eq(), get() and slice() ignore negative indexes; slice(-2) returns the whole set #142

Description

eq(), get() and slice() don't support negative indexes. jQuery counts a negative index back from the end of the set. QueryPath returns nothing for eq() and get(), and the whole set for slice():

$xml = '<?xml version="1.0"?><r><i id="a"/><i id="b"/><i id="c"/><i id="d"/><i id="e"/></r>';
$qp  = qp($xml, 'i');

The slice(-2) result is the worst of these: the call returns a set, just the wrong one, so nothing signals the mistake.

Cause

• DOM::getNthMatch() (src/DOM.php) returns NULL for any $index < 0. Both eq() and get($index) go through it.
• slice() (src/Helpers/QueryFilters.php) keeps every match where $i >= $start. That is true for every $i when $start is negative.

Expected

As in jQuery (and array_slice(), which the slice() DocBlock cites), a negative index counts from the end:

• eq(-n) and get(-n) mean the match at count() - n.
• slice(-n) starts n from the end.
• An index still out of range after that gives an empty set, or NULL from get().

No working code passes a negative index today, so this should be safe to fix in a minor release. The meaning of slice()'s second argument is a separate difference from jQuery (#144).

QueryPath version

main at 4d05c9e. The jQuery values come from jQuery 3.7.1 on jsdom.