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@ 2020-08-27T10:30:26Z

<?php
declare(strict_types=1);

function ct_select(
    bool $returnLeft,
    string $left,
    string $right
): string {
    $length = mb_strlen($left, '8bit');
    if (mb_strlen($right, '8bit') !== $length) {
        throw new Exception('ct_select() expects two strings of equal length');
    }
    
    // Mask byte
    $mask = (-$returnLeft) & 0xff;
    // X
    $x = (string) ($left ^ $right);
    
    // Output = Right XOR (X AND Mask)
    $output = '';
    for ($i = 0; $i < $length; $i++) {
        $rightCharCode = unpack('C', $right[$i])[1];
        $xCharCode = unpack('C', $x[$i])[1];
        $output .= pack(
            'C',
            $rightCharCode ^ ($xCharCode & $mask)
        );
    }
    return $output;
}

var_dump(
    ct_select(true, 'apple', 'tiger'),
    ct_select(false, 'apple', 'tiger'),
);

Output for 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.28, 8.2.0 - 8.2.18, 8.3.0 - 8.3.4, 8.3.6: string(5) "apple" string(5) "tiger"
Output for 8.3.5: Warning: PHP Startup: Unable to load dynamic library 'sodium.so' (tried: /usr/lib/php/8.3.5/modules/sodium.so (libsodium.so.23: cannot open shared object file: No such file or directory), /usr/lib/php/8.3.5/modules/sodium.so.so (/usr/lib/php/8.3.5/modules/sodium.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0 string(5) "apple" string(5) "tiger"
Output for 7.2.0 - 7.2.34: Parse error: syntax error, unexpected ')' in /in/rjqb1 on line 35
Process exited with code 255.

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