Merge pull request #137 from einorler/pipeline_serial_diff

Added serial differencing pipeline aggregation

docs/Aggregation/Pipeline/SerialDifferencing.md

+# Serial Differencing Aggregation
+
+> More info about serial differencing aggregation is in the [official elasticsearch docs][1]
+
+Serial differencing is a technique where values in a time series are subtracted from itself at 
+different time lags or periods
+
+## Simple example
+
+```JSON
+{
+   "aggs": {
+      "my_date_histo": {                  
+         "date_histogram": {
+            "field": "timestamp",
+            "interval": "day"
+         },
+         "aggs": {
+            "the_sum": {
+               "sum": {
+                  "field": "lemmings"     
+               }
+            },
+            "thirtieth_difference": {
+               "serial_diff": {                
+                  "buckets_path": "the_sum",
+                  "lag" : 30
+               }
+            }
+         }
+      }
+   }
+}
+```
+
+And now the query via DSL:
+
+```php
+$search = new Search();
+
+$dateAggregation = new DateHistogramAggregation('my_date_histo', 'timestamp', 'day');
+$dateAggregation->addAggregation(
+    new SumAggregation('the_sum', 'lemmings')
+);
+$diffAggregation = new SerialDifferencingAggregation('thirtieth_difference', 'the_sum');
+$diffAggregation->addParameter('lag', 30);
+$dateAggregation->addAggregation($diffAggregation);
+
+$search->addAggregation($dateAggregation);
+
+$aggArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-pipeline-serialdiff-aggregation.html
\ No newline at end of file

src/Aggregation/Pipeline/SerialDifferencingAggregation.php

+<?php
+
+/*
+ * This file is part of the ONGR package.
+ *
+ * (c) NFQ Technologies UAB <info@nfq.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace ONGR\ElasticsearchDSL\Aggregation\Pipeline;
+
+/**
+ * Class representing Serial Differencing Pipeline Aggregation.
+ *
+ * @link https://goo.gl/46ZR4v
+ */
+class SerialDifferencingAggregation extends AbstractPipelineAggregation
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function getType()
+    {
+        return 'serial_diff';
+    }
+}

tests/Aggregation/Pipeline/SerialDifferencingAggregationTest.php

+<?php
+
+/*
+ * This file is part of the ONGR package.
+ *
+ * (c) NFQ Technologies UAB <info@nfq.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace ONGR\ElasticsearchDSL\Tests\Aggregation\Pipeline;
+
+use ONGR\ElasticsearchDSL\Aggregation\Pipeline\SerialDifferencingAggregation;
+
+/**
+ * Unit test for serial differencing aggregation.
+ */
+class SerialDifferencingAggregationTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * Tests toArray method.
+     */
+    public function testToArray()
+    {
+        $aggregation = new SerialDifferencingAggregation('acme', 'test');
+        $aggregation->addParameter('lag', '7');
+
+        $expected = [
+            'serial_diff' => [
+                'buckets_path' => 'test',
+                'lag' => '7'
+            ],
+        ];
+
+        $this->assertEquals($expected, $aggregation->toArray());
+    }
+}