[lib] Introduce decodeRolePermissionBitmask

Summary:
To reverse `rolePermissionToBitmaskHex` introduced in D9662.

---

Depends on D9662

Test Plan: Unit tests, will add a few more.

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9663

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

@@ -126,4 +126,49 @@ const rolePermissionToBitmaskHex = (threadRolePermission: string): string => {
   return bitmask.toString(16).padStart(3, '0');
 };
 
-export { permissionsToBitmaskHex, hasPermission, rolePermissionToBitmaskHex };
+const inverseBaseRolePermissionEncoding = new Map(
+  Object.entries(baseRolePermissionEncoding).map(([key, value]) => [
+    value,
+    key,
+  ]),
+);
+
+// $FlowIssue bigint-unsupported
+const inversePropagationPrefixes: Map<bigint, string> = new Map(
+  Object.entries(propagationPrefixes).map(([key, value]) => [value, key]),
+);
+// $FlowIssue bigint-unsupported
+const inverseFilterPrefixes: Map<bigint, string> = new Map(
+  Object.entries(filterPrefixes).map(([key, value]) => [value, key]),
+);
+const decodeRolePermissionBitmask = (bitmask: string): string => {
+  const bitmaskInt = BigInt(`0x${bitmask}`);
+  const basePermission = (bitmaskInt >> BigInt(4)) & BigInt(63);
+  const propagationPrefix = (bitmaskInt >> BigInt(2)) & BigInt(3);
+  const filterPrefix = bitmaskInt & BigInt(3);
+
+  const basePermissionString =
+    inverseBaseRolePermissionEncoding.get(basePermission);
+  const propagationPrefixString =
+    inversePropagationPrefixes.get(propagationPrefix) ?? '';
+  const filterPrefixString = inverseFilterPrefixes.get(filterPrefix) ?? '';
+
+  invariant(
+    basePermissionString !== null &&
+      basePermissionString !== undefined &&
+      propagationPrefixString !== null &&
+      propagationPrefixString !== undefined &&
+      filterPrefixString !== null &&
+      filterPrefixString !== undefined,
+    'invalid bitmask',
+  );
+
+  return `${propagationPrefixString}${filterPrefixString}${basePermissionString}`;
+};
+
+export {
+  permissionsToBitmaskHex,
+  hasPermission,
+  rolePermissionToBitmaskHex,
+  decodeRolePermissionBitmask,
+};

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

@@ -1,6 +1,7 @@
 // @flow
 
 import {
+  decodeRolePermissionBitmask,
   hasPermission,
   permissionsToBitmaskHex,
   rolePermissionToBitmaskHex,
@@ -88,3 +89,39 @@ describe('rolePermissionToBitmaskHex', () => {
     expect(rolePermissionToBitmaskHex('child_know_of')).toBe('008');
   });
 });
+
+describe('decodeRolePermissionBitmask', () => {
+  it('should decode `01b` to `child_opentoplevel_visible` successfully', () => {
+    expect(decodeRolePermissionBitmask('01b')).toBe(
+      'child_opentoplevel_visible',
+    );
+  });
+
+  it('should decode `00b` to `child_opentoplevel_know_of` successfully', () => {
+    expect(decodeRolePermissionBitmask('00b')).toBe(
+      'child_opentoplevel_know_of',
+    );
+  });
+
+  it('should decode `01a` to `child_toplevel_visible` successfully', () => {
+    expect(decodeRolePermissionBitmask('01a')).toBe('child_toplevel_visible');
+  });
+
+  it('should decode `00a` to `child_toplevel_know_of` successfully', () => {
+    expect(decodeRolePermissionBitmask('00a')).toBe('child_toplevel_know_of');
+  });
+
+  it('should decode `0ab` to `child_opentoplevel_join_thread` successfully', () => {
+    expect(decodeRolePermissionBitmask('0ab')).toBe(
+      'child_opentoplevel_join_thread',
+    );
+  });
+
+  it('should decode `018` to `child_visible` successfully', () => {
+    expect(decodeRolePermissionBitmask('018')).toBe('child_visible');
+  });
+
+  it('should decode `008` to `child_know_of` successfully', () => {
+    expect(decodeRolePermissionBitmask('008')).toBe('child_know_of');
+  });
+});