Add logic to automatically select the best caching strategy

Based on the number of unique shared strings as well as the available memory amount,
one strategy will be chosen over the other.
The algorithm is based on empirical data and super safe so it may need to be tuned.

tests/Spout/Reader/Helper/XLSX/SharedStringsCaching/CachingStrategyFactoryTest.php

@@ -0,0 +1,99 @@
+<?php
+
+namespace Box\Spout\Reader\Helper\XLSX\SharedStringsCaching;
+
+/**
+ * Class CachingStrategyFactoryTest
+ *
+ * @package Box\Spout\Reader\Helper\XLSX\SharedStringsCaching
+ */
+class CachingStrategyFactoryTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * @return array
+     */
+    public function dataProviderForTestGetBestCachingStrategy()
+    {
+        return [
+            [CachingStrategyFactory::MAX_NUM_STRINGS_PER_TEMP_FILE, -1, 'FileBasedStrategy'],
+            [CachingStrategyFactory::MAX_NUM_STRINGS_PER_TEMP_FILE + 10, -1, 'FileBasedStrategy'],
+            [CachingStrategyFactory::MAX_NUM_STRINGS_PER_TEMP_FILE - 10, -1, 'InMemoryStrategy'],
+            [10 , CachingStrategyFactory::AMOUNT_MEMORY_NEEDED_PER_STRING_IN_KB * 10, 'FileBasedStrategy'],
+            [15, CachingStrategyFactory::AMOUNT_MEMORY_NEEDED_PER_STRING_IN_KB * 10, 'FileBasedStrategy'],
+            [5 , CachingStrategyFactory::AMOUNT_MEMORY_NEEDED_PER_STRING_IN_KB * 10, 'InMemoryStrategy'],
+        ];
+    }
+
+    /**
+     * @dataProvider dataProviderForTestGetBestCachingStrategy
+     *
+     * @param int $sharedStringsUniqueCount
+     * @param int $memoryLimitInKB
+     * @param string $expectedStrategyClassName
+     * @return void
+     */
+    public function testGetBestCachingStrategy($sharedStringsUniqueCount, $memoryLimitInKB, $expectedStrategyClassName)
+    {
+        /** @var CachingStrategyFactory|\PHPUnit_Framework_MockObject_MockObject $factoryStub */
+        $factoryStub = $this
+            ->getMockBuilder('\Box\Spout\Reader\Helper\XLSX\SharedStringsCaching\CachingStrategyFactory')
+            ->disableOriginalConstructor()
+            ->setMethods(['getMemoryLimitInKB'])
+            ->getMock();
+
+        $factoryStub->method('getMemoryLimitInKB')->willReturn($memoryLimitInKB);
+
+        \ReflectionHelper::setStaticValue('\Box\Spout\Reader\Helper\XLSX\SharedStringsCaching\CachingStrategyFactory', 'instance', $factoryStub);
+
+        $strategy = $factoryStub->getBestCachingStrategy($sharedStringsUniqueCount, null);
+
+        $fullExpectedStrategyClassName = 'Box\Spout\Reader\Helper\XLSX\SharedStringsCaching\\' . $expectedStrategyClassName;
+        $this->assertEquals($fullExpectedStrategyClassName, get_class($strategy));
+
+        $strategy->clearCache();
+        \ReflectionHelper::reset();
+    }
+
+    /**
+     * @return array
+     */
+    public function dataProviderForTestGetMemoryLimitInKB()
+    {
+        return [
+            ['-1', -1],
+            ['invalid', -1],
+            ['1024B', 1],
+            ['128K', 128],
+            ['256KB', 256],
+            ['512M', 512 * 1024],
+            ['2MB', 2 * 1024],
+            ['1G', 1 * 1024 * 1024],
+            ['10GB', 10 * 1024 * 1024],
+            ['2T', 2 * 1024 * 1024 * 1024],
+            ['5TB', 5 * 1024 * 1024 * 1024],
+        ];
+    }
+
+    /**
+     * @dataProvider dataProviderForTestGetMemoryLimitInKB
+     *
+     * @param string $memoryLimitFormatted
+     * @param float $expectedMemoryLimitInKB
+     * @return void
+     */
+    public function testGetMemoryLimitInKB($memoryLimitFormatted, $expectedMemoryLimitInKB)
+    {
+        /** @var CachingStrategyFactory|\PHPUnit_Framework_MockObject_MockObject $factoryStub */
+        $factoryStub = $this
+            ->getMockBuilder('\Box\Spout\Reader\Helper\XLSX\SharedStringsCaching\CachingStrategyFactory')
+            ->disableOriginalConstructor()
+            ->setMethods(['getMemoryLimitFromIni'])
+            ->getMock();
+
+        $factoryStub->method('getMemoryLimitFromIni')->willReturn($memoryLimitFormatted);
+
+        $memoryLimitInKB = \ReflectionHelper::callMethodOnObject($factoryStub, 'getMemoryLimitInKB');
+
+        $this->assertEquals($expectedMemoryLimitInKB, $memoryLimitInKB);
+    }
+}

tests/Spout/Reader/Helper/XLSX/SharedStringsHelperTest.php

@@ -87,6 +87,10 @@ class SharedStringsHelperTest extends \PHPUnit_Framework_TestCase
      */
     public function testGetStringAtIndexWithFileBasedStrategy()
     {
+        // force the file-based strategy by setting no memory limit
+        $originalMemoryLimit = ini_get('memory_limit');
+        ini_set('memory_limit', '-1');
+
         $resourcePath = $this->getResourcePath('sheet_with_lots_of_shared_strings.xlsx');
         $sharedStringsHelper = new SharedStringsHelper($resourcePath);
 
@@ -102,5 +106,7 @@ class SharedStringsHelperTest extends \PHPUnit_Framework_TestCase
         $this->assertTrue($usedCachingStrategy instanceof FileBasedStrategy);
 
         $sharedStringsHelper->cleanup();
+
+        ini_set('memory_limit', $originalMemoryLimit);
     }
 }