add unsaturation event to priorityQueue

lib/priorityQueue.js

@@ -53,6 +53,9 @@ export default function(worker, concurrency) {
             if (q.tasks.length === q.concurrency) {
                 q.saturated();
             }
+            if (q.tasks.length <= (q.concurrency - q.buffer) ) {
+                q.unsaturated();
+            }
             setImmediate(q.process);
         });
     }

mocha_test/priorityQueue.js

@@ -0,0 +1,70 @@
+var async = require('../lib');
+var expect = require('chai').expect;
+
+describe('priorityQueue', function() {
+    context('q.unsaturated(): ',function() {
+        it('should have a default buffer property that equals 25% of the concurrenct rate', function(done) {
+            var calls = [];
+            var q = async.priorityQueue(function(task, cb) {
+                // nop
+                calls.push('process ' + task);
+                async.setImmediate(cb);
+            }, 10);
+            expect(q.buffer).to.equal(2.5);
+            done();
+        });
+
+        it('should allow a user to change the buffer property', function(done) {
+            var calls = [];
+            var q = async.priorityQueue(function(task, cb) {
+                // nop
+                calls.push('process ' + task);
+                async.setImmediate(cb);
+            }, 10);
+            q.buffer = 4;
+            expect(q.buffer).to.not.equal(2.5);
+            expect(q.buffer).to.equal(4);
+            done();
+        });
+
+        it('should call the unsaturated callback if tasks length is less than concurrency minus buffer', function(done) {
+            var calls = [];
+            var q = async.priorityQueue(function(task, cb) {
+                calls.push('process ' + task);
+                async.setImmediate(cb);
+            }, 10);
+            q.unsaturated = function() {
+                calls.push('unsaturated');
+            };
+            q.empty = function() {
+                expect(calls.indexOf('unsaturated')).to.be.above(-1);
+                setTimeout(function() {
+                    expect(calls).eql([
+                        'unsaturated',
+                        'unsaturated',
+                        'unsaturated',
+                        'unsaturated',
+                        'unsaturated',
+                        'process foo4',
+                        'process foo3',
+                        'process foo2',
+                        'process foo1',
+                        'process foo0',
+                        'foo4 cb',
+                        'foo3 cb',
+                        'foo2 cb',
+                        'foo1 cb',
+                        'foo0 cb'
+                    ]);
+                    done();
+                }, 50);
+            };
+            q.push('foo0', 5, function () {calls.push('foo0 cb');});
+            q.push('foo1', 4, function () {calls.push('foo1 cb');});
+            q.push('foo2', 3, function () {calls.push('foo2 cb');});
+            q.push('foo3', 2, function () {calls.push('foo3 cb');});
+            q.push('foo4', 1, function () {calls.push('foo4 cb');});
+        });
+    });
+});
+