////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Copyright (c) 2018 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.value.AtomicValue;
import net.sf.saxon.value.NumericValue;

/**
 * An AtomicComparer used for sorting values that are known to be numeric.
 * It also supports a separate method for getting a collation key to test equality of items.
 * This comparator treats NaN values as equal to each other, and less than any other value.
 *
 * @author Michael H. Kay
 */

public class DoubleSortComparer implements AtomicComparer {

    private static DoubleSortComparer THE_INSTANCE = new DoubleSortComparer();

    /**
     * Get the singular instance of this class
     *
     * @return the singular instance
     */

    public static DoubleSortComparer getInstance() {
        return THE_INSTANCE;
    }

    private DoubleSortComparer() {

    }

    /*@Nullable*/
    public StringCollator getCollator() {
        return null;
    }

    /**
     * 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
     */

    public AtomicComparer provideContext(XPathContext context) {
        return this;
    }

    /**
     * Compare two AtomicValue objects according to the rules for their data type.
     *
     * @param a the first object to be compared. It is intended that this should normally 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 java.util.Comparable
     *          interface.
     * @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 &lt;0 if a&lt;b, 0 if a=b, &gt;0 if a&gt;b
     * @throws ClassCastException if the objects are not comparable
     */

    public int compareAtomicValues(AtomicValue a, AtomicValue b) {
        if (a == null) {
            if (b == null) {
                return 0;
            } else {
                return -1;
            }
        } else if (b == null) {
            return +1;
        }

        NumericValue an = (NumericValue) a;
        NumericValue bn = (NumericValue) b;

        if (an.isNaN()) {
            return bn.isNaN() ? 0 : -1;
        } else if (bn.isNaN()) {
            return +1;
        }

        return an.compareTo(bn);
    }

    /**
     * Test whether two values compare equal. Note that for this comparer, NaN is considered equal to itself
     */

    public boolean comparesEqual(AtomicValue a, AtomicValue b) {
        return compareAtomicValues(a, b) == 0;
    }

    /**
     * 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
     */
    public String save() {
        return "DblSC";
    }
}

