Support endorsements for group 1:1 sends

Co-authored-by: trevor-signal <131492920+trevor-signal@users.noreply.github.com>

ts/util/mapEmplace.ts

@@ -0,0 +1,85 @@
+// Copyright 2024 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import type { RequireAtLeastOne } from 'type-fest';
+
+export type MapKey<T extends Map<unknown, unknown>> =
+  T extends Map<infer Key, unknown> ? Key : never;
+
+export type MapValue<T extends Map<unknown, unknown>> =
+  T extends Map<unknown, infer Value> ? Value : never;
+
+export type MapEmplaceOptions<T extends Map<unknown, unknown>> =
+  RequireAtLeastOne<{
+    insert?: (key: MapKey<T>, map: T) => MapValue<T>;
+    update?: (existing: MapValue<T>, key: MapKey<T>, map: T) => MapValue<T>;
+  }>;
+
+/**
+ * Lightweight polyfill of the `Map.prototype.emplace` TC39 Proposal:
+ * @see https://github.com/tc39/proposal-upsert
+ *
+ * @throws If no `insert()` is provided and key is not present
+ *
+ * @example Get or Insert:
+ * ```ts
+ * let pagesByBook = new Map<BookId, Map<PageId, Page>>()
+ * for (let page of pages) {
+ *   let bookPages = mapEmplace(pagesByBook, page.bookId, {
+ *     insert: () => new Map(),
+ *   })
+ *   bookPages.set(page.id, page)
+ * }
+ * ```
+ *
+ * @example Get+Update or Insert:
+ * ```ts
+ * let unreadPages = new Map<BookId, number>()
+ * for (let page of pages) {
+ *   if (page.readAt == null) {
+ *     mapEmplace(unreadPages, page.bookId, {
+ *       insert: () => 1,
+ *       update: (prev) => prev + 1,
+ *     })
+ *   }
+ * }
+ * ```
+ *
+ * @example Get+Update or Throw
+ * ```ts
+ * let PagesCache = new Map<PageId, Page>()
+ *
+ * function onPageReadEvent(pageId: PageId, readAt: number) {
+ *   if (PagesCache.has(pageId)) {
+ *     mapEmplace(PagesCache, pageId, {
+ *       update(page) {
+ *         return { ...page, readAt }
+ *       },
+ *     })
+ *   } else {
+ *     // save for later
+ *     onEarlyPageReadEvent(pageId, readAt)
+ *   }
+ * }
+ * ```
+ */
+export function mapEmplace<T extends Map<unknown, unknown>>(
+  map: T,
+  key: MapKey<T>,
+  options: MapEmplaceOptions<T>
+): MapValue<T> {
+  if (map.has(key)) {
+    let value = map.get(key) as MapValue<T>;
+    if (options.update != null) {
+      value = options.update(value, key, map);
+      map.set(key, value);
+    }
+    return value;
+  }
+  if (options.insert != null) {
+    const value = options.insert(key, map);
+    map.set(key, value);
+    return value;
+  }
+  throw new Error('Key was not present in map, and insert() was not provided');
+}