107 lines
3.7 KiB
C#
107 lines
3.7 KiB
C#
using System.Globalization;
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using System.Security.Cryptography;
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using System.Text;
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using Elternbeirat.Contracts;
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namespace Elternbeirat.Web.Features.Events;
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/// <summary>
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/// Builds an iCalendar (RFC 5545) document from the events so visitors can
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/// subscribe to the calendar in their own app. Times are written as local
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/// wall-clock times without a time zone, which is what a school calendar needs:
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/// "19:30" should read as 19:30 everywhere.
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/// </summary>
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public static class IcsCalendar
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{
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private const string ProductId = "-//Elternbeirat IGMH//Termine//DE";
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/// <summary>
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/// Serialises the given entries into a single VCALENDAR document.
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/// </summary>
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public static string Build(IReadOnlyList<Event> events)
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{
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var sb = new StringBuilder();
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AppendLine(sb, "BEGIN:VCALENDAR");
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AppendLine(sb, "VERSION:2.0");
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AppendLine(sb, $"PRODID:{ProductId}");
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AppendLine(sb, "CALSCALE:GREGORIAN");
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AppendLine(sb, "METHOD:PUBLISH");
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AppendLine(sb, "X-WR-CALNAME:Elternbeirat IGMH");
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foreach (var ev in events)
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{
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AppendEvent(sb, ev);
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}
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AppendLine(sb, "END:VCALENDAR");
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return sb.ToString();
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}
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private static void AppendEvent(StringBuilder sb, Event ev)
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{
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AppendLine(sb, "BEGIN:VEVENT");
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AppendLine(sb, $"UID:{Uid(ev)}");
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// No live timestamp: a stable DTSTAMP keeps the feed byte-identical
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// between requests, so caches and clients do not see spurious changes.
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AppendLine(sb, "DTSTAMP:20000101T000000Z");
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if (ev.HasTime)
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{
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AppendLine(sb, $"DTSTART:{Local(ev.Start)}");
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if (ev.End is { } end)
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{
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AppendLine(sb, $"DTEND:{Local(end)}");
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}
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}
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else
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{
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AppendLine(sb, $"DTSTART;VALUE=DATE:{Date(ev.Start)}");
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// For an all-day event DTEND is exclusive: add one day so a single
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// day shows as one day, and a range covers its last day.
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var end = ev.End ?? ev.Start;
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AppendLine(sb, $"DTEND;VALUE=DATE:{Date(end.AddDays(1))}");
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}
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AppendLine(sb, $"SUMMARY:{Escape(ev.Title)}");
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if (!string.IsNullOrWhiteSpace(ev.Location))
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{
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AppendLine(sb, $"LOCATION:{Escape(ev.Location)}");
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}
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if (!string.IsNullOrWhiteSpace(ev.Note))
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{
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AppendLine(sb, $"DESCRIPTION:{Escape(ev.Note)}");
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}
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AppendLine(sb, "END:VEVENT");
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}
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/// <summary>
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/// A UID that stays the same as long as the entry's title and start do, so a
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/// re-subscribe updates the event instead of creating a duplicate.
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/// </summary>
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private static string Uid(Event ev)
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{
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var seed = $"{ev.Title}|{ev.Start:O}";
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var hash = SHA256.HashData(Encoding.UTF8.GetBytes(seed));
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return $"{Convert.ToHexString(hash)[..16]}@elternbeirat-igmh";
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}
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private static string Local(DateTime value) =>
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value.ToString("yyyyMMdd'T'HHmmss", CultureInfo.InvariantCulture);
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private static string Date(DateTime value) =>
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value.ToString("yyyyMMdd", CultureInfo.InvariantCulture);
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/// <summary>Escapes the characters that are special in an ICS text value.</summary>
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private static string Escape(string value) => value
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.Replace("\\", "\\\\", StringComparison.Ordinal)
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.Replace(";", "\\;", StringComparison.Ordinal)
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.Replace(",", "\\,", StringComparison.Ordinal)
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.Replace("\r\n", "\\n", StringComparison.Ordinal)
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.Replace("\n", "\\n", StringComparison.Ordinal);
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// ICS lines are terminated with CRLF regardless of platform.
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private static void AppendLine(StringBuilder sb, string line) =>
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sb.Append(line).Append("\r\n");
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}
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