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23 lines
745 B
PHP
23 lines
745 B
PHP
<?php
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/**
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* Calculate the fast inverse square root of a number.
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*
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* This function computes the fast inverse square root of a floating-point number
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* using the famous Quake III algorithm, originally developed by id Software.
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*
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* @param float $x The input number for which to calculate the inverse square root.
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* @return float The fast inverse square root of the input number.
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*/
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function fastInvSqrt($x)
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{
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// Convert the input float to an integer
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$i = unpack('l', pack('f', $x))[1];
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// Apply the bit manipulation magic number
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$i = 0x5f3759df - ($i >> 1);
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// Convert the integer back to a float
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$y = unpack('f', pack('l', $i))[1];
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// Perform the final calculation
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return $y * (1.5 - 0.5 * $x * $y * $y);
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}
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