using System.Text;
using System.Globalization;

namespace Socialize.Api.Modules.CalendarIntegrations.Services;

internal sealed record CalendarExportFeedEvent(
    string Uid,
    string Title,
    DateTimeOffset StartsAt,
    DateTimeOffset EndsAt,
    bool IsAllDay,
    string? Description,
    string? Url);

internal static class CalendarExportFeedBuilder
{
    public static string Build(string calendarName, IReadOnlyCollection<CalendarExportFeedEvent> events)
    {
        StringBuilder builder = new();
        builder.AppendLine("BEGIN:VCALENDAR");
        builder.AppendLine("VERSION:2.0");
        builder.AppendLine("PRODID:-//Socialize//User Work Calendar//EN");
        builder.AppendLine("CALSCALE:GREGORIAN");
        builder.AppendLine("METHOD:PUBLISH");
        AppendLineInvariant(builder, $"X-WR-CALNAME:{EscapeText(calendarName)}");

        foreach (CalendarExportFeedEvent feedEvent in events.OrderBy(calendarEvent => calendarEvent.StartsAt))
        {
            builder.AppendLine("BEGIN:VEVENT");
            AppendLineInvariant(builder, $"UID:{EscapeText(feedEvent.Uid)}");
            AppendLineInvariant(builder, $"DTSTAMP:{FormatUtc(DateTimeOffset.UtcNow)}");
            AppendLineInvariant(builder, $"SUMMARY:{EscapeText(feedEvent.Title)}");

            if (feedEvent.IsAllDay)
            {
                AppendLineInvariant(builder, $"DTSTART;VALUE=DATE:{FormatDate(feedEvent.StartsAt)}");
                AppendLineInvariant(builder, $"DTEND;VALUE=DATE:{FormatDate(feedEvent.EndsAt)}");
            }
            else
            {
                AppendLineInvariant(builder, $"DTSTART:{FormatUtc(feedEvent.StartsAt)}");
                AppendLineInvariant(builder, $"DTEND:{FormatUtc(feedEvent.EndsAt)}");
            }

            if (!string.IsNullOrWhiteSpace(feedEvent.Description))
            {
                AppendLineInvariant(builder, $"DESCRIPTION:{EscapeText(feedEvent.Description)}");
            }

            if (!string.IsNullOrWhiteSpace(feedEvent.Url))
            {
                AppendLineInvariant(builder, $"URL:{EscapeText(feedEvent.Url)}");
            }

            builder.AppendLine("END:VEVENT");
        }

        builder.AppendLine("END:VCALENDAR");
        return builder.ToString();
    }

    private static string FormatDate(DateTimeOffset value)
    {
        return value.ToString("yyyyMMdd", System.Globalization.CultureInfo.InvariantCulture);
    }

    private static string FormatUtc(DateTimeOffset value)
    {
        return value.UtcDateTime.ToString("yyyyMMdd'T'HHmmss'Z'", System.Globalization.CultureInfo.InvariantCulture);
    }

    private static string EscapeText(string value)
    {
        return value
            .Replace("\\", "\\\\", StringComparison.Ordinal)
            .Replace("\r\n", "\\n", StringComparison.Ordinal)
            .Replace("\n", "\\n", StringComparison.Ordinal)
            .Replace(";", "\\;", StringComparison.Ordinal)
            .Replace(",", "\\,", StringComparison.Ordinal);
    }

    private static void AppendLineInvariant(StringBuilder builder, FormattableString value)
    {
        builder.AppendLine(value.ToString(CultureInfo.InvariantCulture));
    }
}