Add events calendar with ICS feed and switch content naming to English

2026-09-21 16:22:56 +02:00
1 parent f24a4e73a2
commit 9459d2aeaa
@@ -0,0 +1,105 @@
+using System.Globalization;
+using System.Security.Cryptography;
+using System.Text;
+
+namespace Elternbeirat.Web.Services;
+
+/// <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].ToLowerInvariant()}@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("\\", "\\\\")
+        .Replace(";", "\\;")
+        .Replace(",", "\\,")
+        .Replace("\r\n", "\\n")
+        .Replace("\n", "\\n");
+
+    // ICS lines are terminated with CRLF regardless of platform.
+    private static void AppendLine(StringBuilder sb, string line) =>
+        sb.Append(line).Append("\r\n");
+}