Add post and event cards, shared calendar sheet and per-event ICS; rework past events and next-event card

Elternbeirat.Web/Shared/Markdown.cs

@@ -55,6 +55,12 @@ public static partial class Markdown
     /// </summary>
     public const string LoneLinkClass = "lone-link";
 
+    /// <summary>
+    ///     The default teaser length: about two lines on a card, enough to say what a
+    ///     post is about without turning the list into a wall of text.
+    /// </summary>
+    public const int TeaserLength = 160;
+
     private static readonly MarkdownPipeline Pipeline =
         new MarkdownPipelineBuilder()
             .UseCustomContainers()
@@ -120,6 +126,53 @@ public static partial class Markdown
                 ? UpToSentenceEnd(PlainText(inline)).TrimEnd(':', ' ')
                 : "";
 
+    /// <summary>
+    ///     Builds a short plain-text teaser from a Markdown text, e.g. for a post card.
+    /// </summary>
+    /// <param name="markdown">The editor's Markdown; may be <see langword="null"/>.</param>
+    /// <param name="maxLength">
+    ///     The longest teaser, in characters, before the ellipsis. Defaults to
+    ///     <see cref="TeaserLength"/>.
+    /// </param>
+    /// <returns>
+    ///     The text of all paragraphs (including those in lists, quotes and <c>:::</c>
+    ///     blocks), with all Markdown removed and whitespace collapsed. A longer text is
+    ///     cut at the last word boundary within <paramref name="maxLength"/> and ends
+    ///     in <c>…</c>. An empty string when there is no paragraph text at all.
+    /// </returns>
+    /// <exception cref="ArgumentOutOfRangeException">
+    ///     <paramref name="maxLength"/> is zero or negative.
+    /// </exception>
+    /// <remarks>
+    ///     Headings are skipped: on a card the title already heads the teaser. Unlike
+    ///     <see cref="FirstSentence"/> the teaser does not stop at the first sentence,
+    ///     because a post often opens with a short line ("Liebe Eltern,") that says
+    ///     nothing on its own.
+    ///     <para>
+    ///         Punctuation left dangling at the cut (<c>,</c>, <c>.</c>, <c>:</c> …) is
+    ///         dropped, so the teaser never ends in <c>,…</c>.
+    ///     </para>
+    /// </remarks>
+    /// <example>
+    ///     <code>
+    ///         Markdown.Teaser("## Rückblick\n\nDer **Basar** war ein voller Erfolg.", 20);
+    ///         // "Der Basar war ein…"
+    ///     </code>
+    /// </example>
+    public static string Teaser(string? markdown, int maxLength = TeaserLength) =>
+        maxLength <= 0
+            ? throw new ArgumentOutOfRangeException(nameof(maxLength), maxLength, "The teaser needs room for at least one character.")
+            : string.IsNullOrWhiteSpace(markdown)
+                ? ""
+                : Shorten(
+                    string.Join(
+                        ' ',
+                        Parse(markdown)
+                            .Descendants<ParagraphBlock>()
+                            .Select(paragraph => paragraph.Inline is { } inline ? PlainText(inline) : "")
+                            .Where(text => text.Length > 0)),
+                    maxLength);
+
     /// <summary>
     ///     Parses Markdown with the site's pipeline, for callers that take the document
     ///     apart before rendering it (see <see cref="Render"/>).
@@ -192,6 +245,26 @@ public static partial class Markdown
     private static string UpToSentenceEnd(string text) =>
         SentenceEnd().Match(text) is { Success: true } end ? text[..(end.Index + 1)] : text;
 
+    /// <summary>
+    ///     Cuts a text to a word boundary and marks the cut.
+    /// </summary>
+    /// <param name="text">Plain text.</param>
+    /// <param name="maxLength">The longest result before the ellipsis.</param>
+    /// <returns>
+    ///     The text unchanged if it fits; otherwise everything up to the last space
+    ///     within <paramref name="maxLength"/>, trailing punctuation removed, plus
+    ///     <c>…</c>. A single word longer than <paramref name="maxLength"/> is cut hard.
+    /// </returns>
+    /// <remarks>
+    ///     The space is searched one character past the limit: if the text breaks
+    ///     exactly at <paramref name="maxLength"/>, the last whole word still fits.
+    /// </remarks>
+    private static string Shorten(string text, int maxLength) =>
+        text.Length <= maxLength
+            ? text
+            : (text[..(maxLength + 1)].LastIndexOf(' ') is var space and > 0 ? text[..space] : text[..maxLength])
+              .TrimEnd(' ', ',', ';', ':', '.', '-', '–') + "…";
+
     /// <summary>
     ///     Matches the end mark of a sentence: <c>.</c>, <c>!</c> or <c>?</c> before a
     ///     space or the end, unless it follows a lone letter (<c>z.</c>) or a number