using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Xml.Linq;
using Ical.Net;
using Ical.Net.CalendarComponents;
using Ical.Net.DataTypes;
using Serilog;
using Wino.Core.Domain;
using Wino.Core.Domain.Enums;
using Wino.Core.Domain.Interfaces;
using Wino.Core.Domain.Models.Calendar;

namespace Wino.Services;

public sealed class CalDavClient : ICalDavClient
{
    private static readonly HttpMethod PropFindMethod = new("PROPFIND");
    private static readonly HttpMethod ReportMethod = new("REPORT");
    private static readonly HttpMethod PutMethod = HttpMethod.Put;
    private static readonly HttpMethod DeleteMethod = HttpMethod.Delete;
    private static readonly ILogger Logger = Log.ForContext<CalDavClient>();

    private readonly HttpClient _httpClient;

    public CalDavClient(HttpClient httpClient = null)
    {
        _httpClient = httpClient ?? new HttpClient();
    }

    public async Task<IReadOnlyList<CalDavCalendar>> DiscoverCalendarsAsync(
        CalDavConnectionSettings connectionSettings,
        CancellationToken cancellationToken = default)
    {
        ValidateConnectionSettings(connectionSettings);

        var principalUri = await DiscoverPrincipalUriAsync(connectionSettings, cancellationToken).ConfigureAwait(false);
        var homeSetUri = await DiscoverCalendarHomeSetUriAsync(connectionSettings, principalUri, cancellationToken).ConfigureAwait(false);

        var body = """
            <D:propfind xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav" xmlns:CS="http://calendarserver.org/ns/" xmlns:ICAL="http://apple.com/ns/ical/">
              <D:prop>
                <D:resourcetype />
                <D:displayname />
                <D:current-user-privilege-set />
                <CS:getctag />
                <D:sync-token />
                <C:calendar-description />
                <C:calendar-timezone />
                <C:supported-calendar-component-set />
                <C:schedule-calendar-transp />
                <ICAL:calendar-color />
                <ICAL:calendar-order />
              </D:prop>
            </D:propfind>
            """;

        var responseXml = await SendXmlAsync(
            connectionSettings,
            PropFindMethod,
            homeSetUri,
            depth: "1",
            body,
            cancellationToken).ConfigureAwait(false);

        var calendars = ParseCalendarCollection(responseXml, homeSetUri)
            .GroupBy(c => c.RemoteCalendarId, StringComparer.OrdinalIgnoreCase)
            .Select(g => g.First())
            .ToList();

        return calendars;
    }

    public async Task<IReadOnlyList<CalDavCalendarEvent>> GetCalendarEventsAsync(
        CalDavConnectionSettings connectionSettings,
        CalDavCalendar calendar,
        DateTimeOffset startUtc,
        DateTimeOffset endUtc,
        CancellationToken cancellationToken = default)
    {
        ValidateConnectionSettings(connectionSettings);

        if (calendar == null || string.IsNullOrWhiteSpace(calendar.RemoteCalendarId))
            return [];

        var calendarUri = new Uri(calendar.RemoteCalendarId);
        var startString = startUtc.UtcDateTime.ToString("yyyyMMdd'T'HHmmss'Z'");
        var endString = endUtc.UtcDateTime.ToString("yyyyMMdd'T'HHmmss'Z'");

        var body = $"""
            <C:calendar-query xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
              <D:prop>
                <D:getetag />
                <C:calendar-data />
              </D:prop>
              <C:filter>
                <C:comp-filter name="VCALENDAR">
                  <C:comp-filter name="VEVENT">
                    <C:time-range start="{startString}" end="{endString}" />
                  </C:comp-filter>
                </C:comp-filter>
              </C:filter>
            </C:calendar-query>
            """;

        var responseXml = await SendXmlAsync(
            connectionSettings,
            ReportMethod,
            calendarUri,
            depth: "1",
            body,
            cancellationToken).ConfigureAwait(false);

        var eventResponses = ParseEventResponses(responseXml, calendarUri);
        var result = new List<CalDavCalendarEvent>();

        foreach (var eventResponse in eventResponses)
        {
            result.AddRange(ParseCalendarData(
                eventResponse.CalendarData,
                eventResponse.Href,
                eventResponse.ETag,
                startUtc,
                endUtc));
        }

        // Ensure recurring parents are saved before child occurrences/exceptions.
        return result
            .OrderByDescending(e => e.IsSeriesMaster)
            .ThenBy(e => e.Start)
            .ToList();
    }

    public Task UpsertCalendarEventAsync(
        CalDavConnectionSettings connectionSettings,
        CalDavCalendar calendar,
        string remoteEventId,
        string icsContent,
        CancellationToken cancellationToken = default)
    {
        ValidateConnectionSettings(connectionSettings);

        if (calendar == null || string.IsNullOrWhiteSpace(calendar.RemoteCalendarId))
            throw new ArgumentException("Calendar remote ID is required for CalDAV writes.");

        if (string.IsNullOrWhiteSpace(remoteEventId))
            throw new ArgumentException("Remote event ID is required for CalDAV writes.");

        if (string.IsNullOrWhiteSpace(icsContent))
            throw new ArgumentException("ICS content is required for CalDAV writes.");

        var resourceUri = BuildEventResourceUri(calendar.RemoteCalendarId, remoteEventId);

        return SendAsync(
            connectionSettings,
            PutMethod,
            resourceUri,
            new StringContent(icsContent, Encoding.UTF8, "text/calendar"),
            cancellationToken);
    }

    public Task DeleteCalendarEventAsync(
        CalDavConnectionSettings connectionSettings,
        CalDavCalendar calendar,
        string remoteEventId,
        CancellationToken cancellationToken = default)
    {
        ValidateConnectionSettings(connectionSettings);

        if (calendar == null || string.IsNullOrWhiteSpace(calendar.RemoteCalendarId))
            throw new ArgumentException("Calendar remote ID is required for CalDAV deletes.");

        if (string.IsNullOrWhiteSpace(remoteEventId))
            throw new ArgumentException("Remote event ID is required for CalDAV deletes.");

        var resourceUri = BuildEventResourceUri(calendar.RemoteCalendarId, remoteEventId);

        return SendAsync(
            connectionSettings,
            DeleteMethod,
            resourceUri,
            null,
            cancellationToken);
    }

    private static void ValidateConnectionSettings(CalDavConnectionSettings connectionSettings)
    {
        if (connectionSettings?.ServiceUri == null)
            throw new ArgumentException("Service URI is required for CalDAV.");

        if (string.IsNullOrWhiteSpace(connectionSettings.Username))
            throw new ArgumentException("Username is required for CalDAV.");

        if (string.IsNullOrWhiteSpace(connectionSettings.Password))
            throw new ArgumentException("Password is required for CalDAV.");
    }

    private async Task<Uri> DiscoverPrincipalUriAsync(CalDavConnectionSettings connectionSettings, CancellationToken cancellationToken)
    {
        var body = """
            <D:propfind xmlns:D="DAV:">
              <D:prop>
                <D:current-user-principal />
              </D:prop>
            </D:propfind>
            """;

        var responseXml = await SendXmlAsync(
            connectionSettings,
            PropFindMethod,
            connectionSettings.ServiceUri,
            depth: "0",
            body,
            cancellationToken).ConfigureAwait(false);

        var principalHref = responseXml
            .Descendants()
            .FirstOrDefault(e => e.Name.LocalName == "current-user-principal")
            ?.Descendants()
            .FirstOrDefault(e => e.Name.LocalName == "href")
            ?.Value;

        return string.IsNullOrWhiteSpace(principalHref)
            ? connectionSettings.ServiceUri
            : CreateAbsoluteUri(connectionSettings.ServiceUri, principalHref);
    }

    private async Task<Uri> DiscoverCalendarHomeSetUriAsync(
        CalDavConnectionSettings connectionSettings,
        Uri principalUri,
        CancellationToken cancellationToken)
    {
        var body = """
            <D:propfind xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
              <D:prop>
                <C:calendar-home-set />
              </D:prop>
            </D:propfind>
            """;

        var responseXml = await SendXmlAsync(
            connectionSettings,
            PropFindMethod,
            principalUri,
            depth: "0",
            body,
            cancellationToken).ConfigureAwait(false);

        var homeSetHref = responseXml
            .Descendants()
            .FirstOrDefault(e => e.Name.LocalName == "calendar-home-set")
            ?.Descendants()
            .FirstOrDefault(e => e.Name.LocalName == "href")
            ?.Value;

        return string.IsNullOrWhiteSpace(homeSetHref)
            ? principalUri
            : CreateAbsoluteUri(principalUri, homeSetHref);
    }

    private async Task<XDocument> SendXmlAsync(
        CalDavConnectionSettings connectionSettings,
        HttpMethod method,
        Uri uri,
        string depth,
        string body,
        CancellationToken cancellationToken)
    {
        using var request = new HttpRequestMessage(method, uri);

        request.Headers.Authorization = new AuthenticationHeaderValue(
            "Basic",
            Convert.ToBase64String(Encoding.UTF8.GetBytes($"{connectionSettings.Username}:{connectionSettings.Password}")));
        request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));
        request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("text/xml"));
        request.Headers.Add("Depth", depth);
        request.Content = new StringContent(body, Encoding.UTF8, "application/xml");

        using var response = await _httpClient.SendAsync(request, cancellationToken).ConfigureAwait(false);

        if (response.StatusCode is HttpStatusCode.Unauthorized or HttpStatusCode.Forbidden)
        {
            throw new UnauthorizedAccessException("CalDAV authorization failed.");
        }

        if (!response.IsSuccessStatusCode && response.StatusCode != HttpStatusCode.MultiStatus)
        {
            var failureBody = await response.Content.ReadAsStringAsync(cancellationToken).ConfigureAwait(false);
            throw new HttpRequestException($"CalDAV request failed ({(int)response.StatusCode}): {failureBody}");
        }

        var xml = await response.Content.ReadAsStringAsync(cancellationToken).ConfigureAwait(false);

        if (string.IsNullOrWhiteSpace(xml))
            return new XDocument(new XElement("empty"));

        return XDocument.Parse(xml);
    }

    private async Task SendAsync(
        CalDavConnectionSettings connectionSettings,
        HttpMethod method,
        Uri uri,
        HttpContent content,
        CancellationToken cancellationToken)
    {
        using var request = new HttpRequestMessage(method, uri);
        request.Headers.Authorization = new AuthenticationHeaderValue(
            "Basic",
            Convert.ToBase64String(Encoding.UTF8.GetBytes($"{connectionSettings.Username}:{connectionSettings.Password}")));

        if (content != null)
            request.Content = content;

        using var response = await _httpClient.SendAsync(request, cancellationToken).ConfigureAwait(false);

        if (response.StatusCode is HttpStatusCode.Unauthorized or HttpStatusCode.Forbidden)
            throw new UnauthorizedAccessException("CalDAV authorization failed.");

        if (!response.IsSuccessStatusCode)
        {
            var failureBody = await response.Content.ReadAsStringAsync(cancellationToken).ConfigureAwait(false);
            throw new HttpRequestException($"CalDAV request failed ({(int)response.StatusCode}): {failureBody}");
        }
    }

    private static List<CalDavCalendar> ParseCalendarCollection(XDocument xml, Uri baseUri)
    {
        var result = new List<CalDavCalendar>();

        foreach (var response in xml.Descendants().Where(e => e.Name.LocalName == "response"))
        {
            var href = response.Descendants().FirstOrDefault(e => e.Name.LocalName == "href")?.Value;
            if (string.IsNullOrWhiteSpace(href))
                continue;

            foreach (var prop in GetSuccessProps(response))
            {
                var resourceType = prop.Descendants().FirstOrDefault(e => e.Name.LocalName == "resourcetype");
                if (resourceType == null)
                    continue;

                var isCalendar = resourceType.Descendants().Any(e => e.Name.LocalName == "calendar");
                if (!isCalendar)
                    continue;

                var displayName = prop.Descendants().FirstOrDefault(e => e.Name.LocalName == "displayname")?.Value ?? string.Empty;
                var description = prop.Descendants().FirstOrDefault(e => e.Name.LocalName == "calendar-description")?.Value ?? string.Empty;
                var ctag = prop.Descendants().FirstOrDefault(e => e.Name.LocalName == "getctag")?.Value ?? string.Empty;
                var syncToken = prop.Descendants().FirstOrDefault(e => e.Name.LocalName == "sync-token")?.Value ?? string.Empty;
                var timeZone = ExtractCalendarTimeZoneId(
                    prop.Descendants().FirstOrDefault(e => e.Name.LocalName == "calendar-timezone")?.Value);
                var backgroundColor = NormalizeCalendarColor(
                    prop.Descendants().FirstOrDefault(e => e.Name.LocalName == "calendar-color")?.Value);
                var supportedComponents = prop
                    .Descendants()
                    .Where(e => e.Name.LocalName == "supported-calendar-component-set")
                    .Descendants()
                    .Where(e => e.Name.LocalName == "comp")
                    .Select(e => e.Attribute("name")?.Value?.Trim())
                    .Where(value => !string.IsNullOrWhiteSpace(value))
                    .ToList();
                var supportsEvents = supportedComponents.Count == 0 ||
                                     supportedComponents.Contains("VEVENT", StringComparer.OrdinalIgnoreCase);
                var isReadOnly = IsCalendarReadOnly(prop);
                var defaultShowAs = GetDefaultShowAs(prop);
                var calendarOrder = ParseCalendarOrder(
                    prop.Descendants().FirstOrDefault(e => e.Name.LocalName == "calendar-order")?.Value);
                var remoteUri = CreateAbsoluteUri(baseUri, href).ToString().TrimEnd('/');

                if (!supportsEvents)
                    continue;

                if (string.IsNullOrWhiteSpace(displayName))
                {
                    displayName = WebUtility.UrlDecode(remoteUri.Split('/').LastOrDefault() ?? "Calendar");
                }

                result.Add(new CalDavCalendar
                {
                    RemoteCalendarId = remoteUri,
                    Name = displayName,
                    Description = description,
                    CTag = ctag,
                    SyncToken = syncToken,
                    TimeZone = timeZone,
                    BackgroundColorHex = backgroundColor,
                    IsReadOnly = isReadOnly,
                    SupportsEvents = supportsEvents,
                    DefaultShowAs = defaultShowAs,
                    Order = calendarOrder
                });
            }
        }

        return result;
    }

    private static IEnumerable<CalDavEventResponse> ParseEventResponses(XDocument xml, Uri baseUri)
    {
        foreach (var response in xml.Descendants().Where(e => e.Name.LocalName == "response"))
        {
            var href = response.Descendants().FirstOrDefault(e => e.Name.LocalName == "href")?.Value;
            if (string.IsNullOrWhiteSpace(href))
                continue;

            foreach (var prop in GetSuccessProps(response))
            {
                var calendarData = prop.Descendants().FirstOrDefault(e => e.Name.LocalName == "calendar-data")?.Value;
                if (string.IsNullOrWhiteSpace(calendarData))
                    continue;

                var eTag = prop.Descendants().FirstOrDefault(e => e.Name.LocalName == "getetag")?.Value ?? string.Empty;

                yield return new CalDavEventResponse(
                    CreateAbsoluteUri(baseUri, href).ToString(),
                    eTag,
                    calendarData);
            }
        }
    }

    private static IEnumerable<XElement> GetSuccessProps(XElement response)
    {
        foreach (var propstat in response.Elements().Where(e => e.Name.LocalName == "propstat"))
        {
            var status = propstat.Elements().FirstOrDefault(e => e.Name.LocalName == "status")?.Value ?? string.Empty;
            if (!status.Contains(" 200 ", StringComparison.Ordinal))
                continue;

            var prop = propstat.Elements().FirstOrDefault(e => e.Name.LocalName == "prop");
            if (prop != null)
                yield return prop;
        }
    }

    private static List<CalDavCalendarEvent> ParseCalendarData(
        string icsContent,
        string resourceHref,
        string eTag,
        DateTimeOffset windowStartUtc,
        DateTimeOffset windowEndUtc)
    {
        try
        {
            var calendar = Calendar.Load(icsContent);
            if (calendar?.Events == null || calendar.Events.Count == 0)
                return [];

            var allEvents = calendar.Events.ToList();
            var result = new List<CalDavCalendarEvent>();

            var masters = allEvents
                .Where(e => e != null && !string.IsNullOrWhiteSpace(e.Uid) && GetRecurrenceId(e) == null)
                .GroupBy(e => e.Uid, StringComparer.OrdinalIgnoreCase)
                .Select(g => g.First())
                .ToList();

            var exceptionMap = allEvents
                .Where(e => e != null && !string.IsNullOrWhiteSpace(e.Uid) && GetRecurrenceId(e) != null)
                .GroupBy(e => $"{e.Uid}|{GetOccurrenceKey(GetRecurrenceId(e))}", StringComparer.OrdinalIgnoreCase)
                .ToDictionary(g => g.Key, g => g.First(), StringComparer.OrdinalIgnoreCase);

            var consumedExceptions = new HashSet<string>(StringComparer.OrdinalIgnoreCase);

            foreach (var master in masters)
            {
                var masterRemoteId = BuildRemoteEventId(master.Uid, null);
                var hasRecurrence = HasRecurrence(master);

                if (hasRecurrence)
                {
                    result.Add(CreateCalendarEvent(
                        sourceEvent: master,
                        start: ToDateTimeOffset(master.Start),
                        end: ToDateTimeOffset(master.End),
                        remoteEventId: masterRemoteId,
                        resourceHref: resourceHref,
                        eTag: eTag,
                        icsContent: icsContent,
                        isSeriesMaster: true,
                        isRecurringInstance: false,
                        seriesMasterRemoteEventId: string.Empty,
                        recurrence: BuildRecurrenceString(master)));

                    var occurrences = master
                        .GetOccurrences(
                            new CalDateTime(windowStartUtc.UtcDateTime, true),
                            new Ical.Net.Evaluation.EvaluationOptions())
                        .Where(o => Overlaps(
                            ToDateTimeOffset(o.Period.StartTime),
                            ToDateTimeOffset(o.Period.EndTime),
                            windowStartUtc,
                            windowEndUtc));

                    foreach (var occurrence in occurrences)
                    {
                        var key = GetOccurrenceKey(occurrence.Period.StartTime);
                        var mapKey = $"{master.Uid}|{key}";

                        var sourceEvent = exceptionMap.TryGetValue(mapKey, out var exceptionEvent)
                            ? exceptionEvent
                            : master;

                        if (exceptionEvent != null)
                            consumedExceptions.Add(mapKey);

                        var occurrenceStart = ToDateTimeOffset(occurrence.Period.StartTime);
                        var occurrenceEnd = ToDateTimeOffset(occurrence.Period.EndTime);

                        result.Add(CreateCalendarEvent(
                            sourceEvent: sourceEvent,
                            start: occurrenceStart,
                            end: occurrenceEnd,
                            remoteEventId: BuildRemoteEventId(master.Uid, key),
                            resourceHref: resourceHref,
                            eTag: eTag,
                            icsContent: icsContent,
                            isSeriesMaster: false,
                            isRecurringInstance: true,
                            seriesMasterRemoteEventId: masterRemoteId,
                            recurrence: string.Empty));
                    }
                }
                else
                {
                    var start = ToDateTimeOffset(master.Start);
                    var end = ToDateTimeOffset(master.End);

                    if (!Overlaps(start, end, windowStartUtc, windowEndUtc))
                        continue;

                    result.Add(CreateCalendarEvent(
                        sourceEvent: master,
                        start: start,
                        end: end,
                        remoteEventId: masterRemoteId,
                        resourceHref: resourceHref,
                        eTag: eTag,
                        icsContent: icsContent,
                        isSeriesMaster: false,
                        isRecurringInstance: false,
                        seriesMasterRemoteEventId: string.Empty,
                        recurrence: string.Empty));
                }
            }

            foreach (var exceptionEvent in allEvents.Where(e => e != null && GetRecurrenceId(e) != null && !string.IsNullOrWhiteSpace(e.Uid)))
            {
                var recurrenceId = GetRecurrenceId(exceptionEvent);
                var key = $"{exceptionEvent.Uid}|{GetOccurrenceKey(recurrenceId)}";
                if (consumedExceptions.Contains(key))
                    continue;

                var start = ToDateTimeOffset(exceptionEvent.Start);
                var end = ToDateTimeOffset(exceptionEvent.End);

                if (!Overlaps(start, end, windowStartUtc, windowEndUtc))
                    continue;

                var masterRemoteId = BuildRemoteEventId(exceptionEvent.Uid, null);

                result.Add(CreateCalendarEvent(
                    sourceEvent: exceptionEvent,
                    start: start,
                    end: end,
                    remoteEventId: BuildRemoteEventId(exceptionEvent.Uid, GetOccurrenceKey(recurrenceId)),
                    resourceHref: resourceHref,
                    eTag: eTag,
                    icsContent: icsContent,
                    isSeriesMaster: false,
                    isRecurringInstance: true,
                    seriesMasterRemoteEventId: masterRemoteId,
                    recurrence: string.Empty));
            }

            return result;
        }
        catch (Exception ex)
        {
            Logger.Warning(ex, "Failed to parse CalDAV ICS payload.");
            return [];
        }
    }

    private static bool HasRecurrence(CalendarEvent calendarEvent)
        => (calendarEvent.RecurrenceRules?.Any() ?? false)
           || (calendarEvent.RecurrenceDates?.GetAllPeriods().Any() ?? false);

    private static string BuildRemoteEventId(string uid, string occurrenceKey)
        => string.IsNullOrWhiteSpace(occurrenceKey) ? uid : $"{uid}::{occurrenceKey}";

    private static CalDateTime GetRecurrenceId(CalendarEvent calendarEvent)
        => calendarEvent?.RecurrenceIdentifier?.StartTime;

    private static string GetOccurrenceKey(CalDateTime dateTime)
        => dateTime.AsUtc.ToString("yyyyMMdd'T'HHmmss'Z'");

    private static DateTimeOffset ToDateTimeOffset(CalDateTime dateTime)
    {
        if (dateTime == null)
            return default;

        if (dateTime.IsFloating)
        {
            var floatingValue = DateTime.SpecifyKind(dateTime.Value, DateTimeKind.Unspecified);
            return new DateTimeOffset(floatingValue, TimeZoneInfo.Local.GetUtcOffset(floatingValue));
        }

        return new DateTimeOffset(DateTime.SpecifyKind(dateTime.AsUtc, DateTimeKind.Utc));
    }

    private static bool Overlaps(DateTimeOffset start, DateTimeOffset end, DateTimeOffset windowStart, DateTimeOffset windowEnd)
    {
        if (end <= start)
            end = start.AddHours(1);

        return start < windowEnd && end > windowStart;
    }

    private static CalDavCalendarEvent CreateCalendarEvent(
        CalendarEvent sourceEvent,
        DateTimeOffset start,
        DateTimeOffset end,
        string remoteEventId,
        string resourceHref,
        string eTag,
        string icsContent,
        bool isSeriesMaster,
        bool isRecurringInstance,
        string seriesMasterRemoteEventId,
        string recurrence)
    {
        if (end <= start)
            end = start.AddHours(1);

        var status = MapStatus(sourceEvent.Status);
        var organizerEmail = NormalizeCalendarEmail(sourceEvent.Organizer?.Value);
        var organizerDisplayName = NormalizeOrganizerDisplayName(sourceEvent.Organizer?.CommonName, organizerEmail);
        var attendees = sourceEvent.Attendees?
            .Where(a => a != null && a.Value != null)
            .Select(a => new CalDavEventAttendee
            {
                Email = NormalizeCalendarEmail(a.Value),
                Name = NormalizeAttendeeName(a.CommonName, NormalizeCalendarEmail(a.Value)),
                AttendenceStatus = MapAttendeeStatus(a.ParticipationStatus),
                IsOrganizer = string.Equals(a.Role, "CHAIR", StringComparison.OrdinalIgnoreCase),
                IsOptionalAttendee = string.Equals(a.Role, "OPT-PARTICIPANT", StringComparison.OrdinalIgnoreCase)
            })
            .Where(a => !string.IsNullOrWhiteSpace(a.Email))
            .ToList() ?? [];

        var reminders = sourceEvent.Alarms?
            .Where(a => a?.Trigger != null && a.Trigger.IsRelative && a.Trigger.Duration.HasValue)
            .Select(a => new CalDavEventReminder
            {
                DurationInSeconds = (int)Math.Abs(a.Trigger.Duration.Value.ToTimeSpanUnspecified().TotalSeconds),
                ReminderType = string.Equals(a.Action, "EMAIL", StringComparison.OrdinalIgnoreCase)
                    ? CalendarItemReminderType.Email
                    : CalendarItemReminderType.Popup
            })
            .Where(r => r.DurationInSeconds > 0)
            .ToList() ?? [];

        return new CalDavCalendarEvent
        {
            RemoteEventId = remoteEventId,
            RemoteResourceHref = resourceHref,
            ETag = eTag,
            IcsContent = icsContent,
            Uid = sourceEvent.Uid ?? string.Empty,
            SeriesMasterRemoteEventId = seriesMasterRemoteEventId,
            IsSeriesMaster = isSeriesMaster,
            IsRecurringInstance = isRecurringInstance,
            Title = sourceEvent.Summary ?? string.Empty,
            Description = sourceEvent.Description ?? string.Empty,
            Location = sourceEvent.Location ?? string.Empty,
            Start = start,
            End = end,
            StartTimeZone = ResolveTimeZoneId(sourceEvent.Start, start),
            EndTimeZone = ResolveTimeZoneId(sourceEvent.End, end),
            Recurrence = recurrence,
            OrganizerDisplayName = organizerDisplayName,
            OrganizerEmail = organizerEmail,
            Status = status,
            Visibility = MapVisibility(sourceEvent.Class),
            ShowAs = MapShowAs(sourceEvent.Transparency),
            IsHidden = status == CalendarItemStatus.Cancelled,
            Attendees = attendees,
            Reminders = reminders
        };
    }

    private static string ResolveTimeZoneId(CalDateTime sourceDateTime, DateTimeOffset parsedDateTime)
    {
        var explicitTimeZoneId = sourceDateTime?.TzId;
        if (!string.IsNullOrWhiteSpace(explicitTimeZoneId))
            return explicitTimeZoneId;

        // Explicit UTC values usually don't carry TZID in CalDAV payloads.
        // Preserve UTC so downstream local-time conversion stays correct.
        if (parsedDateTime != default && parsedDateTime.Offset == TimeSpan.Zero)
            return TimeZoneInfo.Utc.Id;

        // Floating times without TZID should remain floating.
        return string.Empty;
    }

    private static string BuildRecurrenceString(CalendarEvent sourceEvent)
    {
        var recurrenceLines = new List<string>();

        if (sourceEvent.RecurrenceRules != null)
        {
            recurrenceLines.AddRange(sourceEvent.RecurrenceRules.Select(r => $"RRULE:{r}"));
        }

        if (sourceEvent.ExceptionDates != null)
        {
            var dates = sourceEvent.ExceptionDates
                .GetAllDates()
                .Where(d => d != null)
                .Select(d => d.AsUtc.ToString("yyyyMMdd'T'HHmmss'Z'"))
                .ToList();

            if (dates.Count > 0)
                recurrenceLines.Add($"EXDATE:{string.Join(",", dates)}");
        }

        if (sourceEvent.RecurrenceDates != null)
        {
            var dates = sourceEvent.RecurrenceDates
                .GetAllPeriods()
                .Where(p => p.StartTime != null)
                .Select(p => p.StartTime.AsUtc.ToString("yyyyMMdd'T'HHmmss'Z'"))
                .ToList();

            if (dates.Count > 0)
                recurrenceLines.Add($"RDATE:{string.Join(",", dates)}");
        }

        return recurrenceLines.Count == 0
            ? string.Empty
            : string.Join(Constants.CalendarEventRecurrenceRuleSeperator, recurrenceLines);
    }

    private static CalendarItemStatus MapStatus(string status)
    {
        if (string.IsNullOrWhiteSpace(status))
            return CalendarItemStatus.Accepted;

        if (string.Equals(status, "CANCELLED", StringComparison.OrdinalIgnoreCase))
            return CalendarItemStatus.Cancelled;

        if (string.Equals(status, "TENTATIVE", StringComparison.OrdinalIgnoreCase))
            return CalendarItemStatus.Tentative;

        return CalendarItemStatus.Accepted;
    }

    private static CalendarItemVisibility MapVisibility(string classValue)
    {
        if (string.IsNullOrWhiteSpace(classValue))
            return CalendarItemVisibility.Default;

        return classValue.ToUpperInvariant() switch
        {
            "PUBLIC" => CalendarItemVisibility.Public,
            "PRIVATE" => CalendarItemVisibility.Private,
            "CONFIDENTIAL" => CalendarItemVisibility.Confidential,
            _ => CalendarItemVisibility.Default
        };
    }

    private static CalendarItemShowAs MapShowAs(string transparency)
    {
        if (string.Equals(transparency, "TRANSPARENT", StringComparison.OrdinalIgnoreCase))
            return CalendarItemShowAs.Free;

        return CalendarItemShowAs.Busy;
    }

    private static AttendeeStatus MapAttendeeStatus(string participationStatus)
    {
        if (string.IsNullOrWhiteSpace(participationStatus))
            return AttendeeStatus.NeedsAction;

        return participationStatus.ToUpperInvariant() switch
        {
            "ACCEPTED" => AttendeeStatus.Accepted,
            "DECLINED" => AttendeeStatus.Declined,
            "TENTATIVE" => AttendeeStatus.Tentative,
            _ => AttendeeStatus.NeedsAction
        };
    }

    private static string NormalizeCalendarEmail(Uri emailUri)
    {
        if (emailUri == null)
            return string.Empty;

        var value = emailUri.OriginalString?.Trim();
        if (string.IsNullOrWhiteSpace(value))
            return string.Empty;

        if (value.StartsWith("mailto:", StringComparison.OrdinalIgnoreCase))
            value = value[7..].Trim();

        // CalDAV providers may use non-mail identifiers (urn:uuid, principal paths, etc.) here.
        // Keep only valid email-like values.
        if (!value.Contains('@'))
            return string.Empty;

        return value;
    }

    private static string NormalizeAttendeeName(string attendeeName, string attendeeEmail)
    {
        var normalizedName = NormalizeOrganizerDisplayName(attendeeName, attendeeEmail);
        return string.IsNullOrWhiteSpace(normalizedName) ? attendeeEmail : normalizedName;
    }

    private static string NormalizeOrganizerDisplayName(string organizerDisplayName, string organizerEmail)
    {
        var normalizedName = organizerDisplayName?.Trim() ?? string.Empty;

        if (string.IsNullOrWhiteSpace(normalizedName))
            return organizerEmail ?? string.Empty;

        if (LooksLikeOpaqueIdentifier(normalizedName))
            return organizerEmail ?? normalizedName;

        return normalizedName;
    }

    private static bool LooksLikeOpaqueIdentifier(string value)
    {
        if (string.IsNullOrWhiteSpace(value))
            return false;

        if (Guid.TryParse(value, out _))
            return true;

        if (value.StartsWith("urn:", StringComparison.OrdinalIgnoreCase))
            return true;

        if (value.Contains("/", StringComparison.Ordinal) && !value.Contains('@'))
            return true;

        return false;
    }

    private static bool IsCalendarReadOnly(XElement prop)
    {
        var privilegeSet = prop.Descendants().FirstOrDefault(e => e.Name.LocalName == "current-user-privilege-set");
        if (privilegeSet == null)
            return false;

        var privilegeNames = privilegeSet
            .Descendants()
            .Where(e => e.Name.LocalName == "privilege")
            .Descendants()
            .Select(e => e.Name.LocalName)
            .ToHashSet(StringComparer.OrdinalIgnoreCase);

        return !privilegeNames.Contains("all")
               && !privilegeNames.Contains("write")
               && !privilegeNames.Contains("write-content");
    }

    private static CalendarItemShowAs GetDefaultShowAs(XElement prop)
    {
        var transparency = prop.Descendants().FirstOrDefault(e => e.Name.LocalName == "schedule-calendar-transp");
        if (transparency?.Descendants().Any(e => e.Name.LocalName == "transparent") == true)
            return CalendarItemShowAs.Free;

        return CalendarItemShowAs.Busy;
    }

    private static double? ParseCalendarOrder(string value)
    {
        if (string.IsNullOrWhiteSpace(value))
            return null;

        return double.TryParse(value.Trim(), System.Globalization.NumberStyles.Float, System.Globalization.CultureInfo.InvariantCulture, out var result)
            ? result
            : null;
    }

    private static string ExtractCalendarTimeZoneId(string value)
    {
        if (string.IsNullOrWhiteSpace(value))
            return string.Empty;

        var unfoldedValue = UnfoldIcsText(TrimCommonIndentation(value));
        foreach (var rawLine in unfoldedValue.Split('\n', StringSplitOptions.RemoveEmptyEntries))
        {
            var line = rawLine.Trim();
            if (string.IsNullOrWhiteSpace(line))
                continue;

            var separatorIndex = line.IndexOf(':');
            if (separatorIndex <= 0)
                continue;

            var propertyName = line[..separatorIndex];
            var propertyValue = line[(separatorIndex + 1)..].Trim();

            if (propertyName.StartsWith("TZID", StringComparison.OrdinalIgnoreCase) && !string.IsNullOrWhiteSpace(propertyValue))
                return propertyValue;

            if (propertyName.StartsWith("X-WR-TIMEZONE", StringComparison.OrdinalIgnoreCase) && !string.IsNullOrWhiteSpace(propertyValue))
                return propertyValue;
        }

        return value.Trim();
    }

    private static string UnfoldIcsText(string value)
    {
        var normalizedValue = value
            .Replace("\r\n", "\n", StringComparison.Ordinal)
            .Replace('\r', '\n');
        var unfoldedLines = new List<string>();

        foreach (var rawLine in normalizedValue.Split('\n'))
        {
            if ((rawLine.StartsWith(' ') || rawLine.StartsWith('\t')) && unfoldedLines.Count > 0)
            {
                unfoldedLines[^1] += rawLine.TrimStart(' ', '\t');
                continue;
            }

            unfoldedLines.Add(rawLine);
        }

        return string.Join("\n", unfoldedLines);
    }

    private static string TrimCommonIndentation(string value)
    {
        var normalizedValue = value
            .Replace("\r\n", "\n", StringComparison.Ordinal)
            .Replace('\r', '\n');
        var lines = normalizedValue.Split('\n');
        var nonEmptyLines = lines
            .Where(line => !string.IsNullOrWhiteSpace(line))
            .ToList();

        if (nonEmptyLines.Count == 0)
            return normalizedValue;

        var commonIndentation = nonEmptyLines
            .Select(line => line.TakeWhile(ch => ch is ' ' or '\t').Count())
            .Min();

        if (commonIndentation <= 0)
            return normalizedValue;

        return string.Join(
            "\n",
            lines.Select(line =>
            {
                if (string.IsNullOrWhiteSpace(line))
                    return string.Empty;

                return line.Length >= commonIndentation
                    ? line[commonIndentation..]
                    : line.TrimStart(' ', '\t');
            }));
    }

    private static string NormalizeCalendarColor(string value)
    {
        if (string.IsNullOrWhiteSpace(value))
            return string.Empty;

        var color = value.Trim();
        if (color.StartsWith('#'))
        {
            color = color[1..];
        }

        if (color.Length == 8)
        {
            color = color[..6];
        }
        else if (color.Length == 3)
        {
            color = string.Concat(color.Select(c => $"{c}{c}"));
        }

        if (color.Length != 6 || !int.TryParse(color, System.Globalization.NumberStyles.HexNumber, System.Globalization.CultureInfo.InvariantCulture, out _))
            return string.Empty;

        return $"#{color.ToUpperInvariant()}";
    }

    private static Uri CreateAbsoluteUri(Uri baseUri, string href)
    {
        if (Uri.TryCreate(href, UriKind.Absolute, out var absolute))
            return absolute;

        return new Uri(baseUri, href);
    }

    private static Uri BuildEventResourceUri(string remoteCalendarId, string remoteEventId)
    {
        var calendarUri = new Uri($"{remoteCalendarId.TrimEnd('/')}/");
        var normalizedEventId = remoteEventId.Split(new[] { "::" }, StringSplitOptions.None)[0];
        var safeEventId = Uri.EscapeDataString(normalizedEventId);
        var fileName = safeEventId.EndsWith(".ics", StringComparison.OrdinalIgnoreCase)
            ? safeEventId
            : $"{safeEventId}.ics";

        return new Uri(calendarUri, fileName);
    }

    private sealed record CalDavEventResponse(string Href, string ETag, string CalendarData);
}