Vendor ical.js instead of loading it from unpkg.com

The upcoming events table pulled its ICS parser straight from a CDN with
`import ICAL from "https://unpkg.com/ical.js/dist/ical.min.js"`. That sends
every visitor of the start page to unpkg.com, which hands their IP address and
user agent to a third party before any of our own code runs. The URL is not
even pinned to a version, so whatever ical.js publishes next is executed on our
site without anybody looking at it, and the start page silently breaks when the
CDN is unreachable.

Check the parser into `assets/js/vendor/` and let Hugo bundle it. This is what
`js.Build` is for: it runs the esbuild that is built into Hugo, so it resolves
the import at build time and needs no node_modules and no extra tooling in the
build environment. The result is minified and fingerprinted like the other
scripts of the site, and the script tag carries a subresource integrity hash.

Since the bundle now has a content hash in its name, its URL cannot be written
by hand in the markdown any more. Move the table and the script tag into an
`upcoming` shortcode, which is the same pattern `calendar.html` already uses,
and move `upcoming.js` from `static/` to `assets/` so Hugo can process it.

The vendored file is the unminified `dist/ical.js` of the pinned release: it
carries the MPL-2.0 header and is the source form of what we ship, and Hugo
minifies it for delivery anyway. `assets/js/vendor/README.md` records the
version, where it came from and how to update it.

The built bundle renders the same table as before, checked against
https://berlin.ccc.de/calendars/all.ics.

Fixes: c28f04c6e8 ("switch to ics files; make calendars work; fix some minor issues")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

assets/js/upcoming.js

@@ -0,0 +1,259 @@
+import ICAL from "./vendor/ical.js";
+
+/**
+ * Read the URL of an event.
+ *
+ * ICAL.Event does not expose the URL property, so read it from the component.
+ *
+ * The value ends up in the href of a link, and the calendar is exported from a
+ * CalDAV server, so whoever may write to it decides what that value is. A
+ * "javascript:" URL there would run on our page as soon as a visitor clicks
+ * the event, so hand on nothing but http and https.
+ *
+ * @param {ICAL.Event} event The event to read the URL of
+ * @returns {string} The URL, empty when the event has none or it is not http(s)
+ */
+function eventUrl(event) {
+  const url = event.component.getFirstPropertyValue("url") ?? "";
+
+  if (!url) {
+    return "";
+  }
+
+  try {
+    // A relative URL is resolved against the page and keeps its scheme.
+    const { protocol } = new URL(url, document.baseURI);
+
+    return protocol === "http:" || protocol === "https:" ? url : "";
+  } catch {
+    // Not a URL at all.
+    return "";
+  }
+}
+
+/**
+ * Group the occurrences that were modified on their own by the UID of the event
+ * they belong to.
+ *
+ * Unless it is told which exceptions belong to an event, ICAL.Event relates
+ * every VEVENT with a RECURRENCE-ID in the file to every recurring event, and
+ * it keys them by the recurrence id alone. Two series that meet at the same
+ * time would therefore take over each other's modifications.
+ *
+ * @param {ICAL.Component[]} components The VEVENTs of the calendar
+ * @returns {Map<string, ICAL.Component[]>} The exceptions per UID
+ */
+function exceptionsByUid(components) {
+  const exceptions = new Map();
+
+  for (const component of components) {
+    if (!component.hasProperty("recurrence-id")) {
+      continue;
+    }
+
+    const uid = component.getFirstPropertyValue("uid");
+    const ofEvent = exceptions.get(uid);
+
+    if (ofEvent) {
+      ofEvent.push(component);
+    } else {
+      exceptions.set(uid, [component]);
+    }
+  }
+
+  return exceptions;
+}
+
+/**
+ * When an occurrence starts, as a point in time.
+ *
+ * A date has no time and no zone, its digits are the day itself. toJSDate()
+ * reads them as midnight in the zone of the browser, which moves an all day
+ * event by the offset that zone has to Berlin and, far enough east or west,
+ * onto the day before or after. Keep the digits and read them as UTC instead,
+ * the table prints an all day event in UTC as well.
+ *
+ * @param {ICAL.Time} time Start of the occurrence
+ * @returns {Date} The point in time to sort and print by
+ */
+function startOf(time) {
+  if (time.isDate) {
+    return new Date(Date.UTC(time.year, time.month - 1, time.day));
+  }
+
+  return time.toJSDate();
+}
+
+/**
+ * How far the recurrences of an event have to be iterated.
+ *
+ * The iteration walks the unmodified recurrence times, so an occurrence that
+ * was moved to an earlier time is only reached through the time it originally
+ * had, which can lie past the end of the window. Keep going for as long as the
+ * largest move towards the past can still carry an occurrence into it.
+ *
+ * @param {ICAL.Event} event The event whose recurrences are iterated
+ * @param {Date} end End of the window
+ * @returns {Date} The recurrence time to stop at
+ */
+function iterationEnd(event, end) {
+  let last = end.getTime();
+
+  for (const exception of Object.values(event.exceptions)) {
+    const movedBy = exception.recurrenceId.toJSDate().getTime()
+      - exception.startDate.toJSDate().getTime();
+
+    if (movedBy > 0) {
+      last = Math.max(last, end.getTime() + movedBy);
+    }
+  }
+
+  return new Date(last);
+}
+
+/**
+ * Parse an ICS calendar and return upcoming event occurrences.
+ *
+ * @param {string} icsText The contents of the .ics file
+ * @param {Date} now Events must still be running at this date
+ * @param {number} maxEvents Maximum number of events to return
+ * @param {number} maxDays Maximum number of days into the future
+ * @returns {{start: Date, allDay: boolean, name: string, url: string}[]} url is empty when the event has no URL
+ */
+function getUpcomingEvents(icsText, now, maxEvents, maxDays) {
+  const jcal = ICAL.parse(icsText);
+  const calendar = new ICAL.Component(jcal);
+
+  const end = new Date(now.getTime());
+  end.setDate(end.getDate() + maxDays);
+
+  const events = [];
+
+  const components = calendar.getAllSubcomponents("vevent");
+  const exceptions = exceptionsByUid(components);
+
+  for (const component of components) {
+    // Occurrences modified via RECURRENCE-ID are reached through the event they
+    // belong to, listing them here as well would show them twice.
+    if (component.hasProperty("recurrence-id")) {
+      continue;
+    }
+
+    const event = new ICAL.Event(component, {
+      exceptions: exceptions.get(component.getFirstPropertyValue("uid")) ?? [],
+    });
+
+    if (!event.startDate) {
+      continue;
+    }
+
+    if (event.isRecurring()) {
+      const iterator = event.iterator();
+      const iterateUntil = iterationEnd(event, end);
+
+      while (true) {
+        const occurrence = iterator.next();
+
+        if (!occurrence) {
+          break;
+        }
+
+        // Recurrences are chronological, so we're done
+        // once we pass the end of our search window.
+        if (occurrence.toJSDate() > iterateUntil) {
+          break;
+        }
+
+        // Details resolve time, name and URL of an occurrence that was
+        // modified via RECURRENCE-ID.
+        const details = event.getOccurrenceDetails(occurrence);
+
+        // A running event stays listed until it is over, so filter on its end.
+        // A modification may have moved the occurrence out of the window, so
+        // judge it by the time it really takes place at.
+        if (details.startDate.toJSDate() <= end && details.endDate.toJSDate() > now) {
+          events.push({
+            start: startOf(details.startDate),
+            allDay: details.startDate.isDate,
+            name: details.item.summary ?? "",
+            url: eventUrl(details.item),
+          });
+        }
+      }
+    } else if (event.startDate.toJSDate() <= end && event.endDate.toJSDate() > now) {
+      events.push({
+        start: startOf(event.startDate),
+        allDay: event.startDate.isDate,
+        name: event.summary ?? "",
+        url: eventUrl(event),
+      });
+    }
+  }
+
+  // We have occurrences from multiple events, so sort them
+  // before applying the maximum event count.
+  events.sort((a, b) => a.start - b.start);
+
+  return events.slice(0, maxEvents);
+}
+
+document.addEventListener("DOMContentLoaded", () => {
+  const ics = "/calendars/all.ics";
+  const max_days = 20;
+  const max_items = 5;
+
+  const now = new Date();
+
+  const table = document.getElementById("upcoming");
+  fetch(ics)
+    .then(response => {
+      // Without this an error page would be handed to the parser below, which
+      // then fails with a confusing complaint about the calendar syntax.
+      if (!response.ok) {
+        throw new Error(`${ics}: ${response.status} ${response.statusText}`);
+      }
+
+      return response.text();
+    })
+    .then(icsText => {
+      getUpcomingEvents(icsText, now, max_items, max_days).forEach(event => {
+        const row = document.createElement("tr");
+
+        const colBegin = document.createElement("td");
+
+        // The events take place in Berlin, so name their time in Berlin time
+        // instead of in the time zone the visitor happens to be in. An all day
+        // event has no time of day and carries its date in UTC, see startOf().
+        const whenFormat = event.allDay
+          ? { timeZone: "UTC" }
+          : { timeZone: "Europe/Berlin", hour: "2-digit", minute: "2-digit" };
+
+        const formattedStart = event.start.toLocaleString("de-DE", {
+          weekday: "long",
+          day: "2-digit",
+          month: "2-digit",
+          ...whenFormat,
+        });
+
+        colBegin.innerText = event.allDay ? formattedStart : `${formattedStart} Uhr`;
+        row.appendChild(colBegin);
+
+        const colName = document.createElement("td");
+
+        if (event.url) {
+          const a = document.createElement("a");
+          a.href = event.url;
+          a.text = event.name;
+
+          colName.appendChild(a);
+        } else {
+          colName.innerText = event.name;
+        }
+
+        row.appendChild(colName);
+
+        table.appendChild(row);
+      });
+    })
+    .catch(err => console.error("Fehler beim Laden der Termine:", err));
+});