net.sf.saxon.expr
Class ExpressionTool

java.lang.Object
  extended by net.sf.saxon.expr.ExpressionTool

public class ExpressionTool
extends Object

This class, ExpressionTool, contains a number of useful static methods for manipulating expressions. Most importantly, it provides the factory method make() for constructing a new expression


Field Summary
static int CALL_EVALUATE_ITEM
           
static int EVALUATE_AND_MATERIALIZE_VARIABLE
           
static int EVALUATE_SUPPLIED_PARAMETER
           
static int EVALUATE_VARIABLE
           
static int ITERATE_AND_MATERIALIZE
           
static int LAZY_TAIL_EXPRESSION
           
static int MAKE_CLOSURE
           
static int MAKE_INDEXED_VARIABLE
           
static int MAKE_MEMO_CLOSURE
           
static int MAKE_SINGLETON_CLOSURE
           
static int NO_EVALUATION_NEEDED
           
static int PROCESS
           
static int RETURN_EMPTY_SEQUENCE
           
static int SHARED_APPEND_EXPRESSION
           
static int UNDECIDED
           
 
Method Summary
static int allocateSlots(Expression exp, int nextFree, SlotManager frame)
          Allocate slot numbers to range variables
static boolean callsFunction(Expression exp, StructuredQName qName)
          Determine whether an expression contains a call on the function with a given fingerprint
static void copyLocationInfo(Expression from, Expression to)
          Copy location information (the line number and reference to the container) from one expression to another
static boolean dependsOnFocus(Expression exp)
          Ask whether an expression has a dependency on the focus
static boolean dependsOnVariable(Expression e, Binding[] bindingList)
          Determine whether an expression depends on any one of a set of variables
static ValueRepresentation eagerEvaluate(Expression exp, XPathContext context)
          Evaluate an expression now; lazy evaluation is not permitted in this case
static int eagerEvaluationMode(Expression exp)
          Determine the method of evaluation to be used when lazy evaluation of an expression is preferred.
static void ebvError(String reason)
          Report an error in computing the effective boolean value of an expression
static boolean effectiveBooleanValue(Item item)
          Determine the effective boolean value of a single item
static boolean effectiveBooleanValue(SequenceIterator iterator)
          Determine the effective boolean value of a sequence, given an iterator over the sequence
static ValueRepresentation evaluate(Expression exp, int evaluationMode, XPathContext context, int ref)
          Do lazy evaluation of an expression.
static int expressionSize(Expression exp)
          Get the size of an expression tree (the number of subexpressions it contains)
static void gatherAllSubExpressions(Expression exp, HashSet set)
          Gather the set of all the subexpressions of an expression (the transitive closure)
static void gatherCalledFunctionNames(Expression e, List list)
          Gather a list of the names of the user-defined functions which a given expression calls directly
static void gatherCalledFunctions(Expression e, List list)
          Gather a list of all the user-defined functions which a given expression calls directly
static void gatherReferencedVariables(Expression e, List list)
          Gather a list of all the variable bindings on which a given expression depends
static void gatherVariableReferences(Expression exp, Binding binding, List list)
          Get a list of all references to a particular variable within a subtree
static int getReferenceCount(Expression exp, Binding binding, boolean inLoop)
          Determine how often a variable is referenced.
static String indent(int level)
          Construct indent string, for diagnostic output
static boolean isAllowedInUpdatingContext(Expression exp)
          Determine whether the expression is either an updating expression, or an expression that is permitted in a context where updating expressions are allowed
static ValueRepresentation lazyEvaluate(Expression exp, XPathContext context, int ref)
          Do lazy evaluation of an expression.
static int lazyEvaluationMode(Expression exp)
          Determine the method of evaluation to be used when lazy evaluation of an expression is preferred.
static Expression make(String expression, StaticContext env, int start, int terminator, int lineNumber, boolean compileWithTracing)
          Parse an expression.
static int markTailFunctionCalls(Expression exp, StructuredQName qName, int arity)
          Scan an expression to find and mark any recursive tail function calls
static void rebindVariableReferences(Expression exp, Binding oldBinding, Binding newBinding)
          Rebind all variable references to a binding
static void resetPropertiesWithinSubtree(Expression exp)
          Reset cached static properties within a subtree, meaning that they have to be recalulated next time they are required
static Expression resolveCallsToCurrentFunction(Expression exp, Configuration config)
          Resolve calls to the XSLT current() function within an expression
static Expression tryToFactorOutDot(Expression exp, ItemType contextItemType)
          Try to factor out dependencies on the context item, by rewriting an expression f(.) as let $dot := .
static Expression unsorted(Optimizer opt, Expression exp, boolean retainAllNodes)
          Remove unwanted sorting from an expression, at compile time
static Expression unsortedIfHomogeneous(Optimizer opt, Expression exp)
          Remove unwanted sorting from an expression, at compile time, if and only if it is known that the result of the expression will be homogeneous (all nodes, or all atomic values).
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

UNDECIDED

public static final int UNDECIDED
See Also:
Constant Field Values

NO_EVALUATION_NEEDED

public static final int NO_EVALUATION_NEEDED
See Also:
Constant Field Values

EVALUATE_VARIABLE

public static final int EVALUATE_VARIABLE
See Also:
Constant Field Values

MAKE_CLOSURE

public static final int MAKE_CLOSURE
See Also:
Constant Field Values

MAKE_MEMO_CLOSURE

public static final int MAKE_MEMO_CLOSURE
See Also:
Constant Field Values

RETURN_EMPTY_SEQUENCE

public static final int RETURN_EMPTY_SEQUENCE
See Also:
Constant Field Values

EVALUATE_AND_MATERIALIZE_VARIABLE

public static final int EVALUATE_AND_MATERIALIZE_VARIABLE
See Also:
Constant Field Values

CALL_EVALUATE_ITEM

public static final int CALL_EVALUATE_ITEM
See Also:
Constant Field Values

ITERATE_AND_MATERIALIZE

public static final int ITERATE_AND_MATERIALIZE
See Also:
Constant Field Values

PROCESS

public static final int PROCESS
See Also:
Constant Field Values

LAZY_TAIL_EXPRESSION

public static final int LAZY_TAIL_EXPRESSION
See Also:
Constant Field Values

SHARED_APPEND_EXPRESSION

public static final int SHARED_APPEND_EXPRESSION
See Also:
Constant Field Values

MAKE_INDEXED_VARIABLE

public static final int MAKE_INDEXED_VARIABLE
See Also:
Constant Field Values

MAKE_SINGLETON_CLOSURE

public static final int MAKE_SINGLETON_CLOSURE
See Also:
Constant Field Values

EVALUATE_SUPPLIED_PARAMETER

public static final int EVALUATE_SUPPLIED_PARAMETER
See Also:
Constant Field Values
Method Detail

make

public static Expression make(String expression,
                              StaticContext env,
                              int start,
                              int terminator,
                              int lineNumber,
                              boolean compileWithTracing)
                       throws XPathException
Parse an expression. This performs the basic analysis of the expression against the grammar, it binds variable references and function calls to variable definitions and function definitions, and it performs context-independent expression rewriting for optimization purposes.

Parameters:
expression - The expression (as a character string)
env - An object giving information about the compile-time context of the expression
start - position of the first significant character in the expression
terminator - The token that marks the end of this expression; typically Tokenizer.EOF, but may for example be a right curly brace
lineNumber - the line number of the start of the expression
compileWithTracing - true if diagnostic tracing during expression parsing is required
Returns:
an object of type Expression
Throws:
XPathException - if the expression contains a static error

copyLocationInfo

public static void copyLocationInfo(Expression from,
                                    Expression to)
Copy location information (the line number and reference to the container) from one expression to another

Parameters:
from - the expression containing the location information
to - the expression to which the information is to be copied

unsorted

public static Expression unsorted(Optimizer opt,
                                  Expression exp,
                                  boolean retainAllNodes)
                           throws XPathException
Remove unwanted sorting from an expression, at compile time

Parameters:
opt - the expression optimizer
exp - the expression to be optimized
retainAllNodes - true if there is a need to retain exactly those nodes returned by exp even if there are duplicates; false if the caller doesn't mind whether duplicate nodes are retained or eliminated
Returns:
the expression after rewriting
Throws:
XPathException

unsortedIfHomogeneous

public static Expression unsortedIfHomogeneous(Optimizer opt,
                                               Expression exp)
                                        throws XPathException
Remove unwanted sorting from an expression, at compile time, if and only if it is known that the result of the expression will be homogeneous (all nodes, or all atomic values). This is done when we need the effective boolean value of a sequence: the EBV of a homogenous sequence does not depend on its order, but this is not true when atomic values and nodes are mixed: (N, AV) is true, but (AV, N) is an error.

Parameters:
opt - the expression optimizer
exp - the expression to be optimized
Returns:
the expression after rewriting
Throws:
XPathException

lazyEvaluationMode

public static int lazyEvaluationMode(Expression exp)
Determine the method of evaluation to be used when lazy evaluation of an expression is preferred. This method is called at compile time, after all optimizations have been done, to determine the preferred strategy for lazy evaluation, depending on the type of expression.

Parameters:
exp - the expression to be evaluated
Returns:
an integer constant identifying the evaluation mode

eagerEvaluationMode

public static int eagerEvaluationMode(Expression exp)
Determine the method of evaluation to be used when lazy evaluation of an expression is preferred. This method is called at compile time, after all optimizations have been done, to determine the preferred strategy for lazy evaluation, depending on the type of expression.

Parameters:
exp - the expression to be evaluated
Returns:
an integer constant identifying the evaluation mode

evaluate

public static ValueRepresentation evaluate(Expression exp,
                                           int evaluationMode,
                                           XPathContext context,
                                           int ref)
                                    throws XPathException
Do lazy evaluation of an expression. This will return a value, which may optionally be a SequenceIntent, which is a wrapper around an iterator over the value of the expression.

Parameters:
exp - the expression to be evaluated
evaluationMode - the evaluation mode for this expression
context - the run-time evaluation context for the expression. If the expression is not evaluated immediately, then parts of the context on which the expression depends need to be saved as part of the Closure
ref - an indication of how the value will be used. The value 1 indicates that the value is only expected to be used once, so that there is no need to keep it in memory. A small value >1 indicates multiple references, so the value will be saved when first evaluated. The special value FILTERED indicates a reference within a loop of the form $x[predicate], indicating that the value should be saved in a way that permits indexing.
Returns:
a value: either the actual value obtained by evaluating the expression, or a Closure containing all the information needed to evaluate it later
Throws:
XPathException - if any error occurs in evaluating the expression

lazyEvaluate

public static ValueRepresentation lazyEvaluate(Expression exp,
                                               XPathContext context,
                                               int ref)
                                        throws XPathException
Do lazy evaluation of an expression. This will return a value, which may optionally be a SequenceIntent, which is a wrapper around an iterator over the value of the expression.

Parameters:
exp - the expression to be evaluated
context - the run-time evaluation context for the expression. If the expression is not evaluated immediately, then parts of the context on which the expression depends need to be saved as part of the Closure
ref - an indication of how the value will be used. The value 1 indicates that the value is only expected to be used once, so that there is no need to keep it in memory. A small value >1 indicates multiple references, so the value will be saved when first evaluated. The special value FILTERED indicates a reference within a loop of the form $x[predicate], indicating that the value should be saved in a way that permits indexing.
Returns:
a value: either the actual value obtained by evaluating the expression, or a Closure containing all the information needed to evaluate it later
Throws:
XPathException - if any error occurs in evaluating the expression

eagerEvaluate

public static ValueRepresentation eagerEvaluate(Expression exp,
                                                XPathContext context)
                                         throws XPathException
Evaluate an expression now; lazy evaluation is not permitted in this case

Parameters:
exp - the expression to be evaluated
context - the run-time evaluation context
Returns:
the result of evaluating the expression
Throws:
XPathException - if any dynamic error occurs evaluating the expression

markTailFunctionCalls

public static int markTailFunctionCalls(Expression exp,
                                        StructuredQName qName,
                                        int arity)
Scan an expression to find and mark any recursive tail function calls

Parameters:
exp - the expression to be analyzed
qName - the name of the containing function
arity - the arity of the containing function
Returns:
0 if no tail call was found; 1 if a tail call to a different function was found; 2 if a tail call to the specified function was found. In this case the UserFunctionCall object representing the tail function call will also have been marked as a tail call.

indent

public static String indent(int level)
Construct indent string, for diagnostic output

Parameters:
level - the indentation level (the number of spaces to return)
Returns:
a string of "level*2" spaces

allocateSlots

public static int allocateSlots(Expression exp,
                                int nextFree,
                                SlotManager frame)
Allocate slot numbers to range variables

Parameters:
exp - the expression whose range variables need to have slot numbers assigned
nextFree - the next slot number that is available for allocation
frame - a SlotManager object that is used to track the mapping of slot numbers to variable names for debugging purposes. May be null.
Returns:
the next unallocated slot number.

effectiveBooleanValue

public static boolean effectiveBooleanValue(SequenceIterator iterator)
                                     throws XPathException
Determine the effective boolean value of a sequence, given an iterator over the sequence

Parameters:
iterator - An iterator over the sequence whose effective boolean value is required
Returns:
the effective boolean value
Throws:
XPathException - if a dynamic error occurs

effectiveBooleanValue

public static boolean effectiveBooleanValue(Item item)
                                     throws XPathException
Determine the effective boolean value of a single item

Parameters:
item - the item whose effective boolean value is required
Returns:
the effective boolean value
Throws:
XPathException - if a dynamic error occurs

ebvError

public static void ebvError(String reason)
                     throws XPathException
Report an error in computing the effective boolean value of an expression

Parameters:
reason - the nature of the error
Throws:
XPathException

dependsOnFocus

public static boolean dependsOnFocus(Expression exp)
Ask whether an expression has a dependency on the focus

Parameters:
exp - the expression
Returns:
true if the value of the expression depends on the context item, position, or size

dependsOnVariable

public static boolean dependsOnVariable(Expression e,
                                        Binding[] bindingList)
Determine whether an expression depends on any one of a set of variables

Parameters:
e - the expression being tested
bindingList - the set of variables being tested
Returns:
true if the expression depends on one of the given variables

gatherReferencedVariables

public static void gatherReferencedVariables(Expression e,
                                             List list)
Gather a list of all the variable bindings on which a given expression depends

Parameters:
e - the expression being tested
list - a list to which the bindings are to be added. The items in this list must implement Binding

callsFunction

public static boolean callsFunction(Expression exp,
                                    StructuredQName qName)
Determine whether an expression contains a call on the function with a given fingerprint

Parameters:
exp - The expression being tested
qName - The name of the function
Returns:
true if the expression contains a call on the function

gatherCalledFunctions

public static void gatherCalledFunctions(Expression e,
                                         List list)
Gather a list of all the user-defined functions which a given expression calls directly

Parameters:
e - the expression being tested
list - a list of the functions that are called. The items in this list must be objects of class UserFunction

gatherCalledFunctionNames

public static void gatherCalledFunctionNames(Expression e,
                                             List list)
Gather a list of the names of the user-defined functions which a given expression calls directly

Parameters:
e - the expression being tested
list - a list of the functions that are called. The items in this list are strings in the format "{uri}local/arity"

resetPropertiesWithinSubtree

public static void resetPropertiesWithinSubtree(Expression exp)
Reset cached static properties within a subtree, meaning that they have to be recalulated next time they are required

Parameters:
exp - the root of the subtree within which static properties should be reset

resolveCallsToCurrentFunction

public static Expression resolveCallsToCurrentFunction(Expression exp,
                                                       Configuration config)
                                                throws XPathException
Resolve calls to the XSLT current() function within an expression

Parameters:
exp - the expression within which calls to current() should be resolved
config - the Saxon configuration
Returns:
the expression after resolving calls to current()
Throws:
XPathException

gatherVariableReferences

public static void gatherVariableReferences(Expression exp,
                                            Binding binding,
                                            List list)
Get a list of all references to a particular variable within a subtree

Parameters:
exp - the expression at the root of the subtree
binding - the variable binding whose references are sought
list - a list to be populated with the references to this variable

getReferenceCount

public static int getReferenceCount(Expression exp,
                                    Binding binding,
                                    boolean inLoop)
Determine how often a variable is referenced. This is the number of times it is referenced at run-time: so a reference in a loop counts as "many". This code currently handles local variables (Let expressions) and function parameters. It is not currently used for XSLT template parameters. It's not the end of the world if the answer is wrong (unless it's wrongly given as zero), but if wrongly returned as 1 then the variable will be repeatedly evaluated.

Parameters:
exp - the expression within which variable references are to be counted
binding - identifies the variable of interest
inLoop - true if the expression is within a loop, in which case a reference counts as many. This should be set to false on the initial call, it may be set to true on an internal recursive call
Returns:
the number of references. The interesting values are 0, 1, "many" (represented by any value >1), and the special value FILTERED, which indicates that there are multiple references and one or more of them is of the form $x[....] indicating that an index might be useful.

gatherAllSubExpressions

public static void gatherAllSubExpressions(Expression exp,
                                           HashSet set)
Gather the set of all the subexpressions of an expression (the transitive closure)

Parameters:
exp - the parent expression
set - the set to be populated; on return it will contain all the subexpressions. Beware that testing for membership of a set of expressions relies on the equals() comparison, which does not test identity.

expressionSize

public static int expressionSize(Expression exp)
Get the size of an expression tree (the number of subexpressions it contains)

Parameters:
exp - the expression whose size is required
Returns:
the size of the expression tree, as the number of nodes

rebindVariableReferences

public static void rebindVariableReferences(Expression exp,
                                            Binding oldBinding,
                                            Binding newBinding)
Rebind all variable references to a binding

Parameters:
exp - the expression whose contained variable references are to be rebound
oldBinding - the old binding for the variable references
newBinding - the new binding to which the variables should be rebound

tryToFactorOutDot

public static Expression tryToFactorOutDot(Expression exp,
                                           ItemType contextItemType)
Try to factor out dependencies on the context item, by rewriting an expression f(.) as let $dot := . return f($dot). This is not always possible, for example where f() is an extension function call that uses XPathContext as an implicit argument. However, doing this increases the chances of distributing a "where" condition in a FLWOR expression to the individual input sequences selected by the "for" clauses.

Parameters:
exp - the expression from which references to "." should be factored out if possible
contextItemType - the static type of the context item
Returns:
either the expression, after binding "." to a local variable and replacing all references to it; or null, if no changes were made.

isAllowedInUpdatingContext

public static boolean isAllowedInUpdatingContext(Expression exp)
Determine whether the expression is either an updating expression, or an expression that is permitted in a context where updating expressions are allowed

Parameters:
exp - the expression under test
Returns:
true if the expression is an updating expression, or an empty sequence, or a call on error()


Copyright (c) Saxonica Limited. All rights reserved.