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Immutable Clojure collections and a Transformation abstraction for Java 8+, immutably, type-safely, and with good performance. Name will change to "Paguro" in November 2016.
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// Copyright 2015 PlanBase Inc. & Glen Peterson
//
// 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.organicdesign.fp.collections;

import java.util.Collection;
import java.util.Set;
import java.util.function.Predicate;

/** An unmodifiable set */
public interface UnmodSet extends UnmodCollection, Set {

    // ========================================== Static ==========================================

    /**
     Implements equals and hashCode() methods compatible with java.util.Set (which ignores order)
     to make defining unmod sets easier, especially for implementing Map.keySet() and such.
     */
    abstract class AbstractUnmodSet extends AbstractUnmodIterable implements UnmodSet {
        @Override public boolean equals(Object other) {
            if (this == other) { return true; }
            if ( !(other instanceof Set) ) { return false; }
            Set that = (Set) other;
            return (size() == that.size()) &&
                   containsAll(that);
        }
    }

    // ========================================= Instance =========================================

    /** Not allowed - this is supposed to be unmodifiable */
    @SuppressWarnings("deprecation")
    @Override @Deprecated default boolean add(E e) {
        throw new UnsupportedOperationException("Modification attempted");
    }
    /** Not allowed - this is supposed to be unmodifiable */
    @SuppressWarnings("deprecation")
    @Override @Deprecated default boolean addAll(Collection c) {
        throw new UnsupportedOperationException("Modification attempted");
    }
    /** Not allowed - this is supposed to be unmodifiable */
    @SuppressWarnings("deprecation")
    @Override @Deprecated default void clear() {
        throw new UnsupportedOperationException("Modification attempted");
    }

    /**
     Returns true if the set contains the given item.  This is the defining method of a set.
     Sets have to override this because the default implementation in UnmodCollection is O(n)
     whereas a sorted set should be O(log n) or O(1).
     */
    @Override boolean contains(Object o);

    /** {@inheritDoc} */
    @Override default boolean containsAll(Collection items) {
        for (Object item : items) {
            if (!this.contains(item)) { return false; }
        }
        return true;
    }
// boolean	equals(Object o)
// int	hashCode()

    /**
     This is a convenience method inherited from Collection that returns true if size() == 0 (if
     this set contains no elements).
     */
    @Override default boolean isEmpty() { return size() == 0; }

    /**
     Iterates over contents with no guarantees about their ordering.
     {@inheritDoc}
     */
    @Override
    UnmodIterator iterator();

    /** Not allowed - this is supposed to be unmodifiable */
    @SuppressWarnings("deprecation")
    @Override @Deprecated default boolean remove(Object o) {
        throw new UnsupportedOperationException("Modification attempted");
    }
    /** Not allowed - this is supposed to be unmodifiable */
    @SuppressWarnings("deprecation")
    @Override @Deprecated default boolean removeAll(Collection c) {
        throw new UnsupportedOperationException("Modification attempted");
    }
    /** Not allowed - this is supposed to be unmodifiable */
    @SuppressWarnings("deprecation")
    @Override @Deprecated default boolean retainAll(Collection c) {
        throw new UnsupportedOperationException("Modification attempted");
    }

//  int size(); // This is limiting for sets, but not so much that it has to be deprecated.
// default Spliterator	spliterator()

    /**
     This method goes against Josh Bloch's Item 25: "Prefer Lists to Arrays", but is provided for
     backwards compatibility in some performance-critical situations.  If you really need an array,
     consider using the somewhat type-safe version of this method instead, but read the caveats
     first.

     {@inheritDoc}
     */
    @Override default Object[] toArray() { return UnmodCollection.super.toArray(); }

    /**
     This method goes against Josh Bloch's Item 25: "Prefer Lists to Arrays", but is provided for
     backwards compatibility in some performance-critical situations.  If you need to create an
     array (you almost always do) then the best way to use this method is:

     MyThing[] things = col.toArray(new MyThing[coll.size()]);

     Calling this method any other way causes unnecessary work to be done - an extra memory
     allocation and potential garbage collection if the passed array is too small, extra effort to
     fill the end of the array with nulls if it is too large.

     {@inheritDoc}
     */
    @SuppressWarnings("SuspiciousToArrayCall")
    @Override default  T[] toArray(T[] as) { return UnmodCollection.super.toArray(as); }

// Methods inherited from interface java.util.Collection
// parallelStream, removeIf, stream
    /** Not allowed - this is supposed to be unmodifiable */
    @SuppressWarnings("deprecation")
    @Override @Deprecated default boolean removeIf(Predicate filter) {
        throw new UnsupportedOperationException("Modification attempted");
    }
// Methods inherited from interface java.lang.Iterable
// forEach
}