CompositeElement.cs

using System.Collections;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using OfficeFlow.DocumentObjectModel.Exceptions;

namespace OfficeFlow.DocumentObjectModel;

public abstract class CompositeElement : Element, IEnumerable<Element>
{
    private readonly ElementCollection _children;

    public int Count
        => _children.Count;
    
    public Element? FirstChild
        => _children.Head;

    public Element? LastChild
        => _children.Tail;

    protected CompositeElement()
        => _children = new ElementCollection(this);

    public void AppendChild(Element element)
        => _children.Append(element);

    public void InsertAfter(Element target, Element element)
        => _children.Insert(
            GetIndexOrThrow(target) + 1,
            element);

    public void InsertBefore(Element target, Element element)
        => _children.Insert(
            GetIndexOrThrow(target),
            element);

    public void PrependChild(Element element)
        => _children.Prepend(element);

    public IEnumerable<Element> GetDescendants()
        => GetDescendants<Element>();

    public IEnumerable<TElement> GetDescendants<TElement>()
        where TElement : Element
    {
        var descedants = new Stack<Element?>(_children);

        while (descedants.Count != 0)
        {
            var nextDescedant = descedants.Pop();

            if (nextDescedant is TElement element)
            {
                yield return element;
            }

            if (nextDescedant is not CompositeElement composite)
            {
                continue;
            }

            foreach (var child in composite)
            {
                descedants.Push(child);
            }
        }
    }

    public void RemoveChild(Element element)
        => _children.Remove(element);

    public void RemoveChildren()
        => _children.Clear();

    public IEnumerator<Element> GetEnumerator()
        => _children.GetEnumerator();

    [ExcludeFromCodeCoverage]
    IEnumerator IEnumerable.GetEnumerator()
        => GetEnumerator();

    private int GetIndexOrThrow(Element element)
    {
        var index = _children.IndexOf(element);

        if (index is -1)
        {
            throw new ElementNotFoundException(
                $"The specified element \"{element.GetType().Name}\" is not part of the current composite element \"{GetType().Name}\"");
        }

        return index;
    }
}