feat(langneg): Order Accept-Language header tags by q values (#262)

Co-authored-by: Eemeli Aro <eemeli@mozilla.com>

Author: blushingpenguin

fluent-langneg/src/accepted_languages.ts

@@ -1,7 +1,20 @@
-export function acceptedLanguages(str: string = ""): Array<string> {
-  if (typeof str !== "string") {
+const entryRegexp =
+  //      locale     ;    q     = qval
+  /(?:^|,)([^,;]+)(?:;\s*[qQ]\s*=([^,;]+))?/g;
+
+export function acceptedLanguages(acceptLanguageHeader = ""): string[] {
+  if (typeof acceptLanguageHeader !== "string") {
     throw new TypeError("Argument must be a string");
   }
-  const tokens = str.split(",").map(t => t.trim());
-  return tokens.filter(t => t !== "").map(t => t.split(";")[0]);
+
+  const langsWithQ: Array<{ lang: string; q: number; index: number }> = [];
+  for (const token of acceptLanguageHeader.matchAll(entryRegexp)) {
+    const q = token[2] ? parseFloat(token[2]) || 0 : 1;
+    langsWithQ.push({ lang: token[1].trim(), q, index: token.index });
+  }
+
+  // order by q descending, keeping the header order for equal weights
+  langsWithQ.sort((a, b) => (a.q === b.q ? a.index - b.index : b.q - a.q));
+
+  return langsWithQ.map(val => val.lang);
 }

fluent-langneg/test/headers_test.js

@@ -18,6 +18,36 @@ 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", "de", "*", "en", "fr-CH"]
+    );
+  });
+
+  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, NaN, Infinity, [], {}];