HTML to plain text converter

HtmlToText.cs

//
// HtmlToText.cs
//
// Author: Jeffrey Stedfast <jeff@xamarin.com>
//
// Copyright (c) 2016 Xamarin Inc. (www.xamarin.com)
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//

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

using MimeKit.Text;

namespace HtmlToTextConverter {
	/// <summary>
	/// An HTML to plain text converter.
	/// </summary>
	/// <remarks>
	/// Used to convert HTML into plain text.
	/// </remarks>
	public class HtmlToText : TextConverter
	{
		/// <summary>
		/// Initializes a new instance of the <see cref="HtmlToText"/> class.
		/// </summary>
		/// <remarks>
		/// Creates a new HTML to plain text converter.
		/// </remarks>
		public HtmlToText ()
		{
		}

		/// <summary>
		/// Get the input format.
		/// </summary>
		/// <remarks>
		/// Gets the input format.
		/// </remarks>
		/// <value>The input format.</value>
		public override TextFormat InputFormat {
			get { return TextFormat.Html; }
		}

		/// <summary>
		/// Get the output format.
		/// </summary>
		/// <remarks>
		/// Gets the output format.
		/// </remarks>
		/// <value>The output format.</value>
		public override TextFormat OutputFormat {
			get { return TextFormat.Text; }
		}

		/// <summary>
		/// Get or set the text that will be appended to the end of the output.
		/// </summary>
		/// <remarks>
		/// <para>Gets or sets the text that will be appended to the end of the output.</para>
		/// <para>The footer must be set before conversion begins.</para>
		/// </remarks>
		/// <value>The footer.</value>
		public string Footer {
			get; set;
		}

		/// <summary>
		/// Get or set the footer format.
		/// </summary>
		/// <remarks>
		/// Gets or sets the footer format.
		/// </remarks>
		/// <value>The footer format.</value>
		public HeaderFooterFormat FooterFormat {
			get; set;
		}

		/// <summary>
		/// Get or set text that will be prepended to the beginning of the output.
		/// </summary>
		/// <remarks>
		/// <para>Gets or sets the text that will be prepended to the beginning of the output.</para>
		/// <para>The header must be set before conversion begins.</para>
		/// </remarks>
		/// <value>The header.</value>
		public string Header {
			get; set;
		}

		/// <summary>
		/// Get or set the header format.
		/// </summary>
		/// <remarks>
		/// Gets or sets the header format.
		/// </remarks>
		/// <value>The header format.</value>
		public HeaderFooterFormat HeaderFormat {
			get; set;
		}

		static void Push (ICollection<HtmlTagId> stack, HtmlTagId id)
		{
			if (id != HtmlTagId.Unknown)
				stack.Add (id);
		}

		static void Pop (IList<HtmlTagId> stack, HtmlTagId id)
		{
			if (id == HtmlTagId.Unknown)
				return;

			for (int i = stack.Count; i > 0; i--) {
				if (stack[i - 1] == id) {
					stack.RemoveAt (i - 1);
					return;
				}
			}
		}

		/// <summary>
		/// Convert the contents of <paramref name="reader"/> from the <see cref="InputFormat"/> to the
		/// <see cref="OutputFormat"/> and uses the <paramref name="writer"/> to write the resulting text.
		/// </summary>
		/// <remarks>
		/// Converts the contents of <paramref name="reader"/> from the <see cref="InputFormat"/> to the
		/// <see cref="OutputFormat"/> and uses the <paramref name="writer"/> to write the resulting text.
		/// </remarks>
		/// <param name="reader">The text reader.</param>
		/// <param name="writer">The text writer.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <para><paramref name="reader"/> is <c>null</c>.</para>
		/// <para>-or-</para>
		/// <para><paramref name="writer"/> is <c>null</c>.</para>
		/// </exception>
		public override void Convert (TextReader reader, TextWriter writer)
		{
			if (reader == null)
				throw new ArgumentNullException ("reader");

			if (writer == null)
				throw new ArgumentNullException ("writer");

			if (!string.IsNullOrEmpty (Header)) {
				if (HeaderFormat == HeaderFooterFormat.Html) {
					var converter = new HtmlToText ();

					using (var sr = new StringReader (Header))
						converter.Convert (sr, writer);
				} else {
					writer.Write (Header);
				}
			}

			var tokenizer = new HtmlTokenizer (reader);
			var stack = new List<HtmlTagId> ();
			HtmlToken token;

			while (tokenizer.ReadNextToken (out token)) {
				switch (token.Kind) {
				case HtmlTokenKind.Tag:
					var tag = (HtmlTagToken) token;

					if (tag.IsEmptyElement || tag.Id.IsEmptyElement ()) {
						if (tag.Id == HtmlTagId.Br || tag.Id == HtmlTagId.P)
							writer.WriteLine ();
					} else if (tag.IsEndTag) {
						if (tag.Id == HtmlTagId.P)
							writer.WriteLine ();
						
						Pop (stack, tag.Id);
					} else {
						if (tag.Id == HtmlTagId.P)
							writer.WriteLine ();

						Push (stack, tag.Id);
					}
					break;
				case HtmlTokenKind.Data:
					var data = (HtmlDataToken) token;

					if (stack.Count == 0)
						break;
					
					switch (stack[stack.Count - 1]) {
					case HtmlTagId.Head:
					case HtmlTagId.Title:
					case HtmlTagId.Meta:
					case HtmlTagId.Table:
					case HtmlTagId.TR:
						// ignore
						break;
					default:
						writer.Write (data.Data);
						break;
					}
					break;
				}
			}

			if (!string.IsNullOrEmpty (Footer)) {
				if (FooterFormat == HeaderFooterFormat.Html) {
					var converter = new HtmlToText ();

					using (var sr = new StringReader (Footer))
						converter.Convert (sr, writer);
				} else {
					writer.Write (Footer);
				}
			}
		}
	}
}