Display user badges

ts/util/userLanguages.ts

@@ -0,0 +1,53 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+// We ["MUST NOT generate more than three digits after the decimal point"][0]. We use a
+//   space-efficient algorithm that runs out of digits after 28 languages. This should be
+//   fine for most users and [the server doesn't parse more than 15 languages, at least
+//   for badges][1].
+//
+// [0]: https://httpwg.org/specs/rfc7231.html#quality.values
+// [1]: https://github.com/signalapp/Signal-Server/blob/d2bc3c736080c3d852c9e88af0bffcb6632d9975/service/src/main/java/org/whispersystems/textsecuregcm/badges/ConfiguredProfileBadgeConverter.java#L29
+const MAX_LANGUAGES_TO_FORMAT = 28;
+
+export function formatAcceptLanguageHeader(
+  languages: ReadonlyArray<string>
+): string {
+  if (languages.length === 0) {
+    return '*';
+  }
+
+  const result: Array<string> = [];
+
+  const length = Math.min(languages.length, MAX_LANGUAGES_TO_FORMAT);
+  for (let i = 0; i < length; i += 1) {
+    const language = languages[i];
+
+    // ["If no 'q' parameter is present, the default weight is 1."][1]
+    //
+    // [1]: https://httpwg.org/specs/rfc7231.html#quality.values
+    if (i === 0) {
+      result.push(language);
+      continue;
+    }
+
+    // These values compute a descending sequence with minimal bytes. See the tests for
+    //   examples.
+    const magnitude = 1 / 10 ** (Math.ceil(i / 9) - 1);
+    const subtractor = (((i - 1) % 9) + 1) * (magnitude / 10);
+    const q = magnitude - subtractor;
+    const formattedQ = q.toFixed(3).replace(/0+$/, '');
+
+    result.push(`${language};q=${formattedQ}`);
+  }
+
+  return result.join(', ');
+}
+
+export function getUserLanguages(
+  defaults: undefined | ReadonlyArray<string>,
+  fallback: string
+): ReadonlyArray<string> {
+  const result = defaults || [];
+  return result.length ? result : [fallback];
+}