104 lines
4.4 KiB
PHP
104 lines
4.4 KiB
PHP
<?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 readonly 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 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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}
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