hrynco-notification-service/HrynCo.NotificationService.Worker.Services/EmailProcessing/NotificationDeliveryErrorFormatter.cs

namespace HrynCo.NotificationService.Worker.Services.EmailProcessing;

using System.Net.Mail;
using System.Net.Sockets;

public static class NotificationDeliveryErrorFormatter
{
    private const int MaximumErrorLength = 2000;
    private const string FallbackError = "Notification delivery failed after all retry attempts.";

    public static string Format(Exception exception)
    {
        string technicalMessage = Normalize(exception.GetBaseException().Message);
        if (string.IsNullOrWhiteSpace(technicalMessage))
        {
            return FallbackError;
        }

        string contextualMessage = CreateContextualMessage(exception, technicalMessage);
        return contextualMessage.Length <= MaximumErrorLength
            ? contextualMessage
            : contextualMessage[..MaximumErrorLength];
    }

    private static string CreateContextualMessage(Exception exception, string technicalMessage)
    {
        if (FindException<SmtpFailedRecipientException>(exception) is not null)
        {
            return "SMTP delivery failed after all retry attempts: " +
                   "the SMTP server rejected the recipient address.";
        }

        SocketException? socketException = FindException<SocketException>(exception);
        if (socketException is not null)
        {
            string reason = socketException.SocketErrorCode switch
            {
                SocketError.HostNotFound or SocketError.NoData =>
                    "the configured SMTP server host could not be resolved",
                SocketError.ConnectionRefused =>
                    "the configured SMTP server refused the connection",
                SocketError.TimedOut =>
                    "the connection to the configured SMTP server timed out",
                _ when technicalMessage.Contains(
                    "Name or service not known",
                    StringComparison.OrdinalIgnoreCase) =>
                    "the configured SMTP server host could not be resolved",
                _ => "the configured SMTP server could not be reached"
            };

            return $"SMTP delivery failed after all retry attempts: {reason} ({technicalMessage}).";
        }

        if (FindException<SmtpException>(exception) is not null)
        {
            return $"SMTP delivery failed after all retry attempts ({technicalMessage}).";
        }

        return $"Notification delivery failed after all retry attempts ({technicalMessage}).";
    }

    private static TException? FindException<TException>(Exception exception)
        where TException : Exception
    {
        for (Exception? current = exception; current is not null; current = current.InnerException)
        {
            if (current is TException typedException)
            {
                return typedException;
            }
        }

        return null;
    }

    private static string Normalize(string message)
    {
        return message.ReplaceLineEndings(" ").Trim();
    }
}