Doubt about the MOD operator in PHP

Question

I have a code PHP that performs a certain calculation, however it is resulting something different than I expect, I performed the calculations in hand and did not generate the same result, anyone could explain to me why? The code for execution is below:

function bsd_rand($seed) {
return function() use (&$seed) {
    return $seed = (1103515245 * $seed + 12345) % (1 << 31);
};
}

$lcg = bsd_rand(0);
echo "BSD ";
for ($i = 0; $i < 10; $i++)
echo $lcg(), " ";
echo "\n";

The first value results in 12345, but doing this same calculation on paper, did not result such value, note:

(1103515245 * 0 + 12345) % (-2147483648) = ?

The result should be -2147471303 and not 12345

Answer 1

The explanation has already been given by @chambelix, apparently, the operator mod (%) does not work very well in PHP when the negative divisor "module" (read the number without the negative sign) is greater than the positive dividend... Or vice versa, see tests:

echo (12345 % (-2147471303))."\n"; // Retorna 12345 (errado)
echo ((-2147471303) % 12345)."\n"; // Retorna -9173 (errado)

To have a better experience with very large negative values, there is a function that solves the problem:

function truemod($num, $mod) {
  return ($mod + ($num % $mod)) % $mod;
}

echo truemod(12345,-2147471303)."\n"; // Retorna -2147471303 (certo)
echo truemod(-2147471303,12345); // Retorna 3172 (certo)

---

Honestly, I think the function you’re using is somewhat confusing.... I would do it that way:

for ($i = 0; $i < 10; $i++){
    $numerador = (1103515245 * $i + 12345);
    $seed = truemod($numerador,-2147471303);
    echo "BSD: " . $seed . "\n";
}
echo "\n";

function truemod($num, $mod) {
  return ($mod + ($num % $mod)) % $mod;
}

---

In that case: &$seed the operator & acts as a reference to the variable $seed which was passed by the parent function...

/* I think it’s crazy to understand this function below, or see a utility, but ok... */

function bsd_rand($seed) {
    return function() use (&$seed) {
        //Retorne essa função, utilizando a mesma referencia de variável que gerou a variável $seed
    }
}

In general, the operator & $variavel then acts as a reference...

Useful links:

What are references?

What references do

Answer 2

No PHP the operator % refers to:

$a % $b Módulo  Resto de $a dividido por $b.

where the module operands are converted to integers (removing the decimal part) before processing... to which I still add:

O resto de $a % $b é negativo se $a for negativo.

Which puts right away the result that should in its impossible view.

Add even though the modulus in PHP if the first operand is lower than the second, then the value of the first operand is returned.