net.sf.saxon.expr

Class PromotionOffer


public class PromotionOffer
extends java.lang.Object

PromotionOffer is an object used transiently during compilation of an expression. It contains information passed by a containing expression to its subexpressions, when looking for subexpressions that can be promoted to a higher level because they are not dependent on the context established by the containing expression. The object is also used to pass back return information when the promotion actually takes place.

Field Summary

static int
EXTRACT_GLOBAL_VARIABLES
EXTRACT_GLOBAL_VARIABLES identifies subexpressions that are not constant, but have no dependencies other than on global variables or parameters (they must also be non-creative).
static int
FOCUS_INDEPENDENT
FOCUS_INDEPENDENT requests promotion of all non-trivial subexpressions that don't depend on the focus.
static int
INLINE_VARIABLE_REFERENCES
Inline variable references causes all references to a variable V to be replaced by the expression E.
static int
RANGE_INDEPENDENT
RANGE_INDEPENDENT requests promotion of all non-trivial subexpressions that don't depend on a specified range variable.
static int
REPLACE_CURRENT
REPLACE_CURRENT causes calls to the XSLT current() function to be replaced by reference to a variable.
static int
UNORDERED
UNORDERED indicates that the containing expression does not require the results to be delivered in any particular order.
boolean
accepted
Flag that is set if the offer has been accepted, that is, if the expression has changed
int
action
action is one of the possible promotion actions, FOCUS_INDEPENDENT, RANGE_INDEPENDENT, INLINE_VARIABLE_REFERENCES, UNORDERED, EXTRACT_GLOBAL_VARIABLES
Binding[]
bindingList
In the case of RANGE_INDEPENDENT, "binding" identifies the range variables whose dependencies we are looking for.
Expression
containingExpression
When a promotion offer is made, containingExpression identifies the level to which the promotion should occur.
boolean
mustEliminateDuplicates
In the case of UNORDERED, "mustEliminateDuplicates" is a boolean that is set to true if the nodes can be delivered in any order so long as there are no duplicates (for example, as required by the count() function).
boolean
promoteDocumentDependent
In the case of FOCUS_INDEPENDENT, "promoteDocumentDependent" is a boolean that, when set to true, indicates that it is safe to promote a subexpression that depends on the context document but not on other aspects of the focus.
boolean
promoteXSLTFunctions
In the case of FOCUS_INDEPENDENT, "promoteXSLTFunctions" is a boolean that, when set to true, indicates that it is safe to promote XSLT functions such as current().
ExpressionVisitor
visitor
The expression visitor in use

Constructor Summary

PromotionOffer(Optimizer optimizer)
Create a PromotionOffer for use with a particular Optimizer

Method Summary

Expression
accept(Expression child)
Method to test whether a subexpression qualifies for promotion, and if so, to accept the promotion.
Optimizer
getOptimizer()
Get the optimizer in use

Field Details

EXTRACT_GLOBAL_VARIABLES

public static final int EXTRACT_GLOBAL_VARIABLES
EXTRACT_GLOBAL_VARIABLES identifies subexpressions that are not constant, but have no dependencies other than on global variables or parameters (they must also be non-creative). Such expressions can be extracted from a function or template and converted into global variables. This optimization is done in Saxon-SA only.
Field Value:
15

FOCUS_INDEPENDENT

public static final int FOCUS_INDEPENDENT
FOCUS_INDEPENDENT requests promotion of all non-trivial subexpressions that don't depend on the focus. This is typically used to extract subexpressions from a filter predicate. The offer is optional - each subexpression can decide whether it's worth the trouble of promoting itself. The offer is normally passed on to subexpressions, except subexpressions that are evaluated with a different focus
Field Value:
10

INLINE_VARIABLE_REFERENCES

public static final int INLINE_VARIABLE_REFERENCES
Inline variable references causes all references to a variable V to be replaced by the expression E. The variable is supplied in the "binding" property; the replacement expression in the containingExpression property. A special case is where the replacement expression is a ContextItemExpression; in this case the offer is not passed on to subexpressions where the context is different.
Field Value:
12

RANGE_INDEPENDENT

public static final int RANGE_INDEPENDENT
RANGE_INDEPENDENT requests promotion of all non-trivial subexpressions that don't depend on a specified range variable. This is typically used to extract subexpressions from the action of a for expression or the condition of a some/every quantified expression. The offer is optional - each subexpression can decide whether it's worth the trouble of promoting itself. The offer is normally passed on to subexpressions, except subexpressions that are evaluated with a different focus or a different range variable, because these may have other dependencies that prevent their promotion.
Field Value:
11

REPLACE_CURRENT

public static final int REPLACE_CURRENT
REPLACE_CURRENT causes calls to the XSLT current() function to be replaced by reference to a variable. The variable binding is the single member of the array bindingList
Field Value:
14

UNORDERED

public static final int UNORDERED
UNORDERED indicates that the containing expression does not require the results to be delivered in any particular order. The boolean mustEliminateDuplicates is set if duplicate items in the result are not allowed.
Field Value:
13

accepted

public boolean accepted
Flag that is set if the offer has been accepted, that is, if the expression has changed

action

public int action
action is one of the possible promotion actions, FOCUS_INDEPENDENT, RANGE_INDEPENDENT, INLINE_VARIABLE_REFERENCES, UNORDERED, EXTRACT_GLOBAL_VARIABLES

bindingList

public Binding[] bindingList
In the case of RANGE_INDEPENDENT, "binding" identifies the range variables whose dependencies we are looking for. For INLINE_VARIABLE_REFERENCES it is a single Binding that we are aiming to inline

containingExpression

public Expression containingExpression
When a promotion offer is made, containingExpression identifies the level to which the promotion should occur. When a subexpression is promoted, an expression of the form let $VAR := SUB return ORIG is created, and this replaces the original containingExpression within the PromotionOffer.

mustEliminateDuplicates

public boolean mustEliminateDuplicates
In the case of UNORDERED, "mustEliminateDuplicates" is a boolean that is set to true if the nodes can be delivered in any order so long as there are no duplicates (for example, as required by the count() function). If this boolean is false, the nodes can be delivered in any order and duplicates are allowed (for example, as required by the boolean() function).

promoteDocumentDependent

public boolean promoteDocumentDependent
In the case of FOCUS_INDEPENDENT, "promoteDocumentDependent" is a boolean that, when set to true, indicates that it is safe to promote a subexpression that depends on the context document but not on other aspects of the focus. This is the case, for example, in a filter expression when it is known that all the nodes selected by the expression will be in the same document - as happens when the filter is applied to a path expression. This allows subexpressions such as key() to be promoted

promoteXSLTFunctions

public boolean promoteXSLTFunctions
In the case of FOCUS_INDEPENDENT, "promoteXSLTFunctions" is a boolean that, when set to true, indicates that it is safe to promote XSLT functions such as current(). This flag is set when rewriting XPath expressions and is unset when rewriting XSLT templates.

visitor

public ExpressionVisitor visitor
The expression visitor in use

Constructor Details

PromotionOffer

public PromotionOffer(Optimizer optimizer)
Create a PromotionOffer for use with a particular Optimizer
Parameters:
optimizer - the optimizer

Method Details

accept

public Expression accept(Expression child)
            throws XPathException
Method to test whether a subexpression qualifies for promotion, and if so, to accept the promotion.
Parameters:
child - the subexpression in question
Returns:
if promotion was done, returns the expression that should be used in place of the child expression. If no promotion was done, returns null. If promotion is determined not to be necessary for this subtree, returns the supplied child expression unchanged

getOptimizer

public Optimizer getOptimizer()
Get the optimizer in use
Returns:
the optimizer