// Copyright (c) Alexandre Mutel. All rights reserved.
// This file is licensed under the BSD-Clause 2 license. 
// See the license.txt file in the project root for more information.

using System;
using System.Collections.Generic;
using System.Diagnostics;
using Markdig.Syntax.Inlines;

namespace Markdig.Syntax
{
    /// <summary>
    /// Extensions for visiting <see cref="Block"/> or <see cref="Inline"/>
    /// </summary>
    public static class MarkdownObjectExtensions
    {
        /// <summary>
        /// Iterates over the descendant elements for the specified markdown element, including <see cref="Block"/> and <see cref="Inline"/>.
        /// <para>The descendant elements are returned in DFS-like order.</para>
        /// </summary>
        /// <param name="markdownObject">The markdown object.</param>
        /// <returns>An iteration over the descendant elements</returns>
        public static IEnumerable<MarkdownObject> Descendants(this MarkdownObject markdownObject)
        {
            Stack<MarkdownObject> stack = new Stack<MarkdownObject>();
            Stack<bool> pushStack = new Stack<bool>();

            stack.Push(markdownObject);
            pushStack.Push(false);

            while (stack.Count > 0)
            {
                var block = stack.Pop();
                if (pushStack.Pop()) yield return block;
                if (block is ContainerBlock containerBlock)
                {
                    int subBlockIndex = containerBlock.Count;
                    while (subBlockIndex-- > 0)
                    {
                        var subBlock = containerBlock[subBlockIndex];
                        if (subBlock is LeafBlock leafBlock)
                        {
                            if (leafBlock.Inline != null)
                            {
                                stack.Push(leafBlock.Inline);
                                pushStack.Push(false);
                            }
                        }
                        stack.Push(subBlock);
                        pushStack.Push(true);
                    }
                }
                else if (block is ContainerInline containerInline)
                {
                    var child = containerInline.LastChild;
                    while (child != null)
                    {
                        stack.Push(child);
                        pushStack.Push(true);
                        child = child.PreviousSibling;
                    }
                }
            }
        }

        /// <summary>
        /// Iterates over the descendant elements for the specified markdown element, including <see cref="Block"/> and <see cref="Inline"/> and filters by the type <typeparamref name="T"/>.
        /// <para>The descendant elements are returned in DFS-like order.</para>
        /// </summary>
        /// <typeparam name="T">Type to use for filtering the descendants</typeparam>
        /// <param name="markdownObject">The markdown object.</param>
        /// <returns>An iteration over the descendant elements</returns>
        public static IEnumerable<T> Descendants<T>(this MarkdownObject markdownObject) where T : MarkdownObject
        {
            if (typeof(T).IsSubclassOf(typeof(Block)))
            {
                if (markdownObject is ContainerBlock containerBlock && containerBlock.Count > 0)
                {
                    return BlockDescendantsInternal<T>(containerBlock);
                }
            }
            else // typeof(T).IsSubclassOf(typeof(Inline)))
            {
                if (markdownObject is ContainerBlock containerBlock)
                {
                    if (containerBlock.Count > 0)
                    {
                        return InlineDescendantsInternal<T>(containerBlock);
                    }
                }
                else if (markdownObject is ContainerInline containerInline && containerInline.FirstChild != null)
                {
                    return containerInline.FindDescendantsInternal<T>();
                }
            }

#if NET452
            return new T[0];
#else
            return Array.Empty<T>();
#endif
        }

        /// <summary>
        /// Iterates over the descendant elements for the specified markdown <see cref="Inline" /> element and filters by the type <typeparamref name="T"/>.
        /// </summary>
        /// <typeparam name="T">Type to use for filtering the descendants</typeparam>
        /// <param name="inline">The inline markdown object.</param>
        /// <returns>
        /// An iteration over the descendant elements
        /// </returns>
        public static IEnumerable<T> Descendants<T>(this ContainerInline inline) where T : Inline
            => inline.FindDescendants<T>();

        /// <summary>
        /// Iterates over the descendant elements for the specified markdown <see cref="Block" /> element and filters by the type <typeparamref name="T"/>.
        /// </summary>
        /// <typeparam name="T">Type to use for filtering the descendants</typeparam>
        /// <param name="block">The markdown object.</param>
        /// <returns>
        /// An iteration over the descendant elements
        /// </returns>
        public static IEnumerable<T> Descendants<T>(this ContainerBlock block) where T : Block
        {
            if (block != null && block.Count > 0)
            {
                return BlockDescendantsInternal<T>(block);
            }
            else
            {
#if NET452
                return new T[0];
#else
                return Array.Empty<T>();
#endif
            }
        }

        private static IEnumerable<T> BlockDescendantsInternal<T>(ContainerBlock block) where T : MarkdownObject
        {
            Debug.Assert(typeof(T).IsSubclassOf(typeof(Block)));

            Stack<Block> stack = new Stack<Block>();

            int childrenCount = block.Count;
            while (childrenCount-- > 0)
            {
                stack.Push(block[childrenCount]);
            }

            while (stack.Count > 0)
            {
                var subBlock = stack.Pop();
                if (subBlock is T subBlockT)
                {
                    yield return subBlockT;
                }

                if (subBlock is ContainerBlock subBlockContainer)
                {
                    childrenCount = subBlockContainer.Count;
                    while (childrenCount-- > 0)
                    {
                        stack.Push(subBlockContainer[childrenCount]);
                    }
                }
            }
        }

        private static IEnumerable<T> InlineDescendantsInternal<T>(ContainerBlock block) where T : MarkdownObject
        {
            Debug.Assert(typeof(T).IsSubclassOf(typeof(Inline)));

            foreach (MarkdownObject descendant in block.Descendants())
            {
                if (descendant is T descendantT)
                {
                    yield return descendantT;
                }
            }
        }
    }
}