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HtmlFormatter.cs

using System.Net;
using System.Text.RegularExpressions;

namespace Nop.Services.Html;

/// 
/// Represents the HTML formatter implementation
/// 
public partial class HtmlFormatter : IHtmlFormatter
{
    #region Fields

    protected readonly IBBCodeHelper _bbCodeHelper;

    #endregion

    #region Ctor

    public HtmlFormatter(IBBCodeHelper bbCodeHelper)
    {
        _bbCodeHelper = bbCodeHelper;
    }

    #endregion

    #region Utilities

    /// 
    /// Ensure only allowed HTML tags
    /// 
    /// Text
    /// Sanitized text with all invalid tags removed
    protected static string EnsureOnlyAllowedHtml(string text)
    {
        if (string.IsNullOrEmpty(text))
            return string.Empty;

        const string allowedTags = "br,hr,b,i,u,a,div,ol,ul,li,blockquote,img,span,p,em,strong,font,pre,h1,h2,h3,h4,h5,h6,address,cite";

        var m = Regex.Matches(text, "<.*?>", RegexOptions.IgnoreCase);
            
        for (var i = m.Count - 1; i >= 0; i--)
        {
            var tag = text[(m[i].Index + 1)..(m[i].Index + m[i].Length)].Trim().ToLower();

            if (!IsValidTag(tag, allowedTags))
                text = text.Remove(m[i].Index, m[i].Length);
        }

        return text;
    }

    /// 
    /// Indicates whether the HTML tag is valid
    /// 
    /// HTML tag to check
    /// List of valid tags
    /// True - if the tag if valid, false otherwise
    protected static bool IsValidTag(string tag, string tags)
    {
        var allowedTags = tags.Split(',');
        if (tag.Contains("javascript", StringComparison.InvariantCultureIgnoreCase))
            return false;
        if (tag.Contains("vbscript", StringComparison.InvariantCultureIgnoreCase))
            return false;
        if (tag.Contains("onclick", StringComparison.InvariantCultureIgnoreCase))
            return false;

        var endChars = new[] { ' ', '>', '/', '\t' };

        var pos = tag.IndexOfAny(endChars, 1);
        if (pos > 0)
            tag = tag[0..pos];
        if (tag[0] == '/')
            tag = tag[1..^0];

        foreach (var aTag in allowedTags)
        {
            if (tag == aTag)
                return true;
        }

        return false;
    }

    #endregion

    #region Methods

    /// 
    /// Formats the text
    /// 
    /// Text
    /// A value indicating whether to strip tags
    /// A value indicating whether HTML is allowed
    /// A value indicating whether HTML is allowed
    /// A value indicating whether BBCode is allowed
    /// A value indicating whether to resolve links
    /// A value indicating whether to add "noFollow" tag
    /// Formatted text
    public virtual string FormatText(string text, bool stripTags,
        bool convertPlainTextToHtml, bool allowHtml,
        bool allowBBCode, bool resolveLinks, bool addNoFollowTag)
    {
        if (string.IsNullOrEmpty(text))
            return string.Empty;

        try
        {
            if (stripTags)
            {
                text = StripTags(text);
            }

            text = allowHtml ? EnsureOnlyAllowedHtml(text) : WebUtility.HtmlEncode(text);

            if (convertPlainTextToHtml)
            {
                text = ConvertPlainTextToHtml(text);
            }

            if (allowBBCode)
            {
                text = _bbCodeHelper.FormatText(text, true, true, true, true, true, true, true);
            }

            if (resolveLinks)
            {
                text = ResolveLinksHelper.FormatText(text);
            }

            if (addNoFollowTag)
            {
                //add noFollow tag. not implemented
            }
        }
        catch (Exception exc)
        {
            text = $"Text cannot be formatted. Error: {exc.Message}";
        }

        return text;
    }

    /// 
    /// Strips tags
    /// 
    /// Text
    /// Formatted text
    public virtual string StripTags(string text)
    {
        if (string.IsNullOrEmpty(text))
            return string.Empty;

        text = Regex.Replace(text, @"(>)(\r|\n)*(<)", "><");
        text = Regex.Replace(text, "(<[^>]*>)([^<]*)", "$2");
        text = Regex.Replace(text, "(&#x?[0-9]{2,4};|"|&| |<|>|€|©|®|‰|‡|†|‹|›|„|”|“|‚|’|‘|—|–|‏|‎|‍|‌| | | |˜|ˆ|Ÿ|š|Š)", "@");

        return text;
    }

    /// 
    /// Replace anchor text (remove a tag from the following URL Name and output only the string "Name")
    /// 
    /// Text
    /// Text
    public virtual string ReplaceAnchorTags(string text)
    {
        if (string.IsNullOrEmpty(text))
            return string.Empty;

        text = Regex.Replace(text, @"]+>([^<]*(?:(?!", "$1", RegexOptions.IgnoreCase);
        return text;
    }

    /// 
    /// Converts plain text to HTML
    /// 
    /// Text
    /// Formatted text
    public virtual string ConvertPlainTextToHtml(string text)
    {
        if (string.IsNullOrEmpty(text))
            return string.Empty;

        text = text.Replace("\r\n", "
");
        text = text.Replace("\r", "
");
        text = text.Replace("\n", "
");
        text = text.Replace("\t", "  ");
        text = text.Replace("  ", "  ");

        return text;
    }

    /// 
    /// Converts HTML to plain text
    /// 
    /// Text
    /// A value indicating whether to decode text
    /// A value indicating whether to replace anchor text (remove a tag from the following URL Name and output only the string "Name")
    /// Formatted text
    public virtual string ConvertHtmlToPlainText(string text,
        bool decode = false, bool replaceAnchorTags = false)
    {
        if (string.IsNullOrEmpty(text))
            return string.Empty;

        if (decode)
            text = WebUtility.HtmlDecode(text);

        text = text.Replace("
", "\n");
        text = text.Replace("
", "\n");
        text = text.Replace("
", "\n");
        text = text.Replace("  ", "\t");
        text = text.Replace("  ", "  ");

        if (replaceAnchorTags)
            text = ReplaceAnchorTags(text);

        return text;
    }

    #endregion
}