using System.Globalization;
using System.Security.Cryptography;
using System.Text;
using Elternbeirat.Web.Services;

namespace Elternbeirat.Web.Features.Events;

/// <summary>
/// Builds an iCalendar (RFC 5545) document from the events so visitors can
/// subscribe to the calendar in their own app. Times are written as local
/// wall-clock times without a time zone, which is what a school calendar needs:
/// "19:30" should read as 19:30 everywhere.
/// </summary>
public static class IcsCalendar
{
    private const string ProductId = "-//Elternbeirat IGMH//Termine//DE";

    /// <summary>
    /// Serialises the given entries into a single VCALENDAR document.
    /// </summary>
    public static string Build(IReadOnlyList<Event> events)
    {
        var sb = new StringBuilder();
        AppendLine(sb, "BEGIN:VCALENDAR");
        AppendLine(sb, "VERSION:2.0");
        AppendLine(sb, $"PRODID:{ProductId}");
        AppendLine(sb, "CALSCALE:GREGORIAN");
        AppendLine(sb, "METHOD:PUBLISH");
        AppendLine(sb, "X-WR-CALNAME:Elternbeirat IGMH");

        foreach (var ev in events)
        {
            AppendEvent(sb, ev);
        }

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

    private static void AppendEvent(StringBuilder sb, Event ev)
    {
        AppendLine(sb, "BEGIN:VEVENT");
        AppendLine(sb, $"UID:{Uid(ev)}");
        // No live timestamp: a stable DTSTAMP keeps the feed byte-identical
        // between requests, so caches and clients do not see spurious changes.
        AppendLine(sb, "DTSTAMP:20000101T000000Z");

        if (ev.HasTime)
        {
            AppendLine(sb, $"DTSTART:{Local(ev.Start)}");
            if (ev.End is { } end)
            {
                AppendLine(sb, $"DTEND:{Local(end)}");
            }
        }
        else
        {
            AppendLine(sb, $"DTSTART;VALUE=DATE:{Date(ev.Start)}");
            // For an all-day event DTEND is exclusive: add one day so a single
            // day shows as one day, and a range covers its last day.
            var end = ev.End ?? ev.Start;
            AppendLine(sb, $"DTEND;VALUE=DATE:{Date(end.AddDays(1))}");
        }

        AppendLine(sb, $"SUMMARY:{Escape(ev.Title)}");
        if (!string.IsNullOrWhiteSpace(ev.Location))
        {
            AppendLine(sb, $"LOCATION:{Escape(ev.Location)}");
        }
        if (!string.IsNullOrWhiteSpace(ev.Note))
        {
            AppendLine(sb, $"DESCRIPTION:{Escape(ev.Note)}");
        }

        AppendLine(sb, "END:VEVENT");
    }

    /// <summary>
    /// A UID that stays the same as long as the entry's title and start do, so a
    /// re-subscribe updates the event instead of creating a duplicate.
    /// </summary>
    private static string Uid(Event ev)
    {
        var seed = $"{ev.Title}|{ev.Start:O}";
        var hash = SHA256.HashData(Encoding.UTF8.GetBytes(seed));
        return $"{Convert.ToHexString(hash)[..16]}@elternbeirat-igmh";
    }

    private static string Local(DateTime value) =>
        value.ToString("yyyyMMdd'T'HHmmss", CultureInfo.InvariantCulture);

    private static string Date(DateTime value) =>
        value.ToString("yyyyMMdd", CultureInfo.InvariantCulture);

    /// <summary>Escapes the characters that are special in an ICS text value.</summary>
    private static string Escape(string value) => value
        .Replace("\\", "\\\\", StringComparison.Ordinal)
        .Replace(";", "\\;", StringComparison.Ordinal)
        .Replace(",", "\\,", StringComparison.Ordinal)
        .Replace("\r\n", "\\n", StringComparison.Ordinal)
        .Replace("\n", "\\n", StringComparison.Ordinal);

    // ICS lines are terminated with CRLF regardless of platform.
    private static void AppendLine(StringBuilder sb, string line) =>
        sb.Append(line).Append("\r\n");
}