Offer "alias" field in room create form

Tries its best to not allow aliases that are too
long (full alias including leading #, : in the
middle, and homeserver domain name must not exceed
255 bytes.

Change-Id: I1e784a94cb599eca7e30736d666b20e37aad5444

src/components/rooms.js

@@ -29,20 +29,73 @@ function generateRoomRecord() {
   return {
     room_name: "",
     public: true,
+    alias: "",
   }
 }
 
 const validateDisplayName = fieldval => fieldval === undefined ? "synapseadmin.rooms.room_name_required" : fieldval.length === 0 ? "synapseadmin.rooms.room_name_required" : undefined;
 
+function approximateAliasLength(alias, homeserver) {
+  /* TODO maybe handle punycode in homeserver URL */
+
+  var te;
+
+  // Support for TextEncoder is quite widespread, but the polyfill is
+  // pretty large; We will only underestimate the size with the regular
+  // length attribute of String, so we never prevent the user from using
+  // an alias that is short enough for the server, but too long for our
+  // heuristic.
+  try {
+    te = new TextEncoder();
+  }
+  catch (err) {
+    if (err instanceof ReferenceError) {
+      te = undefined;
+    }
+  }
+
+  const aliasLength = te === undefined ? alias.length : te.encode(alias).length;
+
+  return (
+    "#".length +
+      aliasLength +
+      ":".length +
+      homeserver.length
+  );
+}
+
+const validateAlias = fieldval => {
+  if (fieldval === undefined) {
+    return undefined;
+  }
+  const homeserver = localStorage.getItem("home_server");
+
+  if (approximateAliasLength(fieldval, homeserver) > 255) {
+    return "synapseadmin.rooms.alias_too_long";
+  }
+}
+
 const removeLeadingWhitespace = fieldVal => fieldVal === undefined ? undefined : fieldVal.trimStart();
 
+const validateHasAliasIfPublic = formdata => {
+  let errors = {};
+  if (formdata.public) {
+    if (formdata.alias === undefined || formdata.alias.trim().length === 0) {
+      errors.alias = "synapseadmin.rooms.alias_required_if_public"
+    }
+  }
+  return errors;
+}
+
 export const RoomCreate = props => (
   <Create record={generateRoomRecord()} {...props}>
-    <SimpleForm>
+    <SimpleForm validate={validateHasAliasIfPublic}>
       <TextInput source="room_name"
-                 label="synapseadmin.rooms.room_name"
                  parse={removeLeadingWhitespace}
                  validate={validateDisplayName}/>
+      <TextInput source="alias"
+                 parse={removeLeadingWhitespace}
+                 validate={validateAlias}/>
       <BooleanInput source="public"
                     label="synapseadmin.rooms.make_public"/>
     </SimpleForm>