Move shared strings caching strategy into its own component

This will help implementing different caching strategies:
- file based
- in-memory

src/Spout/Reader/Helper/XLSX/SharedStringsCaching/CachingStrategyFactory.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace Box\Spout\Reader\Helper\XLSX\SharedStringsCaching;
+
+/**
+ * Class CachingStrategyFactory
+ *
+ * @package Box\Spout\Reader\Helper\XLSX\SharedStringsCaching
+ */
+class CachingStrategyFactory
+{
+    /**
+     * To avoid running out of memory when extracting the shared strings, they will be saved to temporary files
+     * instead of in memory. Then, when accessing a string, the corresponding file contents will be loaded in memory
+     * and the string will be quickly retrieved.
+     * The performance bottleneck is not when creating these temporary files, but rather when loading their content.
+     * Because the contents of the last loaded file stays in memory until another file needs to be loaded, it works
+     * best when the indexes of the shared strings are sorted in the sheet data.
+     * 10,000 was chosen because it creates small files that are fast to be loaded in memory.
+     */
+    const MAX_NUM_STRINGS_PER_TEMP_FILE = 10000;
+
+    /**
+     * Returns the best caching strategy, given the number of unique shared strings
+     * and the amount of memory available.
+     *
+     * @param int $sharedStringsUniqueCount Number of unique shared strings
+     * @param string|void $tempFolder Temporary folder where the temporary files to store shared strings will be stored
+     * @return CachingStrategyInterface The best caching strategy
+     */
+    public function getBestCachingStrategy($sharedStringsUniqueCount, $tempFolder = null)
+    {
+        // TODO add in-memory strategy
+        return new FileBasedStrategy($tempFolder, self::MAX_NUM_STRINGS_PER_TEMP_FILE);
+    }
+}