material grassMaterial;    // shader має підтримувати GPU instancing
    public ComputeShader computeShader;

    public int countX = 256;
    public int countZ = 256;
    public float sizeX = 50f;
    public float sizeZ = 50f;
    public float height = 0f;

    GraphicsBuffer matricesBuffer;
    GraphicsBuffer argsBuffer;
    int totalCount;

    void Start()
    {
        totalCount = countX * countZ;

        // 1) create structured buffer for matrices (float4x4 = 16 floats = 64 bytes)
        matricesBuffer = new GraphicsBuffer(GraphicsBuffer.Target.Structured, totalCount, 16 * sizeof(float));
        // 2) args buffer (5 uints): indexCountPerInstance, instanceCount, startIndexLocation, baseVertexLocation, startInstanceLocation
        uint[] args = new uint[5] { (uint)grassMesh.GetIndexCount(0), (uint)totalCount, (uint)grassMesh.GetIndexStart(0), (uint)grassMesh.GetBaseVertex(0), 0u };
        argsBuffer = new GraphicsBuffer(GraphicsBuffer.Target.IndirectArguments, 1, sizeof(uint) * args.Length);
        argsBuffer.SetData(args);

        // dispatch compute to fill matrices
        int kernel = computeShader.FindKernel("CSMain");
        computeShader.SetInt("_CountX", countX);
        computeShader.SetInt("_CountZ", countZ);
        computeShader.SetFloat("_SizeX", sizeX);
        computeShader.SetFloat("_SizeZ", sizeZ);
        computeShader.SetFloat("_Height", height);
        computeShader.SetBuffer(kernel, "matrices", matricesBuffer);

        int threadGroups = Mathf.CeilToInt(totalCount / 64.0f);
        computeShader.Dispatch(kernel, threadGroups, 1, 1);

        // bind buffer to material for per-instance matrix access (if shader reads it)
        grassMaterial.SetBuffer("matrices", matricesBuffer);
    }

    void OnDisable()
    {
        matricesBuffer?.Dispose();
        argsBuffer?.Dispose();
    }

    void Update()
    {
        // set render params: use simple bounds so Unity can cull
        var bounds = new Bounds(transform.position, new Vector3(sizeX, 10f, sizeZ));
        RenderParams rparams = new RenderParams(grassMaterial);
        // the shader must use UNITY_INDIRECT_DRAW_ARGS / UnityIndirect.cginc if it needs instance id access

        // call indirect draw
        rparams.worldBounds = new Bounds(transform.position, new Vector3(sizeX, 10f, sizeZ));
        Graphics.RenderMeshIndirect(rparams, grassMesh, argsBuffer);
        // parameters: rparams, mesh, argsBuffer, commandCount=1, startCommand=0, worldBounds
    }
}