negative discount for rule leads to floating point error

[xprojects-de]

Isotope 2.9.6, C4.13, PHP 8.1 and also PHP 8.3

A discount with a negative percentage value will randomly lead to incorrect calculations (see screenshots. 139: wrong, 159 ok)

The case here is a product price of 139.00 and a discount of -10% as rule for each Product and normal rounding.

After testing and discussion it seems to be a floating point error for ceil at

core/system/modules/isotope_rules/library/Isotope/Model/ProductCollectionSurcharge/Rule.php

Line 205 in 14c204b

$fltPrice = $fltPrice > 0 ? (floor($fltPrice * 100) / 100) : (ceil($fltPrice * 100) / 100);

As @fritzmg said the error can be reproduced using

$fltPrice = (139.0 / 100) * -10.0;

var_dump([
    $fltPrice,
    $fltPrice * 100,
    ceil($fltPrice * 100),
    ceil($fltPrice * 100) / 100,
]);

More test results:

dd at line 205:

dd(
    $fltPrice,
    $fltPrice * 100,
    ceil($fltPrice * 100),
    ceil($fltPrice * 100) / 100,
);

// Result:
^ -13.9
^ -1390.0
^ -1389.0
^ -13.89
^ -13.9
^ -1390.0
^ -1389.0
^ -13.89

[ausi]

You can round it to the desired precision beforehand (maybe only directly before calling floor or ceil). But it depends on what the code actually wants to have as a result in certain edge cases...

$fltPrice = (139.0 / 100) * -10.0;
$fltPrice = round($fltPrice, 5);

var_dump([
    $fltPrice,
    $fltPrice * 100,
    ceil($fltPrice * 100),
    ceil($fltPrice * 100) / 100,
]);

// array(4) {
//   [0]=>
//   float(-13.9)
//   [1]=>
//   float(-1390)
//   [2]=>
//   float(-1390)
//   [3]=>
//   float(-13.9)
// }