[lib] Introduce invertObjectToMap

Summary:
Context: https://linear.app/comm/issue/ENG-5640/consider-introducing-invertobjecttomap-utility-fn

Thanks @tomek for investigating + resolving the flow issues with this utility!

Test Plan: Consumed in `minimally-encoded-thread-permissions`, where the unit tests continue to succeed.

Reviewers: ashoat, tomek, ginsu, rohan

Reviewed By: ashoat

Subscribers: wyilio, tomek

Differential Revision: https://phab.comm.dev/D9676

lib/permissions/minimally-encoded-thread-permissions.js

@@ -7,7 +7,7 @@ import type {
   ThreadPermission,
   ThreadPermissionsInfo,
 } from '../types/thread-permission-types.js';
-import { entries } from '../utils/objects.js';
+import { entries, invertObjectToMap } from '../utils/objects.js';
 
 // `baseRolePermissionEncoding` maps permission names to indices.
 // These indices represent the 6-bit basePermission part of the 10-bit role
@@ -126,21 +126,18 @@ const rolePermissionToBitmaskHex = (threadRolePermission: string): string => {
   return bitmask.toString(16).padStart(3, '0');
 };
 
-const inverseBaseRolePermissionEncoding = new Map(
-  Object.entries(baseRolePermissionEncoding).map(([key, value]) => [
-    value,
-    key,
-  ]),
+const inverseBaseRolePermissionEncoding = invertObjectToMap(
+  baseRolePermissionEncoding,
 );
 
 // $FlowIssue bigint-unsupported
-const inversePropagationPrefixes: Map<bigint, string> = new Map(
-  Object.entries(propagationPrefixes).map(([key, value]) => [value, key]),
-);
+const inversePropagationPrefixes: Map<bigint, string> =
+  invertObjectToMap(propagationPrefixes);
+
 // $FlowIssue bigint-unsupported
-const inverseFilterPrefixes: Map<bigint, string> = new Map(
-  Object.entries(filterPrefixes).map(([key, value]) => [value, key]),
-);
+const inverseFilterPrefixes: Map<bigint, string> =
+  invertObjectToMap(filterPrefixes);
+
 const decodeRolePermissionBitmask = (bitmask: string): string => {
   const bitmaskInt = BigInt(`0x${bitmask}`);
   const basePermission = (bitmaskInt >> BigInt(4)) & BigInt(63);

lib/utils/objects.js

@@ -129,6 +129,14 @@ function assertObjectsAreEqual<K, T>(
   );
 }
 
+function invertObjectToMap<K, V>(obj: { +[K]: V }): Map<V, K> {
+  const invertedMap = new Map<V, K>();
+  for (const key of Object.keys(obj)) {
+    invertedMap.set(obj[key], key);
+  }
+  return invertedMap;
+}
+
 export {
   findMaximumDepth,
   values,
@@ -137,4 +145,5 @@ export {
   assertObjectsAreEqual,
   deepDiff,
   entries,
+  invertObjectToMap,
 };