median() div swapped for intdiv()

[artumi-richard]

To find the median() the middle element is required in a list. Before this change that was achieved by counting the elements and dividing by 2 using the / operator.

Finding the middle of a list of values using the float div operator (/) is wrong, and can be shown to go wrong at some higher values where the int/2 is not representable as a float to single digit precision.

On a 64bit system it starts going wrong around 2**53, which means the impact of this bug is on collections with more than 2**53 elements. And the impact is that the wrong median value will be out by one.

This is solved with intdiv(). https://www.php.net/manual/en/function.intdiv.php . Here's a proof of the issue:

<?php

function test($i)
{
    $v = $i + 2 ** 53;
    $a1 = (int) ($v / 2);
    $a2 = intdiv($v, 2);

    printf("i = %d, //2**53 + %d\n", $v, $i);
    printf("(int)(i/2) = %d\n", $a1);
    printf("intdiv(i,2) = %d\n", $a2);
    echo ($a1 === $a2 ? "✅ same\n\n" : "❌ DIFFERENT!\n\n");
}

for ($i = 0; $i < 12; $i++) {
    test($i);
}

Which produces the output:

i = 9007199254740992, //2**53 + 0
(int)(i/2) = 4503599627370496
intdiv(i,2) = 4503599627370496
✅ same

i = 9007199254740993, //2**53 + 1
(int)(i/2) = 4503599627370496
intdiv(i,2) = 4503599627370496
✅ same

i = 9007199254740994, //2**53 + 2
(int)(i/2) = 4503599627370497
intdiv(i,2) = 4503599627370497
✅ same

i = 9007199254740995, //2**53 + 3
(int)(i/2) = 4503599627370498
intdiv(i,2) = 4503599627370497
❌ DIFFERENT!

i = 9007199254740996, //2**53 + 4
(int)(i/2) = 4503599627370498
intdiv(i,2) = 4503599627370498
✅ same

i = 9007199254740997, //2**53 + 5
(int)(i/2) = 4503599627370498
intdiv(i,2) = 4503599627370498
✅ same

i = 9007199254740998, //2**53 + 6
(int)(i/2) = 4503599627370499
intdiv(i,2) = 4503599627370499
✅ same

i = 9007199254740999, //2**53 + 7
(int)(i/2) = 4503599627370500
intdiv(i,2) = 4503599627370499
❌ DIFFERENT!

i = 9007199254741000, //2**53 + 8
(int)(i/2) = 4503599627370500
intdiv(i,2) = 4503599627370500
✅ same

i = 9007199254741001, //2**53 + 9
(int)(i/2) = 4503599627370500
intdiv(i,2) = 4503599627370500
✅ same

i = 9007199254741002, //2**53 + 10
(int)(i/2) = 4503599627370501
intdiv(i,2) = 4503599627370501
✅ same

i = 9007199254741003, //2**53 + 11
(int)(i/2) = 4503599627370502
intdiv(i,2) = 4503599627370501
❌ DIFFERENT!

This has been my favourite out by one error for some time!