Move what the two calendar views share into one module

Both views read the same file and ask it the same question, only the
answer is presented differently: the start page lists the next few
occurrences, the calendar page marks the occurrences of one month. Since
they were taught to read a calendar properly they also carry the same
code for it, twice and word for word, some 130 lines of `eventUrl()`,
`exceptionsByUid()`, `iterationEnd()`, the fetch with its check of the
response and the walk over the events of the calendar. Every fix so far
had to be written twice, and the next one that is only written once
leaves the two views disagreeing about the same calendar.

Put it into `assets/js/events.js`, which offers what both need:

- `loadCalendar()` fetches and parses `/calendars/all.ics`
- `occurrencesBetween()` walks the occurrences that touch a window,
  expanding recurring events and resolving the ones that were modified
  on their own
- `eventUrl()` reads the URL of an event

`upcoming.js` and `calendar.js` keep what is really theirs, the shape of
their entries and how they are drawn, and both lose their own import of
ical.js: it is the concern of the module that reads the calendar now.
The two of them shrink from 259 and 549 lines to 105 and 406.

Hugo bundles the module into both scripts, so no shortcode changes and
no second request.

The walk keeps an occurrence when it starts at or before the end of the
window and ends after its start. That is the condition the start page
used; the calendar page compared against the start of the month with "<"
instead of "<=". Its window reaches two days past the month, so the day
this can differ on is nowhere near it.

No behaviour changes with this: the upcoming list over 120 days and
every day of the month view from 2025 to 2028, taken from
https://berlin.ccc.de/calendars/all.ics with the name, URL, start, end
and all day flag of each entry, are identical before and after, and so
are the results of the tests for moved occurrences, for modifications
between two series and for all day events in Berlin, Tokyo, Los Angeles
and Kiritimati.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

assets/js/calendar.js

@@ -1,6 +1,4 @@
-import ICAL from "./vendor/ical.js";
-
-const icsUrl = "/calendars/all.ics";
+import { eventUrl, loadCalendar, occurrencesBetween } from "./events.js";
 
 // The club is in Berlin, so the calendar shows Berlin days and Berlin times,
 // no matter which time zone the browser of the visitor is set to.
@@ -51,29 +49,59 @@ function dayKey(date) {
   return `${parts.year}-${parts.month}-${parts.day}`;
 }
 
+/**
+ * The day an occurrence time falls on in Berlin.
+ *
+ * A date has neither a time nor a zone, its digits are the day itself.
+ * toJSDate() would read them as midnight in the zone of the browser, which
+ * far enough east or west of Berlin lands on the day before or after.
+ *
+ * @param {ICAL.Time} time The time
+ * @returns {string} The day as "YYYY-MM-DD"
+ */
+function timeDayKey(time) {
+  if (time.isDate) {
+    const month = String(time.month).padStart(2, "0");
+    const day = String(time.day).padStart(2, "0");
+
+    return `${time.year}-${month}-${day}`;
+  }
+
+  return dayKey(time.toJSDate());
+}
+
 /**
  * The days an event covers, so that an event running over several days is
  * shown on each of them and not only on the day it starts.
  *
- * @param {Date} start Start of the event
- * @param {Date} end End of the event
+ * @param {ICAL.Time} start Start of the event
+ * @param {ICAL.Time} end End of the event
  * @returns {string[]} The days as "YYYY-MM-DD"
  */
 function daysCovered(start, end) {
   // The end is not part of the event: one ending at midnight belongs to the day
   // before, and an all day event ends on the day before its DTEND.
-  const lastInstant = new Date(Math.max(start.getTime(), end.getTime() - 1));
-  const lastKey = dayKey(lastInstant);
+  const last = end.clone();
+
+  if (end.isDate) {
+    last.adjust(-1, 0, 0, 0);
+  } else {
+    last.adjust(0, 0, 0, -1);
+  }
+
+  const lastKey = timeDayKey(last);
   const days = [];
 
-  let key = dayKey(start);
+  let key = timeDayKey(start);
 
   // The guard keeps a broken event from looping forever, a year of dots on the
   // same event is well past the point where the calendar is still useful.
   while (days.length <= 366) {
     days.push(key);
 
-    if (key === lastKey) {
+    // The keys sort as the days do, so this also stops an event whose end lies
+    // before its start after the day it starts on.
+    if (key >= lastKey) {
       break;
     }
 
@@ -86,25 +114,13 @@ function daysCovered(start, end) {
   return days;
 }
 
-/**
- * Read the URL of an event.
- *
- * ICAL.Event does not expose the URL property, so read it from the component.
- *
- * @param {ICAL.Event} event The event to read the URL of
- * @returns {string} The URL, empty when the event has none
- */
-function eventUrl(event) {
-  return event.component.getFirstPropertyValue("url") ?? "";
-}
-
 /**
  * Reduce one occurrence of an event to what the calendar displays.
  *
  * @param {ICAL.Event} event The event the occurrence belongs to
  * @param {ICAL.Time} startDate Start of this occurrence
  * @param {ICAL.Time} endDate End of this occurrence
- * @returns {{summary: string, description: string, url: string, start: Date, end: Date, allDay: boolean}}
+ * @returns {{summary: string, description: string, url: string, start: Date, end: Date, allDay: boolean, days: string[]}}
  */
 function toOccurrence(event, startDate, endDate) {
   return {
@@ -114,6 +130,9 @@ function toOccurrence(event, startDate, endDate) {
     start: startDate.toJSDate(),
     end: endDate.toJSDate(),
     allDay: startDate.isDate,
+    // Taken from the ICAL times, which still know whether they name a day or a
+    // point in time; the JS dates above no longer do.
+    days: daysCovered(startDate, endDate),
   };
 }
 
@@ -143,7 +162,7 @@ function occurrencesOfMonth(year, month) {
   const monthPrefix = `${year}-${String(month + 1).padStart(2, "0")}-`;
 
   const add = (occurrence) => {
-    for (const key of daysCovered(occurrence.start, occurrence.end)) {
+    for (const key of occurrence.days) {
       if (!key.startsWith(monthPrefix)) {
         continue;
       }
@@ -156,42 +175,20 @@ function occurrencesOfMonth(year, month) {
     }
   };
 
-  for (const component of calendar.getAllSubcomponents("vevent")) {
-    const event = new ICAL.Event(component);
-
-    // Occurrences modified via RECURRENCE-ID are reached through the event they
-    // belong to, handling them here as well would show them twice.
-    if (event.isRecurrenceException() || !event.startDate) {
-      continue;
-    }
-
-    if (event.isRecurring()) {
-      const iterator = event.iterator();
-
-      while (true) {
-        const occurrence = iterator.next();
-
-        // Recurrences are chronological, so we are done once one starts after
-        // the month.
-        if (!occurrence || occurrence.toJSDate() >= to) {
-          break;
-        }
-
-        // Details resolve time, name and URL of an occurrence that was
-        // modified via RECURRENCE-ID.
-        const details = event.getOccurrenceDetails(occurrence);
-
-        if (details.endDate.toJSDate() > from) {
-          add(toOccurrence(details.item, details.startDate, details.endDate));
-        }
-      }
-    } else if (event.startDate.toJSDate() < to && event.endDate.toJSDate() > from) {
-      add(toOccurrence(event, event.startDate, event.endDate));
-    }
+  for (const { event, startDate, endDate } of occurrencesBetween(calendar, from, to)) {
+    add(toOccurrence(event, startDate, endDate));
   }
 
   for (const occurrences of Object.values(byDate)) {
-    occurrences.sort((a, b) => a.start - b.start);
+    occurrences.sort((a, b) => {
+      // An all day event has no time of day to sort by, the JS date of its
+      // start is midnight in the zone of the browser. Put it first instead.
+      if (a.allDay !== b.allDay) {
+        return a.allDay ? -1 : 1;
+      }
+
+      return a.start - b.start;
+    });
   }
 
   return byDate;
@@ -366,7 +363,7 @@ function formatTimeRange(occurrence) {
   const start = timeOfDayFormat.format(occurrence.start);
   const end = timeOfDayFormat.format(occurrence.end);
 
-  return `Start: ${start}, End: ${end}`;
+  return `Beginn: ${start}, Ende: ${end}`;
 }
 
 document.addEventListener("DOMContentLoaded", function() {
@@ -400,18 +397,9 @@ document.addEventListener("DOMContentLoaded", function() {
   currentMonth = today.getMonth();
   updateEventsForMonth(currentYear, currentMonth);
 
-  fetch(icsUrl)
-    .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(`${icsUrl}: ${response.status} ${response.statusText}`);
-      }
-
-      return response.text();
-    })
-    .then(icsText => {
-      calendar = new ICAL.Component(ICAL.parse(icsText));
+  loadCalendar()
+    .then(loaded => {
+      calendar = loaded;
       updateEventsForMonth(currentYear, currentMonth);
     })
     .catch(err => console.error("Fehler beim Laden der ICS-Datei:", err));

assets/js/events.js

@@ -0,0 +1,194 @@
+import ICAL from "./vendor/ical.js";
+
+// The calendar is published next to the site and is not built from this
+// repository, see the README.
+const icsUrl = "/calendars/all.ics";
+
+/**
+ * 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)
+ */
+export 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;
+}
+
+/**
+ * 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} to End of the window
+ * @returns {Date} The recurrence time to stop at
+ */
+function iterationEnd(event, to) {
+  let last = to.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, to.getTime() + movedBy);
+    }
+  }
+
+  return new Date(last);
+}
+
+/**
+ * Whether an occurrence touches a window.
+ *
+ * @param {ICAL.Time} startDate Start of the occurrence
+ * @param {ICAL.Time} endDate End of the occurrence
+ * @param {Date} from Start of the window
+ * @param {Date} to End of the window
+ * @returns {boolean} True when the two overlap
+ */
+function touches(startDate, endDate, from, to) {
+  return startDate.toJSDate() <= to && endDate.toJSDate() > from;
+}
+
+/**
+ * Load and parse the calendar of the club.
+ *
+ * @returns {Promise<ICAL.Component>} The calendar
+ */
+export async function loadCalendar() {
+  const response = await fetch(icsUrl);
+
+  // 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(`${icsUrl}: ${response.status} ${response.statusText}`);
+  }
+
+  return new ICAL.Component(ICAL.parse(await response.text()));
+}
+
+/**
+ * Every occurrence of the calendar that touches the given window.
+ *
+ * A recurring event is expanded, and an occurrence that was modified on its own
+ * is reported with the time, the name and the URL of that modification.
+ *
+ * @param {ICAL.Component} calendar The parsed calendar
+ * @param {Date} from An occurrence has to still be running at this time
+ * @param {Date} to An occurrence has to have started by this time
+ * @yields {{event: ICAL.Event, startDate: ICAL.Time, endDate: ICAL.Time}}
+ */
+export function* occurrencesBetween(calendar, from, to) {
+  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, handling them here as well would report 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()) {
+      if (touches(event.startDate, event.endDate, from, to)) {
+        yield { event, startDate: event.startDate, endDate: event.endDate };
+      }
+
+      continue;
+    }
+
+    const iterator = event.iterator();
+    const iterateUntil = iterationEnd(event, to);
+
+    while (true) {
+      const occurrence = iterator.next();
+
+      // Recurrences are chronological, so we are done once one starts after the
+      // window, and after the occurrences a modification can still move back
+      // into it.
+      if (!occurrence || 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 modification may have moved the occurrence out of the window, so
+      // judge it by the time it really takes place at.
+      if (touches(details.startDate, details.endDate, from, to)) {
+        yield {
+          event: details.item,
+          startDate: details.startDate,
+          endDate: details.endDate,
+        };
+      }
+    }
+  }
+}

assets/js/upcoming.js

@@ -1,88 +1,49 @@
-import ICAL from "./vendor/ical.js";
+import { eventUrl, loadCalendar, occurrencesBetween } from "./events.js";
 
 /**
- * Read the URL of an event.
+ * When an occurrence starts, as a point in time.
  *
- * ICAL.Event does not expose the URL property, so read it from the component.
+ * 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.Event} event The event to read the URL of
- * @returns {string} The URL, empty when the event has none
+ * @param {ICAL.Time} time Start of the occurrence
+ * @returns {Date} The point in time to sort and print by
  */
-function eventUrl(event) {
-  return event.component.getFirstPropertyValue("url") ?? "";
+function startOf(time) {
+  if (time.isDate) {
+    return new Date(Date.UTC(time.year, time.month - 1, time.day));
+  }
+
+  return time.toJSDate();
 }
 
 /**
- * Parse an ICS calendar and return upcoming event occurrences.
+ * The upcoming occurrences of a calendar.
  *
- * @param {string} icsText The contents of the .ics file
+ * @param {ICAL.Component} calendar The parsed calendar
  * @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, name: string, url: string}[]} url is empty when the event has no URL
+ * @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);
-
+function getUpcomingEvents(calendar, now, maxEvents, maxDays) {
   const end = new Date(now.getTime());
   end.setDate(end.getDate() + maxDays);
 
   const events = [];
 
-  for (const component of calendar.getAllSubcomponents("vevent")) {
-    const event = new ICAL.Event(component);
-
-    // Occurrences modified via RECURRENCE-ID are reached through the event they
-    // belong to, listing them here as well would show them twice.
-    if (event.isRecurrenceException()) {
-      continue;
-    }
-
-    if (!event.startDate) {
-      continue;
-    }
-
-    if (event.isRecurring()) {
-      const iterator = event.iterator();
-
-      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() > end) {
-          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.
-        if (details.endDate.toJSDate() > now) {
-          events.push({
-            start: details.startDate.toJSDate(),
-            name: details.item.summary ?? "",
-            url: eventUrl(details.item),
-          });
-        }
-      }
-    } else {
-      const start = event.startDate.toJSDate();
-
-      if (start <= end && event.endDate.toJSDate() > now) {
-        events.push({
-          start,
-          name: event.summary ?? "",
-          url: eventUrl(event),
-        });
-      }
-    }
+  // A running event stays listed until it is over, so the window starts at now
+  // and the walk keeps everything that has not ended yet.
+  for (const { event, startDate } of occurrencesBetween(calendar, now, end)) {
+    events.push({
+      start: startOf(startDate),
+      allDay: startDate.isDate,
+      name: event.summary ?? "",
+      url: eventUrl(event),
+    });
   }
 
   // We have occurrences from multiple events, so sort them
@@ -93,41 +54,34 @@ function getUpcomingEvents(icsText, now, maxEvents, maxDays) {
 }
 
 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 => {
+  loadCalendar()
+    .then(calendar => {
+      getUpcomingEvents(calendar, 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", {
-          // 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.
-          timeZone: "Europe/Berlin",
           weekday: "long",
           day: "2-digit",
           month: "2-digit",
-          hour: "2-digit",
-          minute: "2-digit",
+          ...whenFormat,
         });
 
-        colBegin.innerText = `${formattedStart} Uhr`;
+        colBegin.innerText = event.allDay ? formattedStart : `${formattedStart} Uhr`;
         row.appendChild(colBegin);
 
         const colName = document.createElement("td");

layouts/shortcodes/calendar.html

@@ -1,9 +1,9 @@
 {{ $js := resources.Get "js/calendar.js" | js.Build (dict "minify" true "format" "esm" "target" "es2020") | fingerprint }}
-{{ $css := resources.Get "css/calendar.css" }}
+{{ $css := resources.Get "css/calendar.css" | minify | fingerprint }}
 
 <div class="calendar-container">
   {{ with $css }}
-    <link rel="stylesheet" href="{{ .RelPermalink }}">
+    <link rel="stylesheet" href="{{ .RelPermalink }}" integrity="{{ .Data.Integrity }}">
   {{ end }}
   {{ with $js }}
     <script type="module" src="{{ .RelPermalink }}" integrity="{{ .Data.Integrity }}"></script>