meeting-assistant/MeetingAssistant/Workflow/MeetingWorkflowTemplateRenderer.cs

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

namespace MeetingAssistant.Workflow;

internal sealed class MeetingWorkflowTemplateRenderer
{
    private static readonly Regex EmailAddressPattern = new(
        @"(?<![A-Za-z0-9.!#$%&'*+/=?^_`{|}~-])(?<local>[A-Za-z0-9.!#$%&'*+/=?^_`{|}~-]+)@(?<domain>(?:[A-Za-z0-9](?:[A-Za-z0-9-]{0,61}[A-Za-z0-9])?\.)+[A-Za-z]{2,63})(?![A-Za-z0-9-])",
        RegexOptions.Compiled | RegexOptions.CultureInvariant | RegexOptions.IgnoreCase);

    private readonly RazorLightEngine razorEngine = MeetingWorkflowRazorEngineFactory.Create();

    public async Task<string> RenderAsync(
        string template,
        MeetingWorkflowTemplateModel model)
    {
        if (!ContainsRazorTemplateSyntax(template))
        {
            return template;
        }

        var rendered = await razorEngine.CompileRenderStringAsync(
            Guid.NewGuid().ToString("N"),
            EscapeEmailAddressAtSigns(template),
            model);
        return WebUtility.HtmlDecode(rendered);
    }

    public static bool ContainsRazorTemplateSyntax(string value)
    {
        if (!value.Contains('@', StringComparison.Ordinal))
        {
            return false;
        }

        var emailAtSigns = EmailAddressPattern
            .Matches(value)
            .Select(match => match.Index + match.Value.IndexOf('@', StringComparison.Ordinal))
            .ToHashSet();

        for (var index = 0; index < value.Length; index++)
        {
            if (value[index] != '@')
            {
                continue;
            }

            if (index + 1 < value.Length && value[index + 1] == '@')
            {
                index++;
                continue;
            }

            if (emailAtSigns.Contains(index))
            {
                continue;
            }

            return true;
        }

        return false;
    }

    private static string EscapeEmailAddressAtSigns(string value)
    {
        if (!value.Contains('@', StringComparison.Ordinal))
        {
            return value;
        }

        return EmailAddressPattern.Replace(
            value,
            match => match.Value.Replace("@", "@@", StringComparison.Ordinal));
    }
}