com.unity.demoteam.digital-human/Runtime/MeshTools/LinkedIndexList.cs

using System;
using System.Collections;
using System.Collections.Generic;

[Serializable]
public struct LinkedIndexList
{
    public int head;
    public int size;
}

[Serializable]
public struct LinkedIndexItem
{
    public int next;
    public int prev;
    public int data;
}

[Serializable]
public struct LinkedIndexEnumerable : IEnumerable<int>
{
    public LinkedIndexItem[] items;
    public int headIndex;

    public LinkedIndexEnumerable(LinkedIndexItem[] items, int headIndex)
    {
        this.items = items;
        this.headIndex = headIndex;
    }

    public LinkedIndexEnumerator GetEnumerator()
    {
        return new LinkedIndexEnumerator(items, headIndex, -1);
    }

    IEnumerator<int> IEnumerable<int>.GetEnumerator()
    {
        return GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

[Serializable]
public struct LinkedIndexEnumerator : IEnumerator<int>
{
    public LinkedIndexItem[] items;
    public int headIndex;
    public int itemIndex;

    public LinkedIndexEnumerator(LinkedIndexItem[] items, int headIndex, int itemIndex)
    {
        this.items = items;
        this.headIndex = headIndex;
        this.itemIndex = itemIndex;
    }

    public int Current
    {
        get { return items[itemIndex].data; }
    }

    object IEnumerator.Current
    {
        get { return Current; }
    }

    public bool MoveNext()
    {
        if (itemIndex == -1)
        {
            itemIndex = headIndex;
            return (itemIndex != -1);//return true;
        }
        else
        {
            itemIndex = items[itemIndex].next;
            return (itemIndex != headIndex);
        }
    }

    public int ReadNext()
    {
        if (MoveNext())
            return Current;
        else
            return -1;
    }

    public void Reset()
    {
        itemIndex = headIndex;
    }

    public void Dispose()
    {
        // foo
    }
}