minor symfony#25299 [Serializer] improved CsvEncoder::decode performance (Roman Orlov)

This PR was merged into the 3.4 branch.

Discussion
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[Serializer] improved CsvEncoder::decode performance

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Improved CsvEncoder::decode performance by caching duplicate count calls.
Blackfire profiles before and after change tested on collection of 10000 elements:
[before] https://blackfire.io/profiles/9c08f789-cd29-4eae-92c8-046e3849a2b8/graph
[after] https://blackfire.io/profiles/a17bfb6b-ef82-41ee-9edd-9403f829d6ab/graph

Commits
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3b910a9 [Serializer] improved CsvEncoder::decode performance by caching duplicate count calls

Author: fabpot

src/Symfony/Component/Serializer/Encoder/CsvEncoder.php

@@ -115,6 +115,7 @@ public function decode($data, $format, array $context = array())
 
         $headers = null;
         $nbHeaders = 0;
+        $headerCount = array();
         $result = array();
 
         list($delimiter, $enclosure, $escapeChar, $keySeparator) = $this->getCsvOptions($context);
@@ -126,15 +127,17 @@ public function decode($data, $format, array $context = array())
                 $nbHeaders = $nbCols;
 
                 foreach ($cols as $col) {
-                    $headers[] = explode($keySeparator, $col);
+                    $header = explode($keySeparator, $col);
+                    $headers[] = $header;
+                    $headerCount[] = count($header);
                 }
 
                 continue;
             }
 
             $item = array();
             for ($i = 0; ($i < $nbCols) && ($i < $nbHeaders); ++$i) {
-                $depth = count($headers[$i]);
+                $depth = $headerCount[$i];
                 $arr = &$item;
                 for ($j = 0; $j < $depth; ++$j) {
                     // Handle nested arrays