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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.expr.sort;

import net.sf.saxon.expr.XPathContext;
import net.sf.saxon.lib.StringCollator;
import net.sf.saxon.trans.NoDynamicContextException;
import net.sf.saxon.value.AtomicValue;

/**
 * A Comparer used for comparing descending keys. This simply returns the inverse of the result
 * delivered by the base comparer.
 */

public class DescendingComparer implements AtomicComparer {

    private final AtomicComparer baseComparer;

    public DescendingComparer(AtomicComparer base) {
        baseComparer = base;
    }

    /**
     * Get the underlying (ascending) comparer
     *
     * @return the underlying (ascending) comparer
     */

    public AtomicComparer getBaseComparer() {
        return baseComparer;
    }

    @Override
    public StringCollator getCollator() {
        return baseComparer.getCollator();
    }

    /**
     * Supply the dynamic context in case this is needed for the comparison
     *
     * @param context the dynamic evaluation context
     * @return either the original AtomicComparer, or a new AtomicComparer in which the context
     *         is known. The original AtomicComparer is not modified
     */

    /*@NotNull*/
    @Override
    public AtomicComparer provideContext(XPathContext context) {
        AtomicComparer newBase = baseComparer.provideContext(context);
        if (newBase != baseComparer) {
            return new DescendingComparer(newBase);
        } else {
            return this;
        }
    }

    /**
     * Compare two objects.
     *
     * @return <0 if a<b, 0 if a=b, >0 if a>b
     * @throws ClassCastException if the objects are of the wrong type for this Comparer
     */

    @Override
    public int compareAtomicValues(AtomicValue a, AtomicValue b) throws NoDynamicContextException {
        return 0 - baseComparer.compareAtomicValues(a, b);
    }

    /**
     * Compare two AtomicValue objects for equality according to the rules for their data type. UntypedAtomic
     * values are compared by converting to the type of the other operand.
     *
     * @param a the first object to be compared. It is intended that this should be an instance
     *          of AtomicValue, though this restriction is not enforced. If it is a StringValue, the
     *          collator is used to compare the values, otherwise the value must implement the equals() method.
     * @param b the second object to be compared. This must be comparable with the first object: for
     *          example, if one is a string, they must both be strings.
     * @return true if the values are equal, false if not
     * @throws ClassCastException if the objects are not comparable
     */

    @Override
    public boolean comparesEqual(AtomicValue a, AtomicValue b) throws NoDynamicContextException {
        return baseComparer.comparesEqual(a, b);
    }

    /**
     * Create a string representation of this AtomicComparer that can be saved in a compiled
     * package and used to reconstitute the AtomicComparer when the package is reloaded
     *
     * @return a string representation of the AtomicComparer
     */
    @Override
    public String save() {
        return "DESC|" + baseComparer.save();
    }
}