Use a neutral compact switch codec and clarify provider size limits

src/data/idSwitches.js

@@ -0,0 +1,47 @@
+const MAX_SWITCHES_LENGTH = 256;
+const ENTRY = /^([a-z][a-z0-9-]*)-([01])$/;
+
+/** Canonical, bounded boolean switches keyed by stable slugs. */
+export function normalizeIdSwitches(value, reservedIds = []) {
+  if (typeof value !== 'string' || value.length > MAX_SWITCHES_LENGTH)
+    return null;
+  if (!value) return '';
+  const entries = new Map();
+  for (const field of value.split('*')) {
+    const match = ENTRY.exec(field);
+    if (!match || reservedIds.includes(match[1]) || entries.has(match[1]))
+      return null;
+    entries.set(match[1], match[2] === '1');
+  }
+  return [...entries]
+    .sort(([a], [b]) => (a < b ? -1 : a > b ? 1 : 0))
+    .map(([id, on]) => `${id}-${on ? '1' : '0'}`)
+    .join('*');
+}
+
+/** Encode by stable IDs rather than registration order or generated tokens. */
+export function encodeIdSwitches(switches) {
+  const value = [...switches]
+    .map(([id, on]) => `${id}-${on === true ? '1' : '0'}`)
+    .join('*');
+  if (value.length > MAX_SWITCHES_LENGTH)
+    throw new RangeError(
+      `Switch IDs exceed the ${MAX_SWITCHES_LENGTH}-character share-link limit`,
+    );
+  const normalized = normalizeIdSwitches(value);
+  if (normalized === null) throw new TypeError('Invalid boolean switch IDs');
+  return normalized;
+}
+
+/** A Map keeps switch IDs separate from object prototype properties. */
+export function decodeIdSwitches(value) {
+  const normalized = normalizeIdSwitches(value);
+  return new Map(
+    normalized
+      ? normalized.split('*').map((entry) => {
+          const [, id, on] = ENTRY.exec(entry);
+          return [id, on === '1'];
+        })
+      : [],
+  );
+}