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<?php
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declare(strict_types=1);
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namespace OpenSpout\Reader\XLSX\Manager\SharedStringsCaching;
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/**
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 * @internal
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 */
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final class CachingStrategyFactory implements CachingStrategyFactoryInterface
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{
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    /**
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     * The memory amount needed to store a string was obtained empirically from this data:.
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     *
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     *        ------------------------------------
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     *        | Number of chars⁺ | Memory needed |
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     *        ------------------------------------
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     *        |           3,000  |         1 MB  |
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     *        |          15,000  |         2 MB  |
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     *        |          30,000  |         5 MB  |
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     *        |          75,000  |        11 MB  |
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     *        |         150,000  |        21 MB  |
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     *        |         300,000  |        43 MB  |
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     *        |         750,000  |       105 MB  |
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     *        |       1,500,000  |       210 MB  |
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     *        |       2,250,000  |       315 MB  |
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     *        |       3,000,000  |       420 MB  |
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     *        |       4,500,000  |       630 MB  |
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     *        ------------------------------------
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     *
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     *        ⁺ All characters were 1 byte long
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     *
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     * This gives a linear graph where each 1-byte character requires about 150 bytes to be stored.
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     * Given that some characters can take up to 4 bytes, we need 600 bytes per character to be safe.
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     * Also, there is on average about 20 characters per cell (this is entirely empirical data...).
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     *
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     * This means that in order to store one shared string in memory, the memory amount needed is:
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     *   => 20 * 600 ≈ 12KB
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     */
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    public const AMOUNT_MEMORY_NEEDED_PER_STRING_IN_KB = 12;
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    /**
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     * To avoid running out of memory when extracting a huge number of shared strings, they can be saved to temporary files
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     * instead of in memory. Then, when accessing a string, the corresponding file contents will be loaded in memory
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     * and the string will be quickly retrieved.
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     * The performance bottleneck is not when creating these temporary files, but rather when loading their content.
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     * Because the contents of the last loaded file stays in memory until another file needs to be loaded, it works
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     * best when the indexes of the shared strings are sorted in the sheet data.
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     * 10,000 was chosen because it creates small files that are fast to be loaded in memory.
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     */
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    public const MAX_NUM_STRINGS_PER_TEMP_FILE = 10000;
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    private readonly MemoryLimit $memoryLimit;
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    public function __construct(MemoryLimit $memoryLimit)
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    {
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        $this->memoryLimit = $memoryLimit;
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    }
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    /**
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     * Returns the best caching strategy, given the number of unique shared strings
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     * and the amount of memory available.
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     *
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     * @param null|int $sharedStringsUniqueCount Number of unique shared strings (NULL if unknown)
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     * @param string   $tempFolder               Temporary folder where the temporary files to store shared strings will be stored
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     *
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     * @return CachingStrategyInterface The best caching strategy
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     */
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    public function createBestCachingStrategy(?int $sharedStringsUniqueCount, string $tempFolder): CachingStrategyInterface
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    {
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        if ($this->isInMemoryStrategyUsageSafe($sharedStringsUniqueCount)) {
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            return new InMemoryStrategy($sharedStringsUniqueCount);
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        }
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        return new FileBasedStrategy($tempFolder, self::MAX_NUM_STRINGS_PER_TEMP_FILE);
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    }
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    /**
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     * Returns whether it is safe to use in-memory caching, given the number of unique shared strings
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     * and the amount of memory available.
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     *
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     * @param null|int $sharedStringsUniqueCount Number of unique shared strings (NULL if unknown)
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     */
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    private function isInMemoryStrategyUsageSafe(?int $sharedStringsUniqueCount): bool
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    {
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        // if the number of shared strings in unknown, do not use "in memory" strategy
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        if (null === $sharedStringsUniqueCount) {
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            return false;
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        }
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        $memoryAvailable = $this->memoryLimit->getMemoryLimitInKB();
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        if (-1 === (int) $memoryAvailable) {
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            // if cannot get memory limit or if memory limit set as unlimited, don't trust and play safe
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            $isInMemoryStrategyUsageSafe = ($sharedStringsUniqueCount < self::MAX_NUM_STRINGS_PER_TEMP_FILE);
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        } else {
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            $memoryNeeded = $sharedStringsUniqueCount * self::AMOUNT_MEMORY_NEEDED_PER_STRING_IN_KB;
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            $isInMemoryStrategyUsageSafe = ($memoryAvailable > $memoryNeeded);
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        }
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        return $isInMemoryStrategyUsageSafe;
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    }
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}