Count compared to zero

count($x) > 0 and count($x) = 0 ask the processor to walk the whole sequence and tally it, only to find out whether it holds anything. The question is existence, and every version of XPath can state it directly — and let the engine stop at the first item instead of counting every one.

Incorrect:

<xsl:if test="count($items) &gt; 0">
<xsl:if test="count($items) = 0">

Correct, on XSLT 2.0 and later:

<xsl:if test="exists($items)">
<xsl:if test="empty($items)">

Correct, on XSLT 1.0 (where exists()/empty() do not exist):

<xsl:if test="$items">
<xsl:if test="not($items)">

A node-set in a boolean context is already true exactly when it is non-empty, so in an xsl:if/xsl:when @test the bare $items says it; in a @select (or any value context) write boolean($items), and for the empty case not($items) either way.

The --fix picks the form the stylesheet's version can run: exists()/empty() on 2.0/3.0, and boolean()/not() — or the bare node-set in a whole @test — on 1.0. The 1.0 forms are valid in every version, so an unversioned stylesheet gets them too.

The call is the standard fn:count however its namespace is spelled: bare, as the default function namespace; behind a prefix bound to that namespace, which is the idiomatic fn:count($items) = 0 of a 2.0 stylesheet; or with the namespace written inline. A function of your own that happens to be called count is another function and is left alone, and so is a call spelling no argument or several, fn:count taking exactly one.

Nor does the operator's spelling. XSLT 2.0 brought the value comparisons, which ask the same six questions in words, so count($items) eq 0, count($items) ne 0 and 0 lt count($items) are the existence tests = 0, != 0 and 0 < are, and collapse to the same exists()/empty() — the direct form is a call and carries no operator, so nothing of the original spelling is lost in dropping it.

The operand order does not matter: 0 < count($items) and 0 = count($items) are flagged the same way. A comparison that is not an existence test — count($x) > 1, count($x) = 5 — is a genuine count and is left alone. So is a 0 or 1 that is only part of a wider operand: in $max + 1 > count($x) the left side is $max + 1, and in count($x) > 0 + $n the right side is 0 + $n, so neither compares the call against a bare 0 or 1.

The comparison is caught in the XPath and pattern attributes of XSLT elements and inside an attribute value template, where <div empty="{count($items) = 0}"/> holds a real expression; both a comparison and its rewrite print true/false there, so the fix stays safe. An attribute of your output vocabulary that happens to be called test or select carries text for the result tree, not XPath, and is never touched.