// https://docs.unity3d.com/560/Documentation/ScriptReference/Graphics.DrawMeshInstancedIndirect.html

using UnityEngine;

public class InstancedIndirectExample1 : MonoBehaviour
{
    public int instanceCount = 100000;
    public Mesh instanceMesh;
    public Material instanceMaterial;

    private int cachedInstanceCount = -1;
    private ComputeBuffer positionBuffer;
    private ComputeBuffer argsBuffer;
    private uint[] args = new uint[5] { 0, 0, 0, 0, 0 };

    void Start()
    {

        argsBuffer = new ComputeBuffer(1, args.Length * sizeof(uint), ComputeBufferType.IndirectArguments);
        UpdateBuffers();
    }

    void Update()
    {

        // Update starting position buffer
        if (cachedInstanceCount != instanceCount)
            UpdateBuffers();

        // Pad input
        //if (Input.GetAxisRaw("Horizontal") != 0.0f)
           // instanceCount = (int)Mathf.Clamp(instanceCount + Input.GetAxis("Horizontal") * 40000, 1.0f, 5000000.0f);

        // Render
        Graphics.DrawMeshInstancedIndirect(instanceMesh, 0, instanceMaterial, new Bounds(Vector3.zero, new Vector3(100.0f, 100.0f, 100.0f)), argsBuffer);
    }

    void OnGUI()
    {

        GUI.Label(new Rect(265, 25, 200, 30), "Instance Count: " + instanceCount.ToString());
        instanceCount = (int)GUI.HorizontalSlider(new Rect(25, 20, 200, 30), (float)instanceCount, 1.0f, 5000000.0f);
    }

    void UpdateBuffers()
    {

        // positions
        if (positionBuffer != null)
            positionBuffer.Release();
        positionBuffer = new ComputeBuffer(instanceCount, 16);
        Vector4[] positions = new Vector4[instanceCount];
        for (int i = 0; i < instanceCount; i++)
        {
            float angle = Random.Range(0.0f, Mathf.PI * 2.0f);
            float distance = Random.Range(20.0f, 100.0f);
            float height = Random.Range(-2.0f, 2.0f);
            float size = Random.Range(0.05f, 0.25f);
            positions[i] = new Vector4(Mathf.Sin(angle) * distance, height, Mathf.Cos(angle) * distance, size);
        }
        positionBuffer.SetData(positions);
        instanceMaterial.SetBuffer("positionBuffer", positionBuffer);

        // indirect args
        uint numIndices = (instanceMesh != null) ? (uint)instanceMesh.GetIndexCount(0) : 0;
        args[0] = numIndices;
        args[1] = (uint)instanceCount;
        argsBuffer.SetData(args);

        cachedInstanceCount = instanceCount;
    }

    void OnDisable()
    {

        if (positionBuffer != null)
            positionBuffer.Release();
        positionBuffer = null;

        if (argsBuffer != null)
            argsBuffer.Release();
        argsBuffer = null;
    }

}