RandomInstancing.cs

using System.Collections;
using UnityEngine;
using System.Collections.Generic;
using UnityEngine.AI;

namespace Unity.AI.Navigation.Samples
{
    /// <summary>
    /// Fill 5x5 tiles around the local position procedurally by instantiating prefabs at random positions/orientations
    /// </summary>
    [DefaultExecutionOrder(-200)]
    public class RandomInstancing : MonoBehaviour
    {
        public GameObject m_Prefab;
        public int m_PoolSize = 250;
        public int m_InstancesPerTile = 10;
        public bool m_RandomPosition = true;
        public bool m_RandomOrientation = true;
        public float m_Height;

        public int m_BaseHash = 347652783;
        public float m_Size = 100.0f;

        [SerializeField]
        Transform parent;
        [SerializeField]
        NavMeshAgent trackedAgent;

        List<Transform> m_Instances = new List<Transform>();
        int m_Used;
        int m_LocX, m_LocZ;

        void Awake()
        {
            for (int i = 0; i < m_PoolSize; ++i)
            {
                var go = Instantiate(m_Prefab, Vector3.zero, Quaternion.identity, parent) as GameObject;
                go.SetActive(false);
                m_Instances.Add(go.transform);
            }
        }

        void OnEnable()
        {
            m_LocX = ~0;
            m_LocZ = ~0;
            UpdateInstances();
        }

        void OnDestroy()
        {
            for (int i = 0; i < m_Instances.Count; ++i)
            {
                if (m_Instances[i])
                    Destroy(m_Instances[i].gameObject);
            }
            m_Instances.Clear();
        }

        void Update()
        {
            if (trackedAgent.remainingDistance > 0.1f)
            {
                UpdateInstances();
            }
        }

        void UpdateInstances()
        {
            var x = (int)Mathf.Floor(transform.position.x / m_Size);
            var z = (int)Mathf.Floor(transform.position.z / m_Size);
            if (x == m_LocX && z == m_LocZ)
                return;

            m_LocX = x;
            m_LocZ = z;

            m_Used = 0;
            for (var i = x - 2; i <= x + 2; ++i)
            {
                for (var j = z - 2; j <= z + 2; ++j)
                {
                    var count = UpdateTileInstances(i, j);
                    if (count != m_InstancesPerTile)
                        return;
                }
            }

            // Deactivate the remaining active elements in the pool.
            // Here we assume all active elements are contiguous and first in the list.
            for (int i = m_Used; i < m_PoolSize && m_Instances[i].gameObject.activeSelf; ++i)
                m_Instances[i].gameObject.SetActive(false);
        }

        int UpdateTileInstances(int i, int j)
        {
            var seed = Hash2(i, j) ^ m_BaseHash;
            var count = System.Math.Min(m_InstancesPerTile, m_PoolSize - m_Used);
            for (var end = m_Used + count; m_Used < end; ++m_Used)
            {
                float x = 0;
                float y = 0;

                if (m_RandomPosition)
                {
                    x = Random(ref seed);
                    y = Random(ref seed);
                }
                var pos = new Vector3((i + x) * m_Size, m_Height, (j + y) * m_Size);

                if (m_RandomOrientation)
                {
                    float r = 360.0f * Random(ref seed);
                    m_Instances[m_Used].rotation = Quaternion.AngleAxis(r, Vector3.up);
                }
                m_Instances[m_Used].position = pos;
                m_Instances[m_Used].gameObject.SetActive(true);
            }

            if (count < m_InstancesPerTile)
                Debug.LogWarning("Pool exhausted", this);

            return count;
        }

        static int Hash2(int i, int j)
        {
            return (i * 73856093) ^ (j * 19349663);
        }

        static float Random(ref int seed)
        {
            seed = (seed ^ 123459876);
            var k = seed / 127773;
            seed = 16807 * (seed - k * 127773) - 2836 * k;
            if (seed < 0)
                seed = seed + 2147483647;
            float ran0 = seed * 1.0f / 2147483647.0f;
            seed = (seed ^ 123459876);
            return ran0;
        }
    }
}