Added fold and filter methods

src/Collection/Collection.php

@@ -357,6 +357,39 @@ public function deduplicate()
         return $this;
     }
 
+    /**
+     * Return a new collection with only filtered keys/values
+     *
+     * The callback accepts value, key, index and should return true if the item should be added to the returned
+     * collection
+     *
+     * @param callable $callback
+     *
+     * @return Collection
+     */
+    public function filter(callable $callback)
+    {
+        $collection = static::factory();
+        $index = 0;
+        foreach ($this as $key => $value) {
+            if ($callback($value, $key, $index++)) {
+                $collection->set($key, $value);
+            }
+        }
+
+        return $collection;
+    }
+
+    public function fold($callback, $initial = null)
+    {
+        $index = 0;
+        $folded = $initial;
+        foreach ($this as $key => $val) {
+            $folded = $callback($folded, $val, $key, $index++);
+        }
+        return $folded;
+    }
+
     /** ++++                  ++++ **/
     /** ++ Interface Compliance ++ **/
     /** ++++                  ++++ **/

tests/Tests/CollectionTest.php

@@ -173,6 +173,58 @@ public function testDeduplicateRemovesDuplicatesInPlace()
         ], $col->toArray());
     }
 
+    public function testFilterKeepsOnlyItemsPassingTest()
+    {
+        $arr = $this->getFixture('numwords');
+        $col = new Collection($arr);
+
+        $filtered = $col->filter(function($val, $key) {
+            return is_int($val);
+        });
+        $this->assertEquals([
+            'two' => 2,
+            'four' => 4,
+            'five' => 5
+        ], $filtered->toArray());
+
+        $filtered = $col->filter(function($val, $key) {
+            return is_int($key);
+        });
+        $this->assertEquals([
+            0 => 'zero',
+            1 => 'one',
+            3 => 'three',
+            4 => 'four'
+        ], $filtered->toArray());
+
+        $filtered = $col->filter(function($val, $key, $i) {
+            return $i % 2 == 0;
+        });
+        $this->assertEquals([
+            0 => 'zero',
+            'two' => 2,
+            'four' => 4,
+            4 => 'four'
+        ], $filtered->toArray());
+    }
+
+    public function testFoldReturnsOneValue()
+    {
+        $arr = $this->getFixture('numwords');
+        $col = new Collection($arr);
+
+        $this->assertEquals(11, $col->fold(function($accum, $val, $key, $i) {
+            if (is_int($val)) {
+                return $accum + $val;
+            }
+            return $accum;
+        }));
+
+        $this->assertEquals('[[[[[[[init-zero-0-0]-one-1-1]-2-two-2]-three-3-3]-4-four-4]-5-five-5]-four-4-6]', $col->fold(function($accum, $val, $key, $i) {
+            return "[{$accum}-{$val}-{$key}-{$i}]";
+        }, 'init'), 'Initial value passed in should be the first value of $accum');
+    }
+
     /**
      * @expectedException RuntimeException
      */

tests/Tests/TestCase.php

@@ -30,6 +30,15 @@ public function setUp()
             'secondthree' => 3,
             'thirdzero' => 0
         ];
+        $this->fixtures['numwords'] = [
+            0 => 'zero',
+            1 => 'one',
+            'two' => 2,
+            3 => 'three',
+            'four' => 4,
+            'five' => 5,
+            4 => 'four'
+        ];
     }
 
     public function tearDown()