Validate RFC 5545 calendar feed

Author: HeyItsGilbert

.github/workflows/build.yml

@@ -96,3 +96,6 @@ jobs:
             --gc \
             --minify \
             --destination public
+
+      - name: Validate iCalendar feed
+        run: node scripts/validate-calendar.mjs public/calendar/calendar.ics

hugo.yaml

@@ -146,7 +146,7 @@ params:
     branch: "main"
 
   ical:
-    timezone: UTC
+    timezone: Etc/UTC
 
 module:
   imports:

layouts/list.calendar.ics

@@ -0,0 +1,21 @@
+{{- $calendar := "BEGIN:VCALENDAR\n" -}}
+{{- $calendar = printf "%s%s" $calendar (partial "header.ics" .) -}}
+{{- $timezones := slice -}}
+{{- range .Pages -}}
+  {{- if .Params.startDate -}}
+    {{- $timezone := partial "ical/get_timezone.ics" . -}}
+    {{- if not (in $timezones $timezone) -}}
+      {{- $timezones = $timezones | append $timezone -}}
+    {{- end -}}
+  {{- end -}}
+{{- end -}}
+{{- range $timezones -}}
+  {{- $calendar = printf "%s%s" $calendar (partial "timezone.ics" .) -}}
+{{- end -}}
+{{- range .Pages -}}
+  {{- if .Params.startDate -}}
+    {{- $calendar = printf "%s%s" $calendar (partial "event.ics" .) -}}
+  {{- end -}}
+{{- end -}}
+{{- $calendar = printf "%sEND:VCALENDAR\n" $calendar -}}
+{{- replace $calendar "\n" "\r\n" -}}

scripts/validate-calendar.mjs

@@ -0,0 +1,105 @@
+import { readFile } from 'node:fs/promises';
+
+const input = process.argv[2];
+
+if (!input) {
+  throw new Error('Usage: node scripts/validate-calendar.mjs <calendar.ics|URL>');
+}
+
+const calendar = new URL(input, 'file:').protocol === 'file:'
+  ? await readFile(input, 'utf8')
+  : await (await fetch(input)).text();
+
+const errors = [];
+const fail = (message) => errors.push(message);
+
+if (/(^|[^\r])\n/.test(calendar)) {
+  fail('Content lines must use CRLF line endings.');
+}
+
+const lines = calendar.replace(/\r?\n[ \t]/g, '').split(/\r?\n/).filter(Boolean);
+const components = [];
+const events = [];
+const timezones = new Set();
+const timezoneReferences = new Set();
+let event;
+
+for (const line of lines) {
+  if (line.startsWith('BEGIN:')) {
+    const component = line.slice('BEGIN:'.length);
+    components.push(component);
+    if (component === 'VEVENT') {
+      event = new Map();
+      events.push(event);
+    }
+    continue;
+  }
+
+  if (line.startsWith('END:')) {
+    const component = line.slice('END:'.length);
+    if (components.pop() !== component) {
+      fail(`Mismatched component terminator: ${line}`);
+    }
+    if (component === 'VEVENT') {
+      event = undefined;
+    }
+    continue;
+  }
+
+  const separator = line.indexOf(':');
+  if (separator < 1) {
+    fail(`Malformed content line: ${line}`);
+    continue;
+  }
+
+  const declaration = line.slice(0, separator);
+  const [name, ...parameters] = declaration.split(';');
+  const value = line.slice(separator + 1);
+
+  for (const parameter of parameters) {
+    if (parameter.startsWith('TZID=')) {
+      timezoneReferences.add(parameter.slice('TZID='.length));
+    }
+  }
+
+  if (components.at(-1) === 'VTIMEZONE' && name === 'TZID') {
+    timezones.add(value);
+  }
+
+  if (event) {
+    event.set(name, [...(event.get(name) ?? []), value]);
+  }
+}
+
+if (components.length) {
+  fail(`Unclosed component: ${components.at(-1)}`);
+}
+
+if (lines[0] !== 'BEGIN:VCALENDAR' || lines.at(-1) !== 'END:VCALENDAR') {
+  fail('Calendar must be wrapped in BEGIN:VCALENDAR and END:VCALENDAR.');
+}
+
+for (const [index, properties] of events.entries()) {
+  for (const property of ['UID', 'DTSTAMP', 'DTSTART']) {
+    if (properties.get(property)?.length !== 1) {
+      fail(`VEVENT ${index + 1} must contain exactly one ${property}.`);
+    }
+  }
+
+  const uid = properties.get('UID')?.[0];
+  if (uid && events.some((other) => other !== properties && other.get('UID')?.[0] === uid)) {
+    fail(`VEVENT ${index + 1} reuses UID ${uid}.`);
+  }
+}
+
+for (const timezone of timezoneReferences) {
+  if (!timezones.has(timezone)) {
+    fail(`TZID=${timezone} has no matching VTIMEZONE component.`);
+  }
+}
+
+if (errors.length) {
+  throw new Error(`Invalid RFC 5545 calendar:\n- ${errors.join('\n- ')}`);
+}
+
+console.log(`Validated ${events.length} VEVENT components.`);