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////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Copyright (c) 2018-2022 Saxonica Limited
// This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0.
// If a copy of the MPL was not distributed with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
// This Source Code Form is "Incompatible With Secondary Licenses", as defined by the Mozilla Public License, v. 2.0.
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
package net.sf.saxon.om;
import net.sf.saxon.expr.parser.ExpressionTool;
import net.sf.saxon.str.EmptyUnicodeString;
import net.sf.saxon.str.UnicodeString;
import net.sf.saxon.trans.XPathException;
import net.sf.saxon.transpile.CSharpInjectMembers;
import net.sf.saxon.tree.iter.SingletonIterator;
import net.sf.saxon.value.EmptySequence;
/**
* A value that is a sequence containing zero or one items.
*/
@CSharpInjectMembers(code=
"public static net.sf.saxon.om.ZeroOrOne empty() where U : net.sf.saxon.om.Item { return new ZeroOrOne(default(U)); }")
public class ZeroOrOne implements GroundedValue {
private final T item; // may be null, to represent an empty sequence
@SuppressWarnings("rawtypes")
private static final ZeroOrOne EMPTY = new ZeroOrOne<>(null);
/**
* Return the instance of ZeroOrOne that represents the empty sequence
* @param the static type of the empty sequence
* @return a representation of the empty sequence that satisfies the type ZeroOrOne
*/
@SuppressWarnings("unchecked")
public static ZeroOrOne empty() {
return EMPTY;
}
/**
* Create a sequence containing zero or one items
*
* @param item The item to be contained in the sequence, or null if the sequence
* is to be empty
*/
public ZeroOrOne(T item) {
this.item = item;
}
/**
* Convert the value to a string, using the serialization rules.
* For atomic values this is the same as a cast; for sequence values
* it gives a space-separated list. For QNames and NOTATIONS, or lists
* containing them, it fails.
* @return the string value
*/
/*@NotNull*/
@Override
public UnicodeString getUnicodeStringValue() {
return item == null ? EmptyUnicodeString.getInstance() : item.getUnicodeStringValue();
}
@Override
public String getStringValue() {
return item == null ? "" : item.getStringValue();
}
/**
* Get the first item in the sequence.
*
* @return the first item in the sequence if there is one, or null if the sequence
* is empty
*/
@Override
public T head() {
return item;
}
/**
* Get the length of the sequence
*/
@Override
public int getLength() {
return item == null ? 0 : 1;
}
/**
* Get the n'th item in the sequence (starting from 0). This is defined for all
* SequenceValues, but its real benefits come for a SequenceValue stored extensionally
* (or for a MemoClosure, once all the values have been read)
* @param n the index of the required item, with 0 representing the first item in the sequence
* @return the n'th item if it exists, or null otherwise
*/
/*@Nullable*/
@Override
public T itemAt(int n) {
if (n == 0 && item != null) {
return item;
} else {
return null;
}
}
/**
* Get a subsequence of the value
*
* @param start the index of the first item to be included in the result, counting from zero.
* A negative value is taken as zero. If the value is beyond the end of the sequence, an empty
* sequence is returned
* @param length the number of items to be included in the result. Specify Integer.MAX_VALUE to
* get the subsequence up to the end of the base sequence. If the value is negative, an empty sequence
* is returned. If the value goes off the end of the sequence, the result returns items up to the end
* of the sequence
* @return the required subsequence. If min is
*/
/*@NotNull*/
@Override
public GroundedValue subsequence(int start, int length) {
if (item != null && start <= 0 && start + length > 0) {
return this;
} else {
return EmptySequence.getInstance();
}
}
/**
* Return an iterator over this value.
*/
@Override
public SequenceIterator iterate() {
return SingletonIterator.makeIterator(item);
}
/**
* Get the effective boolean value
*/
@Override
public boolean effectiveBooleanValue() throws XPathException {
return ExpressionTool.effectiveBooleanValue(item);
}
/**
* Returns a string representation of the object (used only for diagnostics).
*
* @return a string representation of the object.
*/
@Override
public String toString() {
return item==null ? "null" : item.toString();
}
/**
* Reduce the sequence to its simplest form. If the value is an empty sequence, the result will be
* EmptySequence.getInstance(). If the value is a single atomic value, the result will be an instance
* of AtomicValue. If the value is a single item of any other kind, the result will be an instance
* of SingletonItem. Otherwise, the result will typically be unchanged.
*
* @return the simplified sequence
*/
@Override
public GroundedValue reduce() {
if (item == null) {
return EmptySequence.getInstance();
} else {
return item;
}
}
}