make acceptedLanguages order the header by q values descending

fluent-langneg/src/accepted_languages.js

@@ -1,7 +1,25 @@
-export default function acceptedLanguages(string = "") {
-  if (typeof string !== "string") {
+function parseAcceptLanguageEntry(entry) {
+  const langWithQ = entry.split(";").map(u => u.trim());
+  let q = 1.0;
+  if (langWithQ.length > 1) {
+    const qVal = langWithQ[1].split("=").map(u => u.trim());
+    if (qVal.length === 2 && qVal[0].toLowerCase() === "q") {
+      const qn = Number(qVal[1]);
+      q = isNaN(qn) ? 0.0 : qn;
+    }
+  }
+  return { lang: langWithQ[0], q };
+}
+
+export default function acceptedLanguages(acceptLanguageHeader = "") {
+  if (typeof acceptLanguageHeader !== "string") {
     throw new TypeError("Argument must be a string");
   }
-  const tokens = string.split(",").map(t => t.trim());
-  return tokens.filter(t => t !== "").map(t => t.split(";")[0]);
+  const tokens = acceptLanguageHeader.split(",").map(t => t.trim())
+    .filter(t => t !== "");
+  const langsWithQ = Array.from(tokens.map(parseAcceptLanguageEntry).entries());
+  // order by q descending, keeping the header order for equal weights
+  langsWithQ.sort(([aidx, aval], [bidx, bval]) =>
+    aval.q === bval.q ? aidx - bidx : bval.q - aval.q);
+  return langsWithQ.map(([, val]) => val.lang);
 }

fluent-langneg/test/headers_test.js

@@ -2,6 +2,12 @@ import assert from 'assert';
 import acceptedLanguages from '../src/accepted_languages';
 
 suite('parse headers', () => {
+  test('without an argument', () => {
+    assert.deepStrictEqual(
+      acceptedLanguages(), []
+    );
+  });
+
   test('without quality values', () => {
     assert.deepStrictEqual(
       acceptedLanguages('en-US, fr, pl'), [
@@ -29,6 +35,53 @@ suite('parse headers', () => {
     );
   });
 
+  test('with out of order quality values', () => {
+    assert.deepStrictEqual(
+      acceptedLanguages('en;q=0.8, fr;q=0.9, de;q=0.7, *;q=0.5, fr-CH'), [
+        'fr-CH',
+        'fr',
+        'en',
+        'de',
+        '*'
+      ]
+    );
+  });
+
+  test('with equal q values', () => {
+    assert.deepStrictEqual(
+      acceptedLanguages('en;q=0.1, fr;q=0.1, de;q=0.1, *;q=0.1'), [
+        'en',
+        'fr',
+        'de',
+        '*'
+      ]
+    );
+  });
+
+  test('with duff q values', () => {
+    assert.deepStrictEqual(
+      acceptedLanguages('en;q=no, fr;z=0.9, de;q=0.7;q=9, *;q=0.5, fr-CH;q=a=0.1'), [
+        'fr',
+        'fr-CH',
+        'de',
+        '*',
+        'en'
+      ]
+    );
+  });
+
+  test('with empty entries', () => {
+    assert.deepStrictEqual(
+      acceptedLanguages('en;q=0.8,,, fr;q=0.9,, de;q=0.7, *;q=0.5, fr-CH'), [
+        'fr-CH',
+        'fr',
+        'en',
+        'de',
+        '*'
+      ]
+    );
+  });
+
   test('edge cases', () => {
     const args = [
       null,