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 * Copyright (c) 2018, 2019, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package org.graalvm.compiler.lir.hashing;

import java.util.Comparator;
import java.util.HashSet;
import java.util.Optional;
import java.util.Set;
import java.util.TreeSet;

import org.graalvm.compiler.lir.gen.ArithmeticLIRGenerator;

import jdk.vm.ci.meta.JavaConstant;
import jdk.vm.ci.meta.Value;

/**
 * This class holds a hash function at a specific cardinality and min value (lowest key). The
 * cardinality is the required size of the hash table to make the hasher injective for the provided
 * keys.
 */
public final class Hasher {

    /**
     * Tries to find a hash function without conflicts for the provided keys.
     *
     * @param keys the keys
     * @param minDensity the minimum density of the switch table. Used to determine the maximum
     *            cardinality of the hash function
     * @return an optional hasher
     */
    public static Optional<Hasher> forKeys(JavaConstant[] keys, double minDensity) {
        if (keys.length <= 2) {
            return Optional.empty();
        } else {
            int maxCardinality = (int) Math.round(keys.length / minDensity);
            assertSorted(keys);
            TreeSet<Hasher> candidates = new TreeSet<>(new Comparator<Hasher>() {
                @Override
                public int compare(Hasher o1, Hasher o2) {
                    int d = o1.cardinality - o2.cardinality;
                    if (d != 0) {
                        return d;
                    } else {
                        return o1.effort() - o2.effort();
                    }
                }
            });
            long min = keys[0].asLong();
            for (HashFunction f : HashFunction.instances()) {
                for (int cardinality = keys.length; cardinality < maxCardinality; cardinality++) {
                    if (isValid(keys, min, f, cardinality)) {
                        candidates.add(new Hasher(f, cardinality, min));
                        break;
                    }
                }
            }
            if (candidates.isEmpty()) {
                return Optional.empty();
            } else {
                return Optional.of(candidates.first());
            }
        }
    }

    private static void assertSorted(JavaConstant[] keys) {
        for (int i = 1; i < keys.length; i++) {
            assert keys[i - 1].asLong() < keys[i].asLong();
        }
    }

    private static boolean isValid(JavaConstant[] keys, long min, HashFunction function, int cardinality) {
        Set<Integer> seen = new HashSet<>(keys.length);
        for (JavaConstant key : keys) {
            int hash = function.apply(key.asLong(), min) & (cardinality - 1);
            if (!seen.add(hash)) {
                return false;
            }
        }
        return true;
    }

    private final HashFunction function;
    private final int cardinality;
    private final long min;

    private Hasher(HashFunction function, int cardinality, long min) {
        this.function = function;
        this.cardinality = cardinality;
        this.min = min;
    }

    /**
     * Applies the hash function.
     *
     * @param value the value to be hashed
     * @return the hash value
     */
    public int hash(long value) {
        return function.apply(value, min) & (cardinality - 1);
    }

    /**
     * Applies the hash function to a lir value.
     *
     * @param value the value to be hashed
     * @param gen the lir generator
     * @return the hashed lir value
     */
    public Value hash(Value value, ArithmeticLIRGenerator gen) {
        Value h = function.gen(value, gen.getLIRGen().emitJavaConstant(JavaConstant.forLong(min)), gen);
        return gen.emitAnd(h, gen.getLIRGen().emitJavaConstant(JavaConstant.forInt(cardinality - 1)));
    }

    /**
     * @return the hashing effort
     */
    public int effort() {
        return function.effort() + 1;
    }

    /**
     * @return the cardinality of the hash function that should match the size of the table switch.
     */
    public int cardinality() {
        return cardinality;
    }

    /**
     * @return the hash function
     */
    public HashFunction function() {
        return function;
    }

    @Override
    public String toString() {
        return "Hasher[function=" + function + ", effort=" + effort() + ", cardinality=" + cardinality + "]";
    }
}