net.sf.saxon.expr
Class TailExpression

java.lang.Object
  extended by net.sf.saxon.expr.Expression
      extended by net.sf.saxon.expr.TailExpression
All Implemented Interfaces:
Serializable, SourceLocator, EvaluableItem, SequenceIterable, InstructionInfoProvider

public class TailExpression
extends Expression

A TailExpression represents a FilterExpression of the form EXPR[position() > n] Here n is usually 2, but we allow other values

See Also:
Serialized Form

Field Summary
 
Fields inherited from class net.sf.saxon.expr.Expression
EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD
 
Constructor Summary
TailExpression(Expression base, int start)
          Construct a TailExpression, representing a filter expression of the form $base[position() >= $start]
 
Method Summary
 int computeCardinality()
          Compute the static cardinality of this expression
 int computeSpecialProperties()
          Compute the special properties of this expression.
 Expression copy()
          Copy an expression.
 boolean equals(Object other)
          Compare two expressions to see if they are equal
 void explain(ExpressionPresenter destination)
          Diagnostic print of expression structure.
 Expression getBaseExpression()
          Get the base expression (of which this expression returns the tail part of the value)
 ItemType getItemType(TypeHierarchy th)
          Determine the data type of the expression, if possible.
 int getStart()
          Get the start offset
 int hashCode()
           
 SequenceIterator iterate(XPathContext context)
          Return an Iterator to iterate over the values of a sequence.
 Iterator iterateSubExpressions()
          Get the immediate sub-expressions of this expression.
 Expression optimize(ExpressionVisitor visitor, ItemType contextItemType)
          Perform optimisation of an expression and its subexpressions.
 Expression promote(PromotionOffer offer)
          Offer promotion for this subexpression.
 boolean replaceSubExpression(Expression original, Expression replacement)
          Replace one subexpression by a replacement subexpression
 Expression typeCheck(ExpressionVisitor visitor, ItemType contextItemType)
          Perform type checking of an expression and its subexpressions.
 
Methods inherited from class net.sf.saxon.expr.Expression
addToPathMap, adoptChildExpression, checkPermittedContents, computeDependencies, computeStaticProperties, display, doPromotion, effectiveBooleanValue, evaluateAsString, evaluateItem, explain, findParentOf, getCardinality, getColumnNumber, getContainer, getDependencies, getExecutable, getHostLanguage, getImplementationMethod, getInstructionInfo, getIntrinsicDependencies, getLineNumber, getLocationId, getLocationProvider, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, hasLoopingSubexpression, implementsStaticTypeCheck, iterateEvents, markTailFunctionCalls, process, setContainer, setFiltered, setFlattened, setLocationId, simplify, staticTypeCheck, suppressValidation, toString
 
Methods inherited from class java.lang.Object
getClass, notify, notifyAll, wait, wait, wait
 

Constructor Detail

TailExpression

public TailExpression(Expression base,
                      int start)
Construct a TailExpression, representing a filter expression of the form $base[position() >= $start]

Parameters:
base - the expression to be filtered
start - the position (1-based) of the first item to be included
Method Detail

typeCheck

public Expression typeCheck(ExpressionVisitor visitor,
                            ItemType contextItemType)
                     throws XPathException
Description copied from class: Expression
Perform type checking of an expression and its subexpressions. This is the second phase of static optimization.

This checks statically that the operands of the expression have the correct type; if necessary it generates code to do run-time type checking or type conversion. A static type error is reported only if execution cannot possibly succeed, that is, if a run-time type error is inevitable. The call may return a modified form of the expression.

This method is called after all references to functions and variables have been resolved to the declaration of the function or variable. However, the types of such functions and variables may not be accurately known if they have not been explicitly declared.

If the implementation returns a value other than "this", then it is required to ensure that the parent pointer and location information in the returned expression have been set up correctly. It should not rely on the caller to do this, although for historical reasons many callers do so.

Overrides:
typeCheck in class Expression
Parameters:
visitor - an expression visitor
contextItemType - the static type of "." at the point where this expression is invoked. The parameter is set to null if it is known statically that the context item will be undefined. If the type of the context item is not known statically, the argument is set to Type.ITEM_TYPE
Returns:
the original expression, rewritten to perform necessary run-time type checks, and to perform other type-related optimizations
Throws:
XPathException - if an error is discovered during this phase (typically a type error)

optimize

public Expression optimize(ExpressionVisitor visitor,
                           ItemType contextItemType)
                    throws XPathException
Description copied from class: Expression
Perform optimisation of an expression and its subexpressions. This is the third and final phase of static optimization.

This method is called after all references to functions and variables have been resolved to the declaration of the function or variable, and after all type checking has been done.

Overrides:
optimize in class Expression
Parameters:
visitor - an expression visitor
contextItemType - the static type of "." at the point where this expression is invoked. The parameter is set to null if it is known statically that the context item will be undefined. If the type of the context item is not known statically, the argument is set to Type.ITEM_TYPE
Returns:
the original expression, rewritten if appropriate to optimize execution
Throws:
XPathException - if an error is discovered during this phase (typically a type error)

promote

public Expression promote(PromotionOffer offer)
                   throws XPathException
Description copied from class: Expression
Offer promotion for this subexpression. The offer will be accepted if the subexpression is not dependent on the factors (e.g. the context item) identified in the PromotionOffer. By default the offer is not accepted - this is appropriate in the case of simple expressions such as constant values and variable references where promotion would give no performance advantage. This method is always called at compile time.

Overrides:
promote in class Expression
Parameters:
offer - details of the offer, for example the offer to move expressions that don't depend on the context to an outer level in the containing expression
Returns:
if the offer is not accepted, return this expression unchanged. Otherwise return the result of rewriting the expression to promote this subexpression
Throws:
XPathException - if any error is detected

computeSpecialProperties

public int computeSpecialProperties()
Description copied from class: Expression
Compute the special properties of this expression. These properties are denoted by a bit-significant integer, possible values are in class StaticProperty. The "special" properties are properties other than cardinality and dependencies, and most of them relate to properties of node sequences, for example whether the nodes are in document order.

Returns:
the special properties, as a bit-significant integer

copy

public Expression copy()
Copy an expression. This makes a deep copy.

Specified by:
copy in class Expression
Returns:
the copy of the original expression

getItemType

public ItemType getItemType(TypeHierarchy th)
Description copied from class: Expression
Determine the data type of the expression, if possible. All expression return sequences, in general; this method determines the type of the items within the sequence, assuming that (a) this is known in advance, and (b) it is the same for all items in the sequence.

This method should always return a result, though it may be the best approximation that is available at the time.

Specified by:
getItemType in class Expression
Parameters:
th - the type hierarchy cache
Returns:
a value such as Type.STRING, Type.BOOLEAN, Type.NUMBER, Type.NODE, or Type.ITEM (meaning not known at compile time)

computeCardinality

public int computeCardinality()
Description copied from class: Expression
Compute the static cardinality of this expression

Returns:
the computed cardinality, as one of the values StaticProperty.ALLOWS_ZERO_OR_ONE, StaticProperty.EXACTLY_ONE, StaticProperty.ALLOWS_ONE_OR_MORE, StaticProperty.ALLOWS_ZERO_OR_MORE

iterateSubExpressions

public Iterator iterateSubExpressions()
Description copied from class: Expression
Get the immediate sub-expressions of this expression. Default implementation returns a zero-length array, appropriate for an expression that has no sub-expressions.

Overrides:
iterateSubExpressions in class Expression
Returns:
an iterator containing the sub-expressions of this expression

replaceSubExpression

public boolean replaceSubExpression(Expression original,
                                    Expression replacement)
Replace one subexpression by a replacement subexpression

Overrides:
replaceSubExpression in class Expression
Parameters:
original - the original subexpression
replacement - the replacement subexpression
Returns:
true if the original subexpression is found

getBaseExpression

public Expression getBaseExpression()
Get the base expression (of which this expression returns the tail part of the value)

Returns:
the base expression

getStart

public int getStart()
Get the start offset

Returns:
the one-based start offset (returns 2 if all but the first item is being selected)

equals

public boolean equals(Object other)
Compare two expressions to see if they are equal

Overrides:
equals in class Object
Parameters:
other - the other expression
Returns:
true if the expressions are equivalent

hashCode

public int hashCode()
Overrides:
hashCode in class Object

iterate

public SequenceIterator iterate(XPathContext context)
                         throws XPathException
Description copied from class: Expression
Return an Iterator to iterate over the values of a sequence. The value of every expression can be regarded as a sequence, so this method is supported for all expressions. This default implementation handles iteration for expressions that return singleton values: for non-singleton expressions, the subclass must provide its own implementation.

Specified by:
iterate in interface SequenceIterable
Overrides:
iterate in class Expression
Parameters:
context - supplies the context for evaluation
Returns:
a SequenceIterator that can be used to iterate over the result of the expression
Throws:
XPathException - if any dynamic error occurs evaluating the expression

explain

public void explain(ExpressionPresenter destination)
Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.

Specified by:
explain in class Expression
Parameters:
destination - the expression presenter used to display the structure


Copyright (C) Michael H. Kay. All rights reserved.