Added an option preferred encodings array

README.md

@@ -172,6 +172,10 @@ Returns the most preferred encoding from the client.
 
 Returns the most preferred encoding from a list of available encodings.
 
+##### encoding(availableEncodings, preferred)
+
+Returns the most preferred encoding from a list of available encodings, while prioritizing based on `preferred` array between same-quality encodings.
+
 ##### encodings()
 
 Returns an array of preferred encodings ordered by the client preference.
@@ -181,6 +185,11 @@ Returns an array of preferred encodings ordered by the client preference.
 Returns an array of preferred encodings ordered by priority from a list of
 available encodings.
 
+##### encodings(availableEncodings, preferred)
+
+Returns an array of preferred encodings ordered by priority from a list of
+available encodings, while prioritizing based on `preferred` array between same-quality encodings.
+
 ## See Also
 
 The [accepts](https://npmjs.org/package/accepts#readme) module builds on

index.js

@@ -44,13 +44,13 @@ Negotiator.prototype.charsets = function charsets(available) {
   return preferredCharsets(this.request.headers['accept-charset'], available);
 };
 
-Negotiator.prototype.encoding = function encoding(available) {
-  var set = this.encodings(available);
+Negotiator.prototype.encoding = function encoding(available, preferred) {
+  var set = this.encodings(available, preferred);
   return set && set[0];
 };
 
-Negotiator.prototype.encodings = function encodings(available) {
-  return preferredEncodings(this.request.headers['accept-encoding'], available);
+Negotiator.prototype.encodings = function encodings(available, preferred) {
+  return preferredEncodings(this.request.headers['accept-encoding'], available, preferred);
 };
 
 Negotiator.prototype.language = function language(available) {

lib/encoding.js

@@ -96,7 +96,7 @@ function parseEncoding(str, i) {
  */
 
 function getEncodingPriority(encoding, accepted, index) {
-  var priority = {o: -1, q: 0, s: 0};
+  var priority = {encoding: encoding, o: -1, q: 0, s: 0};
 
   for (var i = 0; i < accepted.length; i++) {
     var spec = specify(encoding, accepted[i], index);
@@ -123,6 +123,7 @@ function specify(encoding, spec, index) {
   }
 
   return {
+    encoding: encoding,
     i: index,
     o: spec.i,
     q: spec.q,
@@ -135,14 +136,34 @@ function specify(encoding, spec, index) {
  * @public
  */
 
-function preferredEncodings(accept, provided) {
+function preferredEncodings(accept, provided, preferred) {
   var accepts = parseAcceptEncoding(accept || '');
 
+  var comparator = preferred ? function comparator (a, b) {
+    if (a.q !== b.q) {
+      return b.q - a.q // higher quality first
+    }
+
+    var aPreferred = preferred.indexOf(a.encoding)
+    var bPreferred = preferred.indexOf(b.encoding)
+
+    if (aPreferred === -1 && bPreferred === -1) {
+      // consider the original specifity/order
+      return (b.s - a.s) || (a.o - b.o) || (a.i - b.i)
+    }
+
+    if (aPreferred !== -1 && bPreferred !== -1) {
+      return aPreferred - bPreferred // consider the preferred order
+    }
+
+    return aPreferred === -1 ? 1 : -1 // preferred first
+  } : compareSpecs;
+
   if (!provided) {
     // sorted list of all encodings
     return accepts
       .filter(isQuality)
-      .sort(compareSpecs)
+      .sort(comparator)
       .map(getFullEncoding);
   }
 
@@ -151,7 +172,7 @@ function preferredEncodings(accept, provided) {
   });
 
   // sorted list of accepted encodings
-  return priorities.filter(isQuality).sort(compareSpecs).map(function getEncoding(priority) {
+  return priorities.filter(isQuality).sort(comparator).map(function getEncoding(priority) {
     return provided[priorities.indexOf(priority)];
   });
 }
@@ -162,7 +183,7 @@ function preferredEncodings(accept, provided) {
  */
 
 function compareSpecs(a, b) {
-  return (b.q - a.q) || (b.s - a.s) || (a.o - b.o) || (a.i - b.i) || 0;
+  return (b.q - a.q) || (b.s - a.s) || (a.o - b.o) || (a.i - b.i);
 }
 
 /**

test/encoding.js

@@ -190,6 +190,14 @@ describe('negotiator.encoding(array)', function () {
     it('should return first client-preferred encoding', function () {
       assert.strictEqual(this.negotiator.encoding(['deflate', 'compress']), 'deflate')
     })
+
+    it('should return developer-preferred encodings', function () {
+      assert.strictEqual(this.negotiator.encoding(['gzip', 'deflate'], ['deflate']), 'deflate')
+      assert.strictEqual(this.negotiator.encoding(['deflate', 'gzip'], ['deflate']), 'deflate')
+      assert.strictEqual(this.negotiator.encoding(['gzip', 'deflate'], ['gzip']), 'gzip')
+      assert.strictEqual(this.negotiator.encoding(['deflate', 'gzip'], ['gzip']), 'gzip')
+      assert.strictEqual(this.negotiator.encoding(['gzip'], ['gzip']), 'gzip')
+    })
   })
 
   whenAcceptEncoding('gzip;q=0.8, deflate', function () {
@@ -204,6 +212,14 @@ describe('negotiator.encoding(array)', function () {
     it('should return most client-preferred encoding', function () {
       assert.strictEqual(this.negotiator.encoding(['gzip']), 'gzip')
       assert.strictEqual(this.negotiator.encoding(['compress', 'identity']), 'identity')
+      assert.strictEqual(this.negotiator.encoding(['gzip', 'deflate'], ['deflate']), 'gzip')
+      assert.strictEqual(this.negotiator.encoding(['deflate', 'gzip'], ['deflate']), 'gzip')
+    })
+
+    it('should return developer-preferred encodings', function () {
+      assert.strictEqual(this.negotiator.encoding(['gzip', 'deflate'], ['gzip']), 'gzip')
+      assert.strictEqual(this.negotiator.encoding(['deflate', 'gzip'], ['gzip']), 'gzip')
+      assert.strictEqual(this.negotiator.encoding(['gzip'], ['gzip']), 'gzip')
     })
   })
 })
@@ -401,6 +417,14 @@ describe('negotiator.encodings(array)', function () {
       assert.deepEqual(this.negotiator.encodings(['deflate', 'gzip']), ['gzip', 'deflate'])
       assert.deepEqual(this.negotiator.encodings(['identity']), ['identity'])
     })
+
+    it('should return developer-preferred encodings', function () {
+      assert.deepEqual(this.negotiator.encodings(['gzip', 'deflate'], ['deflate']), ['deflate', 'gzip'])
+      assert.deepEqual(this.negotiator.encodings(['deflate', 'gzip'], ['deflate']), ['deflate', 'gzip'])
+      assert.deepEqual(this.negotiator.encodings(['gzip', 'deflate'], ['gzip']), ['gzip', 'deflate'])
+      assert.deepEqual(this.negotiator.encodings(['deflate', 'gzip'], ['gzip']), ['gzip', 'deflate'])
+      assert.deepEqual(this.negotiator.encodings(['gzip'], ['gzip']), ['gzip'])
+    })
   })
 
   whenAcceptEncoding('gzip;q=0.8, deflate', function () {
@@ -415,6 +439,14 @@ describe('negotiator.encodings(array)', function () {
     it('should return client-preferred encodings', function () {
       assert.deepEqual(this.negotiator.encodings(['gzip']), ['gzip'])
       assert.deepEqual(this.negotiator.encodings(['identity', 'gzip', 'compress']), ['gzip', 'identity', 'compress'])
+      assert.deepEqual(this.negotiator.encodings(['gzip', 'deflate'], ['deflate']), ['gzip', 'deflate'])
+      assert.deepEqual(this.negotiator.encodings(['deflate', 'gzip'], ['deflate']), ['gzip', 'deflate'])
+    })
+
+    it('should return developer-preferred encodings', function () {
+      assert.deepEqual(this.negotiator.encodings(['gzip', 'deflate'], ['gzip']), ['gzip', 'deflate'])
+      assert.deepEqual(this.negotiator.encodings(['deflate', 'gzip'], ['gzip']), ['gzip', 'deflate'])
+      assert.deepEqual(this.negotiator.encodings(['gzip'], ['gzip']), ['gzip'])
     })
   })
 })