Fix generator xrange() example

[KodyWiremane]

First, the increment part in the else branch's loop must be +=, not -=, because $step is negative there.
Second, $start == $limit is OK for range(), the resulting sequence will simply contain only one value.

Also, I've noticed some deviation from how range() behaves: it ignores $step's sign, and derives it from which of $start and $limit is greater. It also emits a warning if $start != $limit && abs($step) > abs($limit - $start) , and returns false (as well as if $step == 0).

If xrange() is thought of as an equivalent of range(), this should be addressed. Though, personally, I more like xrange() the way it is (except for the two errors initially mentioned), as it looks more consistent.

To check for [dis]similarities between range() and xrange(), I've used

the following code

define('MAX_ITERATIONS', 20);

function testFunction($func, $args) {
    list($from, $to, $step) = $args;
    $counter = 0;
    try {
        foreach ($func($from, $to, $step) as $number) {
            echo " $number";
            $counter++;
            if ($counter > MAX_ITERATIONS) {
                echo ' INT' . PHP_EOL;
                break;
            }
        }
    } catch (Throwable $e) {
        echo " EXP: {$e->getMessage()}";
    }
    echo PHP_EOL;
}

$cases = [
    [2, 11, 2],
    [2, 11, -2],
    [2, 11, 0],
    [2, 2, 0],
    [11, 2, 2],
    [11, 2, -2],
    [2, 2, 2],
    [2, -2, -2],
    [2, 4, 10]
];

foreach ($cases as $case) {

    $caseAsString = implode(', ', $case);

    echo "Testing [$caseAsString]:" . PHP_EOL;
    echo 'range(): ';
    testFunction('range', $case);
    echo 'xrange():';
    testFunction('xrange', $case);
    echo PHP_EOL;

}

[nikic]

Thanks! I don't think it's necessary to match range() exactly, as long as the code is correct and illustrates the generator usage.

@php-pulls merge