apps/pi-demo/tor/src/main/java/org/onosproject/pi/demo/app/tor/Combination.java

/*
 * Copyright 2017-present Open Networking Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.onosproject.pi.demo.app.tor;

import com.google.common.collect.AbstractIterator;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.google.common.math.IntMath;

import java.util.AbstractSet;
import java.util.BitSet;
import java.util.Collection;
import java.util.Iterator;
import java.util.Set;

import static com.google.common.base.Preconditions.checkArgument;

/**
 * Combination algorithm implementation, taken from Guava 23.
 */
final class Combination {

    private Combination() {
        // Hides constructor.
    }

    /**
     * Returns a map from the ith element of list to i.
     */
    private static <E> ImmutableMap<E, Integer> indexMap(Collection<E> list) {
        ImmutableMap.Builder<E, Integer> builder = ImmutableMap.builder();
        int i = 0;
        for (E e : list) {
            builder.put(e, i++);
        }
        return builder.build();
    }

    /**
     * Returns the set of all subsets of {@code set} of size {@code size}. For example, {@code
     * combinations(ImmutableSet.of(1, 2, 3), 2)} returns the set {@code {{1, 2}, {1, 3}, {2, 3}}}.
     * <p>
     * <p>Elements appear in these subsets in the same iteration order as they appeared in the input set. The order in
     * which these subsets appear in the outer set is undefined.
     * <p>
     * <p>The returned set and its constituent sets use {@code equals} to decide whether two elements are identical,
     * even if the input set uses a different concept of equivalence.
     * <p>
     * <p><i>Performance notes:</i> the memory usage of the returned set is only {@code O(n)}. When the result set is
     * constructed, the input set is merely copied. Only as the result set is iterated are the individual subsets
     * created. Each of these subsets occupies an additional O(n) memory but only for as long as the user retains a
     * reference to it. That is, the set returned by {@code combinations} does not retain the individual subsets.
     *
     * @param set  the set of elements to take combinations of
     * @param size the number of elements per combination
     * @return the set of all combinations of {@code size} elements from {@code set}
     * @throws IllegalArgumentException if {@code size} is not between 0 and {@code set.size()} inclusive
     * @throws NullPointerException     if {@code set} is or contains {@code null}
     * @since 23.0
     */
    static <E> Set<Set<E>> combinations(Set<E> set, final int size) {
        final ImmutableMap<E, Integer> index = indexMap(set);
        checkArgument(size > 0, "size");
        checkArgument(size <= index.size(), "size (%s) must be <= set.size() (%s)", size, index.size());
        if (size == 0) {
            return ImmutableSet.of(ImmutableSet.<E>of());
        } else if (size == index.size()) {
            return ImmutableSet.of(index.keySet());
        }
        return new AbstractSet<Set<E>>() {
            @Override
            public boolean contains(Object o) {
                if (o instanceof Set) {
                    Set<?> s = (Set<?>) o;
                    return s.size() == size && index.keySet().containsAll(s);
                }
                return false;
            }

            @Override
            public Iterator<Set<E>> iterator() {
                return new AbstractIterator<Set<E>>() {
                    final BitSet bits = new BitSet(index.size());

                    @Override
                    protected Set<E> computeNext() {
                        if (bits.isEmpty()) {
                            bits.set(0, size);
                        } else {
                            int firstSetBit = bits.nextSetBit(0);
                            int bitToFlip = bits.nextClearBit(firstSetBit);

                            if (bitToFlip == index.size()) {
                                return endOfData();
                            }

                          /*
                           * The current set in sorted order looks like
                           * {firstSetBit, firstSetBit + 1, ..., bitToFlip - 1, ...}
                           * where it does *not* contain bitToFlip.
                           *
                           * The next combination is
                           *
                           * {0, 1, ..., bitToFlip - firstSetBit - 2, bitToFlip, ...}
                           *
                           * This is lexicographically next if you look at the combinations in descending order
                           * e.g. {2, 1, 0}, {3, 1, 0}, {3, 2, 0}, {3, 2, 1}, {4, 1, 0}...
                           */

                            bits.set(0, bitToFlip - firstSetBit - 1);
                            bits.clear(bitToFlip - firstSetBit - 1, bitToFlip);
                            bits.set(bitToFlip);
                        }
                        final BitSet copy = (BitSet) bits.clone();
                        return new AbstractSet<E>() {
                            @Override
                            public boolean contains(Object o) {
                                Integer i = index.get(o);
                                return i != null && copy.get(i);
                            }

                            @Override
                            public Iterator<E> iterator() {
                                return new AbstractIterator<E>() {
                                    int i = -1;

                                    @Override
                                    protected E computeNext() {
                                        i = copy.nextSetBit(i + 1);
                                        if (i == -1) {
                                            return endOfData();
                                        }
                                        return index.keySet().asList().get(i);
                                    }
                                };
                            }

                            @Override
                            public int size() {
                                return size;
                            }
                        };
                    }
                };
            }

            @Override
            public int size() {
                return IntMath.binomial(index.size(), size);
            }

            @Override
            public String toString() {
                return "Sets.combinations(" + index.keySet() + ", " + size + ")";
            }
        };
    }

}