XSLTC Predicate Handling
Definition
According to Michael Kay's "XSLT Programmer's Reference" page 736, a predicate is "An expression used to filter which nodes are selected by a particular step in a path expression, or to select a subset of the nodes in a node-set. A Boolean expression selects the nodes for which the predicate is true; a numeric expression selects the node at the position given by the value of the expression, for example '[1]' selects the first node.". Note that a predicate containing a boolean expression can return zero, one or more nodes, while a predicate containing a numeric expression can return only zero or one node.
Examples
I'll list a few examples that I can refer back to later on in this document. All examples will use this XML document:
<?xml version="1.0"?>
<doc>
<foo location="Drumcondra">
<bar name="Cat and Cage"/>
<bar name="Fagan's"/>
<bar name="Gravedigger's"/>
<bar name="Ivy House"/>
<foo>
<foo location="Town">
<bar name="Peter's Pub"/>
<bar name="Grogan's"/>
<bar name="Hogans's"/>
<bar name="Brogan's"/>
</foo>
</doc>
Here are some examples of a predicate with boolean expressions:
<xsl:for-each select="//bar[contains(@name,'ogan')]">
<xsl:for-each select="//bar[parent::*/@location = 'Drumcondra']">
<xsl:for-each select="//bar[@name = 'Cat and Cage']">
The first two select more than one node, while the last selects only one. The last expression could select more nodes if the input document was different. Now, here are a few examples of predicates with numeric expressions:
<xsl:value-of select="//bar[1]">
<xsl:value-of select="/doc/foo[2]/bar[1]">
<xsl:value-of select="/doc/foo[2]/bar">
The last expression will return more than one node, but the step that
contains the predicate returns only one (the second <foo>
element).
The above are the basic types of predicates. These can be grouped to create a predicate pipeline, where the first predicate reduces the node-set that the second predicate filters, and so on. Here are some examples:
A: <for-each select="//bar[contains(@name,'ogan')][2]">
C: <for-each select="//bar[2][contains(@name,'ogan')]">
B: <for-each select="//bar[position() > 3][2]">
It is easier to figure out which nodes these expressions should return if
one goes through the steps and predicates one by one. In expression
A: we first get all <bar> elements from the
whole document. Then the first predicate selects from that node-set only
those elements that have a @name attribute that contains
"ogan", and we're left with these elements:
<bar name="Grogan's">
<bar name="Hogans's">
<bar name="Brogan's">
And finally, the last predicate then selects the second of those elements:
<bar name="Hogans's">
Expression B: contains the same predicates as A:,
but the resulting node set if completely different. We start off with the same
set of <bar> elements, but we apply the
"[2]" predicate first, and end up with this
element:
<bar name="Fagan's">
Fagan's is the bar where the Irish Taoiseach (prime minister) drinks his
pints, but its name does not contain the string "ogan",
so the resulting node-set is empty.
The third expressions also starts off with all <bar>
elements, applies the predicate "[position() > 3]",
and reduces the node set to these:
<bar name="Ivy House">
<bar name="Peter's Pub">
<bar name="Grogan's">
<bar name="Hogans's">
<bar name="Brogan's">
The last predicate "[2]" is applied to this node-set
and set is further reduced to:
<bar name="Peter's Pub">
Categories
From the examples in the last chapter we can try to categorize predicate chains/pipelines to simplify our implementation. We can speed up processing significantly if we can avoid using a data-structure (iterator) to represent the intermediate step between predicates. The goal of setting up these categories is to pinpoint those cases where an intermediate iterator has to be used and when it can be avoided.
Single predicate expressions
Expressions containing just a single predicate have no intermediate step and there is no need for any extra iterator. The expression inside the predicate can be applied directly to the original iterator. We call this category SIMPLE_CONTEXT.
Expressions containing only non-position predicates
Predicate-order is significant when the predicate-chain contains one or
more predicate with an expression similar to
"position() > 3" or "2". This
is because the position() and last() explicitly
refer to the intermediate step between applying each predicate. The
expression:
<xsl:for-each select="//bar[contains(@name,'ogan')][parent::*/@location = 'Town']">
has two predicates that can be applied in any order and still produce the desired node-set. Such predicates can be merged to:
<xsl:for-each select="//bar[contains(@name,'ogan') & (parent::*/@location = 'Town')]">
We call this category NO_CONTEXT.
Expressions containing position predicates
A predicate-chain, whose predicates' expressions contain any use of the
position() or last() functions require some way
of representing the intermediate step in an iterator. The first predicate
is applied to the original node-set, and the resulting node-set must then
be stored in some other iterator, from which the second predicate can get
the current position from the iterator's getPosition() and
getLast() methods. We call this category
GENERAL_CONTEXT
Expressions containing one position predicate
There is one expection from the GENERAL_CONTEXT category. If the predicate-chain contains only one position-predicate, and that predicate is the very first one, then that predicate can call the iterator that contains the first node-set directly. Just look:
<xsl:for-each select="//bar[2][parent::*/@location = 'Drumcondra']">
The [2] predicate can be applied to the original iterator
for the //bar step. And so can the
[parent::*/@location = 'Drumcondra'] predicate as well. This
is only the case when the position predicate is first in the predicate
chain. These types of predicate chains belong in the
NO_CONTEXT category.
Design details
Predicates are handled quite differently in step expressions and step patterns. Step expressions are not implemented with the various contexts in mind and use a specialised iterator to wrap the code for each predicate. Step patterns are more complicated and CPU (or should I say JVM?) exhaustive. Step patterns containing predicates are analysed to determine context type and compiled accordingly.
Predicates and Step expressions
The basic behaviour for a predicate is to compile a filter. This
filter is an auxiliary class that implements the
org.apache.xalan.xsltc.dom.CurrentNodeListFilter interface. The
Step or StepPattern that uses the predicate will
create a org.apache.xalan.xsltc.dom.CurrentNodeListFilter. This
iterator contains the nodes that pass through the predicate. The compiled
filter is used by the iterator to determine which nodes that should be
included. The org.apache.xalan.xsltc.dom.CurrentNodeListFilter
interface contains only a single method:
public interface CurrentNodeListFilter {
public abstract boolean test(int node, int position, int last, int current,
AbstractTranslet translet, NodeIterator iter);
}
The code that is compiled into the test() method is the
code for the predicate's expression. The Predicate class
compiles the filter class and a test() method skeleton, while
some sub-class of the Expression class compiles the actual
code that goes into this method.
The iterator is initialised with a filter that implements this interface:
public CurrentNodeListIterator(NodeIterator source,
CurrentNodeListFilter filter,
int currentNode,
AbstractTranslet translet) {
this(source, !source.isReverse(), filter, currentNode, translet);
}
The iterator will use its source iterator to provide it with the initial
node-set. Each node that is returned from this set is passed through the
filter before returned by the next() method. Note that the
source iterator can also be a current node-list iterator (if two or more
predicates are chained together).
Optimisations in Step expressions
Node-value iterators
Some simple predicates that test for node values are handled by the
NodeValueIterator class at runtime. These are:
A: foo[@attr = <value>]
B: foo[bar = <value>]
C: foo/bar[. = <value>]
The first case is handled by creating an iterator that represents
foo/@attr, then passing this iterator and a test-value to
a NodeValueIterator. The <value> is an
expression that is compiled and passed to the iterator as a string. It
does not have to be a literal string as the string value is
found at runtime. The last two cases are similarly handled by creating an
iterator for foo/bar and passing that and the test-value to
a NodeValueIterator.
Nth descendant iterators
The Step class is also optimised for position-predicates
that are applied to descendant iterators:
<xsl:for-each select="//bar[3]">
Such step/predicate combinations are handled by the internal DOM's
inner class NthDescendantIterator.
Nth position iterators
Similarly, the Step class is optimised for
position-predicates that are applied to basic steps:
<xsl:for-each select="bar[3]">
Such step/predicate combinations are handled by the internal DOM's
inner class NthPositionIterator.
Node test
The predicate class contains a method that tells you if it is a boolean test:
public boolean isBooleanTest();
This can be, but it currently is not, used by the Step class
to compile in optimised code. Some work to be done here!
Predicates and StepPatterns
Using predicates in patterns is slow on any XSLT processor, and XSLTC
is no exception. This is why the predicate context is carefully analysed
by the StepPattern class, so that the compiled code is
specialised to handle the specific predicate(s) in use. First we should
consider the basic step pattern.
Basic pattern handling
All patterns are grouped (by the Mode class) according to
their
kernel
-node type. The kernel node-type is node-type of the
last step in a pattern:
<xsl:template match="foo/bar/baz"> ... <xsl:template>
In this case the type for elements <baz> is the
kernel type. This step is not compiled as a step pattern. The node
type is passed to the Mode class and is used to place the
remainder of the pattern code inside the big switch() statement
in the translet's applyTemplates() method. The whole pattern
is then reduced to:
match="foo/bar"
The StepPattern representing the <bar>
element test is compiled under the appropriate case: section
of the switch() statement. The code compiled for the step
pattern is basically just a call to the DOM's getType()
method and a test for the desired node type. There are two special cases
for:
<xsl:template match="foo/*/baz"> ... <xsl:template>
<xsl:template match="foo/*@[2]"> ... <xsl:template>
In the first case we call isElement() and in the second case
we call isAttribute(), instead of getType().
Patterns with predicates
The typeCheck() method of the StepPattern
invokes a method that analyses the predicates of the step pattern and
determines their context. (Note that this method needs to be updated to
handle the exception to the GENERAL_CONTEXT metioned in the section
Expressions containing one position predicate earlier in this document.) The translate() method of the
StepPattern class contains a switch() statement
that calls methods that are tailored for compiling code for the various
predicate contexts.