Dépôt officiel du code source de l'ERP OpenConcerto

/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.

 *

 * Copyright 2011-2019 OpenConcerto, by ILM Informatique. All rights reserved.

 *

 * The contents of this file are subject to the terms of the GNU General Public License Version 3

 * only ("GPL"). You may not use this file except in compliance with the License. You can obtain a

 * copy of the License at http://www.gnu.org/licenses/gpl-3.0.html See the License for the specific

 * language governing permissions and limitations under the License.

 *

 * When distributing the software, include this License Header Notice in each file.

 */

 package org.openconcerto.utils;

import org.openconcerto.utils.cc.CustomEquals;

import org.openconcerto.utils.cc.IClosure;

import org.openconcerto.utils.cc.IPredicate;

import org.openconcerto.utils.cc.ITransformer;

import org.openconcerto.utils.cc.ITransformerExn;

import org.openconcerto.utils.cc.IdentityHashSet;

import org.openconcerto.utils.cc.IdentitySet;

import org.openconcerto.utils.cc.LinkedIdentitySet;

import org.openconcerto.utils.cc.Transformer;

import java.io.Serializable;

import java.util.AbstractSet;

import java.util.ArrayList;

import java.util.Arrays;

import java.util.Collection;

import java.util.Collections;

import java.util.HashMap;

import java.util.HashSet;

import java.util.IdentityHashMap;

import java.util.Iterator;

import java.util.LinkedHashMap;

import java.util.LinkedHashSet;

import java.util.LinkedList;

import java.util.List;

import java.util.ListIterator;

import java.util.Map;

import java.util.Map.Entry;

import java.util.NoSuchElementException;

import java.util.RandomAccess;

import java.util.Set;

import java.util.SortedMap;

import java.util.SortedSet;

import java.util.TreeMap;

import java.util.TreeSet;

import java.util.function.Function;

import java.util.function.Supplier;

import java.util.regex.Pattern;

import java.util.stream.Collectors;

import java.util.stream.Stream;

/**

 * Une classe regroupant des méthodes utilitaires pour les collections.

 *

 * @author ILM Informatique 30 sept. 2004

 */

public class CollectionUtils {

    /**

     * Concatene une collection. Cette méthode va appliquer un transformation sur chaque élément

     * avant d'appeler toString(). join([-1, 3, 0], " ,", doubleTransformer) == "-2, 6, 0"

     *

     * @param <E> type of items

     * @param c la collection a concaténer.

     * @param sep le séparateur entre chaque élément.

     * @param tf la transformation à appliquer à chaque élément.

     * @return la chaine composée de chacun des éléments séparés par <code>sep</code>.

     */

    static public final <E> String join(final Collection<E> c, final String sep, final ITransformer<? super E, ?> tf) {

        final int size = c.size();

        if (size == 0)

            return "";

        final StringBuffer res = new StringBuffer(size * 4);

        if (c instanceof RandomAccess && c instanceof List) {

            final List<E> list = (List<E>) c;

            for (int i = 0; i < size; i++) {

                res.append(tf.transformChecked(list.get(i)));

                if (i < size - 1)

                    res.append(sep);

            }

        } else {

            final Iterator<E> iter = c.iterator();

            while (iter.hasNext()) {

                final E elem = iter.next();

                res.append(tf.transformChecked(elem));

                if (iter.hasNext())

                    res.append(sep);

            }

        }

        return res.toString();

    }

    /**

     * Concatene une collection en appelant simplement toString() sur chaque élément.

     *

     * @param <T> type of collection

     * @param c la collection a concaténer.

     * @param sep le séparateur entre chaque élément.

     * @return la chaine composée de chacun des éléments séparés par <code>sep</code>.

     * @see #join(Collection, String, ITransformer)

     */

    static public <T> String join(Collection<T> c, String sep) {

        return join(c, sep, org.openconcerto.utils.cc.Transformer.<T> nopTransformer());

    }

    static public <T, U, C extends Collection<? super U>> C transform(final Collection<T> c, final ITransformer<? super T, U> transf, final C res) {

        return transformAndFilter(c, transf, IPredicate.truePredicate(), res);

    }

    static public <T, U, C extends Collection<? super U>> C transformAndFilter(final Collection<T> c, final ITransformer<? super T, U> transf, final IPredicate<? super U> filterAfter, final C res) {

        return filterTransformAndFilter(c, IPredicate.truePredicate(), transf, filterAfter, res);

    }

    static public <T, U, C extends Collection<? super U>> C filterAndTransform(final Collection<T> c, final IPredicate<? super T> filterBefore, final ITransformer<? super T, U> transf, final C res) {

        return filterTransformAndFilter(c, filterBefore, transf, IPredicate.truePredicate(), res);

    }

    static public <T, U, C extends Collection<? super U>> C filterTransformAndFilter(final Collection<T> c, final IPredicate<? super T> filterBefore, final ITransformer<? super T, U> transf,

            final IPredicate<? super U> filterAfter, final C res) {

        iterate(c, filterBefore, new IClosure<T>() {

            @Override

            public void executeChecked(T input) {

                final U item = transf.transformChecked(input);

                if (filterAfter.evaluateChecked(item))

                    res.add(item);

            }

        });

        return res;

    }

    static public <T> void iterate(final Collection<T> c, final IClosure<T> cl) {

        iterate(c, IPredicate.truePredicate(), cl);

    }

    static public <T> void iterate(final Collection<T> c, final IPredicate<? super T> filterBefore, final IClosure<T> cl) {

        if (c instanceof RandomAccess && c instanceof List) {

            final List<T> list = (List<T>) c;

            final int size = c.size();

            for (int i = 0; i < size; i++) {

                final T item = list.get(i);

                if (filterBefore.evaluateChecked(item))

                    cl.executeChecked(item);

            }

        } else {

            final Iterator<T> iter = c.iterator();

            while (iter.hasNext()) {

                final T item = iter.next();

                if (filterBefore.evaluateChecked(item))

                    cl.executeChecked(item);

            }

        }

    }

    private static final Pattern COMMA = Pattern.compile("\\p{Space}*,\\p{Space}*");

    static public List<String> split(String s) {

        return split(s, COMMA);

    }

    static public List<String> split(String s, String sep) {

        return split(s, Pattern.compile(sep));

    }

    /**

     * Split a string into a list based on a pattern.

     *

     * @param s the string to split.

     * @param pattern the pattern where to cut the string.

     * @return the splitted string, empty list if <code>s</code> is "".

     */

    static public List<String> split(String s, Pattern pattern) {

        return s.length() == 0 ? Collections.<String> emptyList() : Arrays.asList(pattern.split(s));

    }

    /**

     * Return an index between <code>0</code> and <code>l.size()</code> inclusive. If <code>i</code>

     * is negative, it is added to <code>l.size()</code> (i.e. -1 is the index of the last item and

     * -size is the first) :

     *

     * <pre>

     *    a  b  c  a  b  c

     *   -3 -2 -1  0  1  2  3

     * </pre>

     *

     * @param l the list, e.g. a list of 3 items.

     * @param i the virtual index, e.g. -1.

     * @return the real index, e.g. 2.

     * @throws IndexOutOfBoundsException if not <code>0 &le; abs(i) &le; l.size()</code>

     */

    static public int getValidIndex(final List<?> l, final int i) throws IndexOutOfBoundsException {

        return getValidIndex(l, i, true);

    }

    /**

     * Return an index between <code>0</code> and <code>l.size()</code> inclusive. If <code>i</code>

     * is negative, it is added to <code>l.size()</code> (bounded to 0 if <code>strict</code> is

     * <code>false</code>), i.e. for a list of 3 items, -1 is the index of the last item ; -3 and -4

     * are both the first. If <code>i</code> is greater than <code>l.size()</code> then

     * <code>l.size()</code> is returned if not <code>strict</code>.

     *

     * @param l the list, e.g. a list of 3 items.

     * @param i the virtual index, e.g. -1.

     * @param strict if <code>true</code> <code>abs(i)</code> must be between <code>0</code> and

     *        <code>l.size()</code>, else values are bounded between <code>0</code> and

     *        <code>l.size()</code>.

     * @return the real index between <code>0</code> and <code>l.size()</code>, e.g. 2.

     * @throws IndexOutOfBoundsException if <code>strict</code> is <code>true</code> and not

     *         <code>0 &le; abs(i) &le; l.size()</code>

     */

    static public int getValidIndex(final List<?> l, final int i, final boolean strict) throws IndexOutOfBoundsException {

        final int size = l.size();

        if (i > size) {

            if (strict)

                throw new IndexOutOfBoundsException("Too high : " + i + " > " + size);

            return size;

        } else if (i < -size) {

            if (strict)

                throw new IndexOutOfBoundsException("Too low : " + i + " < " + -size);

            return 0;

        } else if (i >= 0) {

            return i;

        } else {

            return size + i;

        }

    }

    /**

     * Deletes a slice of a list. Pass indexes to {@link #getValidIndex(List, int, boolean)} to

     * allow delete(l, 0, -1) to clear l or delete(l, -2, -2) to remove the penultimate item.

     *

     * @param l the list to delete from.

     * @param from the first index to be removed (inclusive).

     * @param to the last index to be removed (inclusive).

     */

    static public void delete(List<?> l, int from, int to) {

        if (!l.isEmpty())

            l.subList(getValidIndex(l, from, false), getValidIndex(l, to, false) + 1).clear();

    }

    /**

     * Deletes the tail of a list. The resulting list will have a size of <code>from</code>.

     *

     * @param l the list to delete from.

     * @param from the first index to be removed (inclusive).

     */

    static public void delete(List<?> l, int from) {

        delete(l, from, -1);

    }

    public static <T> boolean exists(Collection<T> collection, IPredicate<? super T> predicate) {

        return collection.stream().anyMatch(predicate::evaluateChecked);

    }

    /**

     * Convertit une map en 2 listes, une pour les clefs, une pour les valeurs.

     *

     * @param map la Map à convertir.

     * @return un tuple de 2 List, en 0 les clefs, en 1 les valeurs.

     * @param <K> type of key

     * @param <V> type of value

     */

    static public <K, V> Tuple2<List<K>, List<V>> mapToLists(Map<K, V> map) {

        final List<K> keys = new ArrayList<K>(map.size());

        final List<V> vals = new ArrayList<V>(map.size());

        for (final Map.Entry<K, V> e : map.entrySet()) {

            keys.add(e.getKey());

            vals.add(e.getValue());

        }

        return Tuple2.create(keys, vals);

    }

    /**

     * Add entries from <code>toAdd</code> into <code>map</code> only if the key is not already

     * present.

     *

     * @param <K> type of keys.

     * @param <V> type of values.

     * @param map the map to fill.

     * @param toAdd the entries to add.

     * @return <code>map</code>.

     */

    static public <K, V> Map<K, V> addIfNotPresent(Map<K, V> map, Map<? extends K, ? extends V> toAdd) {

        for (final Map.Entry<? extends K, ? extends V> e : toAdd.entrySet()) {

            if (!map.containsKey(e.getKey()))

                map.put(e.getKey(), e.getValue());

        }

        return map;

    }

    // compared to computeIfAbsent() :

    // 1. allow exceptions

    // 2. allow nulls

    static public <K, V, X extends Exception> V computeIfAbsent(final Map<K, V> map, final K key, final ITransformerExn<K, V, X> function, final boolean allowNullValue) throws X {

        V res = map.get(key);

        final boolean contains;

        if (allowNullValue) {

            contains = res != null || map.containsKey(key);

        } else {

            contains = res != null;

        }

        if (!contains) {

            res = function.transformChecked(key);

            if (res == null && !allowNullValue)

                throw new IllegalStateException("Null value computed for key '" + key + "'");

            map.put(key, res);

        }

        assert allowNullValue || res != null;

        return res;

    }

    /**

     * Compute the index that have changed (added or removed) between 2 lists. One of the lists MUST

     * be a sublist of the other, ie the to go from one to the other we just add or remove items but

     * we don't do both.

     *

     * @param oldList the first list.

     * @param newList the second list.

     * @return a list of Integer.

     * @param <E> type of item

     * @throws IllegalStateException if one list is not a sublist of the other.

     */

    static public <E> List<Integer> getIndexesChanged(List<E> oldList, List<E> newList) {

        final List<E> longer;

        final List<E> shorter;

        if (newList.size() > oldList.size()) {

            longer = new ArrayList<E>(newList);

            shorter = new ArrayList<E>(oldList);

        } else {

            longer = new ArrayList<E>(oldList);

            shorter = new ArrayList<E>(newList);

        }

        final List<Integer> res = new ArrayList<Integer>();

        int offset = 0;

        while (shorter.size() > 0) {

            if (longer.size() < shorter.size())

                throw new IllegalStateException(shorter + " is not a sublist of " + longer);

            // compare nulls

            if (CompareUtils.equals(shorter.get(0), longer.get(0))) {

                shorter.remove(0);

                longer.remove(0);

            } else {

                longer.remove(0);

                res.add(offset);

            }

            offset++;

        }

        for (int i = 0; i < longer.size(); i++) {

            res.add(i + offset);

        }

        return res;

    }

    /**

     * Aggregate a list of ints into a list of intervals. Eg aggregate([-1,0,1,2,5]) returns

     * [[-1,2], [5,5]].

     *

     * @param ints a list of Integer strictly increasing.

     * @return a list of int[2].

     */

    static public List<int[]> aggregate(Collection<? extends Number> ints) {

        final List<int[]> res = new ArrayList<int[]>();

        int[] currentInterval = null;

        for (final Number n : ints) {

            final int index = n.intValue();

            if (currentInterval == null || index != currentInterval[1] + 1) {

                currentInterval = new int[2];

                currentInterval[0] = index;

                currentInterval[1] = currentInterval[0];

                res.add(currentInterval);

            } else {

                currentInterval[1] = index;

            }

        }

        return res;

    }

    /**

     * Test whether col2 is contained in col1.

     *

     * @param <T> type of collection

     * @param col1 the first collection

     * @param col2 the second collection

     * @return <code>null</code> if col1 contains all of col2, else return the extra items that col2

     *         have.

     */

    static public <T> Set<T> contains(final Set<T> col1, final Set<T> col2) {

        if (col1.containsAll(col2))

            return null;

        else {

            final Set<T> names = new HashSet<T>(col2);

            names.removeAll(col1);

            return names;

        }

    }

    static public <C extends Collection<?>> boolean containsAny(final C coll1, final C coll2) {

        return !Collections.disjoint(coll1, coll2);

    }

    static public final boolean identityContains(final Collection<?> coll, final Object item) {

        final int size = coll.size();

        if (size > 0) {

            if (coll instanceof RandomAccess && coll instanceof List) {

                final List<?> list = (List<?>) coll;

                for (int i = 0; i < size; i++) {

                    final Object v = list.get(i);

                    if (item == v)

                        return true;

                }

            } else {

                for (final Object v : coll) {

                    if (item == v)

                        return true;

                }

            }

        }

        return false;

    }

    static public final boolean identityRemove(final Collection<?> coll, final Object item) {

        final int size = coll.size();

        if (size > 0) {

            if (coll instanceof RandomAccess && coll instanceof List) {

                final List<?> list = (List<?>) coll;

                for (int i = 0; i < size; i++) {

                    final Object v = list.get(i);

                    if (item == v) {

                        list.remove(i);

                        return true;

                    }

                }

            } else {

                for (Iterator<?> i = coll.iterator(); i.hasNext();) {

                    if (item == i.next()) {

                        i.remove();

                        return true;

                    }

                }

            }

        }

        return false;

    }

    static public final <T> int identityIndexOf(final List<T> coll, final T item) {

        return CustomEquals.indexOf(coll, item, CustomEquals.getIdentity());

    }

    static public final boolean identityEquals(final List<?> coll1, final List<?> coll2) {

        if (coll1 == coll2)

            return true;

        final int size = coll1.size();

        if (size != coll2.size())

            return false;

        if (size == 0)

            return true;

        final Iterator<?> iter1 = coll1.iterator();

        final Iterator<?> iter2 = coll2.iterator();

        while (iter1.hasNext()) {

            final Object elem1 = iter1.next();

            final Object elem2 = iter2.next();

            if (elem1 != elem2)

                return false;

        }

        assert !iter2.hasNext();

        return true;

    }

    /**

     * Convert an array to a list of a different type.

     *

     * @param <U> type of array

     * @param <T> type of list

     * @param array the array to convert, eg new Object[]{"a", "b"}.

     * @param clazz the class of the list items, eg String.class.

     * @return all items of <code>array</code> into a list, eg ["a", "b"].

     * @throws ClassCastException if some item of <code>array</code> is not a <code>T</code>.

     */

    static public <U, T extends U> List<T> castToList(U[] array, Class<T> clazz) throws ClassCastException {

        final List<T> res = new ArrayList<T>(array.length);

        for (final U item : array) {

            res.add(clazz.cast(item));

        }

        return res;

    }

    /**

     * Cast list

     *

     * @param <E> type of result list

     * @param list the list to convert

     * @param c the class of the result list

     * @return a new ArrayList create from <code>list</code> or null if <code>list</code> is null

     * @throws ClassCastException if some item of <code>list</code> is not a <code>E</code>.

     */

    public static <E> List<E> castList(final List<?> list, Class<E> c) throws ClassCastException {

        if (list == null) {

            return null;

        }

        final List<E> result;

        if (list instanceof RandomAccess) {

            final int size = list.size();

            result = new ArrayList<>(size);

            for (int i = 0; i < size; i++) {

                result.add(c.cast(list.get(i)));

            }

        } else {

            result = new LinkedList<>();

            for (final Object o : list) {

                result.add(c.cast(o));

            }

        }

        return result;

    }

    /**

     * Cast Map

     *

     * @param <E> type of key

     * @param <F> type of value

     * @param map the map to convert

     * @param cKey the class of key

     * @param cValue the class of value

     * @return a new HashMap create from the <code>map</code> or null if <code>map</code> is null

     * @throws ClassCastException if some item of <code>map</code> have a key which the type is not

     *         a <code>E</code> or a value which the type is not a <code>F</code>.

     */

    public static <E, F> Map<E, F> castMap(final Map<?, ?> map, Class<E> cKey, Class<F> cValue) throws ClassCastException {

        return castMap(map, cKey, cValue, HashMap::new);

    }

    public static <E, F> Map<E, F> castMap(final Map<?, ?> map, Class<E> cKey, Class<F> cValue, final Supplier<Map<E, F>> ctor) throws ClassCastException {

        if (map == null) {

            return null;

        }

        final Map<E, F> result = ctor.get();

        for (final Entry<?, ?> mapEntry : map.entrySet()) {

            final E key;

            try {

                key = cKey.cast(mapEntry.getKey());

            } catch (final ClassCastException ex) {

                throw new ClassCastException("Key " + mapEntry.getKey().toString() + " is not valid: " + ex.getMessage());

            }

            final F value;

            try {

                value = cValue.cast(mapEntry.getValue());

            } catch (final ClassCastException ex) {

                throw new ClassCastException("Value " + mapEntry.getKey().toString() + " is not valid: " + ex.getMessage());

            }

            result.put(key, value);

        }

        return result;

    }

    public static <T> Stream<T> subclassFilter(final Collection<? super T> coll, final Class<T> subclass) {

        return coll.stream().filter(subclass::isInstance).map(subclass::cast);

    }

    public static <T> List<T> subclassFilterToList(final Collection<? super T> coll, final Class<T> subclass) {

        return subclassFilter(coll, subclass).collect(Collectors.toList());

    }

    /**

     * The number of equals item between a and b, starting from the end.

     *

     * @param <T> type of items.

     * @param a the first list, eg [a, b, c].

     * @param b the second list, eg [a, null, z, c].

     * @return the number of common items, eg 1.

     */

    public static <T> int equalsFromEnd(final List<T> a, final List<T> b) {

        return equals(a, b, true, null);

    }

    public static <T> int equalsFromStart(final List<T> a, final List<T> b) {

        return equals(a, b, false, null);

    }

    /**

     * The number of equals item between a and b, starting from the choosen end.

     *

     * @param <A> type of the first list.

     * @param <B> type of the second list.

     * @param a the first list, eg [a, b, c].

     * @param b the second list, eg [a, null, z, c].

     * @param fromEnd whether search from the start or the end, <code>true</code>.

     * @param transf how items of <code>a</code> should be transformed before being compared, can be

     *        <code>null</code>.

     * @return the number of common items, eg 1.

     */

    public final static <A, B> int equals(final List<A> a, final List<B> b, boolean fromEnd, ITransformer<A, B> transf) {

        final int sizeA = a.size();

        final int sizeB = b.size();

        final int lastI = Math.min(sizeA, sizeB);

        for (int i = 0; i < lastI; i++) {

            final A itemA = a.get(fromEnd ? sizeA - 1 - i : i);

            final B itemB = b.get(fromEnd ? sizeB - 1 - i : i);

            if (!CompareUtils.equals(transf == null ? itemA : transf.transformChecked(itemA), itemB))

                return i;

        }

        return lastI;

    }

    public final static <T> int getEqualsCount(final Iterator<? extends T> a, final Iterator<? extends T> b) {

        return getEqualsCount(a, b, null);

    }

    public final static <A, B> int getEqualsCount(final Iterator<A> a, final Iterator<B> b, final ITransformer<A, B> transf) {

        int res = 0;

        while (a.hasNext() && b.hasNext()) {

            final A itemA = a.next();

            final B itemB = b.next();

            if (!CompareUtils.equals(transf == null ? itemA : transf.transformChecked(itemA), itemB))

                break;

            res++;

        }

        return res;

    }

    public static <T> Collection<T> select(final Collection<T> a, final IPredicate<? super T> pred) {

        return select(a, pred, new ArrayList<T>());

    }

    public static <T, C extends Collection<? super T>> C select(final Collection<T> a, final IPredicate<? super T> pred, final C b) {

        for (final T item : a)

            if (pred.evaluateChecked(item))

                b.add(item);

        return b;

    }

    // avoid name collision causing eclipse bug https://bugs.eclipse.org/bugs/show_bug.cgi?id=319603

    @SuppressWarnings("unchecked")

    public static <T> Collection<T> intersection(final Collection<T> a, final Collection<T> b) {

        return org.apache.commons.collections.CollectionUtils.intersection(a, b);

    }

    /**

     * Compute the intersection of a and b. <code>null</code>s are ignored : x ∩ null = x.

     *

     * @param <T> type of collection.

     * @param a the first set, can be <code>null</code>.

     * @param b the second set, can be <code>null</code>.

     * @return the intersection.

     */

    @SuppressWarnings("unchecked")

    public static <T> Set<T> inter(final Set<T> a, final Set<T> b) {

        return (Set<T>) interSubtype(a, b);

    }

    public static <T> Set<? extends T> interSubtype(final Set<? extends T> a, final Set<? extends T> b) {

        if (a == b)

            return a;

        else if (a == null)

            return b;

        else if (b == null)

            return a;

        else if (a.size() > b.size()) {

            return interSubtype(b, a);

        }

        final Set<T> res = new HashSet<T>();

        for (final T item : a) {

            if (b.contains(item))

                res.add(item);

        }

        return res;

    }

    public static <T> Set<T> inter(final Set<T>... sets) {

        return inter(Arrays.asList(sets));

    }

    public static <T> Set<T> inter(final List<Set<T>> sets) {

        final List<Set<T>> mutable = new ArrayList<Set<T>>(sets.size());

        for (final Set<T> s : sets) {

            // ignore nulls

            if (s != null)

                mutable.add(s);

        }

        if (mutable.isEmpty())

            return null;

        else if (mutable.size() == 1)

            return mutable.get(0);

        final int indexMin = indexOfMinSize(mutable);

        if (indexMin != 0) {

            mutable.add(0, mutable.remove(indexMin));

            return inter(mutable);

        }

        if (mutable.get(0).isEmpty())

            return Collections.emptySet();

        // replace the first 2 by their intersection

        // (inter will swap as appropriate if java doesn't evalute args in source order)

        mutable.add(0, inter(mutable.remove(0), mutable.remove(0)));

        return inter(mutable);

    }

    private static final <T> int indexOfMinSize(final List<Set<T>> sets) {

        if (sets.isEmpty())

            throw new IllegalArgumentException("empty sets");

        int res = 0;

        for (int i = 1; i < sets.size(); i++) {

            if (sets.get(i).size() < sets.get(res).size())

                res = i;

        }

        return res;

    }

    /**

     * Returns a {@link Set} containing the union of the given {@link Set}s.

     *

     * @param <T> type of items.

     * @param a the first set, must not be <code>null</code>

     * @param b the second set, must not be <code>null</code>

     * @return the union of the two.

     */

    public static <T> Set<T> union(final Set<? extends T> a, final Set<? extends T> b) {

        final Set<T> res = new HashSet<T>(a);

        if (a != b)

            res.addAll(b);

        return res;

    }

    public static <T> Set<T> union(final Collection<? extends Collection<? extends T>> colls) {

        return union(new HashSet<T>(), colls);

    }

    public static <T> List<T> cat(final Collection<? extends Collection<? extends T>> colls) {

        return union(new ArrayList<T>(), colls);

    }

    public static <T, C extends Collection<? super T>> C union(final C collector, final Collection<? extends Collection<? extends T>> colls) {

        return union(collector, colls, Transformer.<Collection<? extends T>> nopTransformer());

    }

    public static <T, C extends Collection<? super T>, A> C union(final C collector, final Collection<? extends A> colls, final ITransformer<? super A, ? extends Collection<? extends T>> transf) {

        for (final A coll : colls) {

            collector.addAll(transf.transformChecked(coll));

        }

        return collector;

    }

    public static <T> Collection<T> subtract(final Collection<T> a, final Collection<? extends T> b) {

        final Collection<T> res = a instanceof IdentitySet ? new LinkedIdentitySet<>(a) : new ArrayList<>(a);

        for (Iterator<? extends T> it = b.iterator(); it.hasNext();) {

            res.remove(it.next());

        }

        return res;

    }

    public static <T> Collection<T> substract(final Collection<T> a, final Collection<? extends T> b) {

        return subtract(a, b);

    }

    public static final <T> T coalesce(T o1, T o2) {

        return o1 != null ? o1 : o2;

    }

    public static final <T> T coalesce(T... objects) {

        for (T o : objects) {

            if (o != null)

                return o;

        }

        return null;

    }

    /**

     * Return the one and only item of <code>l</code>, otherwise <code>null</code>.

     *

     * @param <T> type of list.

     * @param l the list.

     * @param atMostOne <code>true</code> if the passed collection must not have more than one item.

     * @return the only item of <code>l</code>, <code>null</code> if there's not exactly one.

     * @throws IllegalArgumentException if <code>atMostOne</code> and <code>l.size() > 1</code>.

     */

    public static <T> T getSole(Collection<T> l, final boolean atMostOne) throws IllegalArgumentException {

        final int size = l.size();

        if (atMostOne && size > 1)

            throw new IllegalArgumentException("More than one");

        if (size != 1)

            return null;

        else if (l instanceof List)

            return ((List<T>) l).get(0);

        else

            return l.iterator().next();

    }

    public static <T> T getSole(Collection<T> l) {

        return getSole(l, false);

    }

    public static <T> T getFirst(Collection<T> l) {

        return l.size() > 0 ? l.iterator().next() : null;

    }

    /**

     * Return the first item of <code>l</code> if it isn't empty, otherwise <code>null</code>.

     *

     * @param <T> type of list.

     * @param l the list.

     * @return the first item of <code>l</code> or <code>null</code>.

     */

    public static <T> T getFirst(List<T> l) {

        return getNoExn(l, 0);

    }

    /**

     * Return the last item of <code>l</code> if it isn't empty, otherwise <code>null</code>.

     *

     * @param <T> type of list.

     * @param l the list.

     * @return the last item of <code>l</code> or <code>null</code>.

     */

    public static <T> T getLast(List<T> l) {

        return getNoExn(l, l.size() - 1);

    }

    /**

     * Return the item no <code>index</code> of <code>l</code> if it exists, otherwise

     * <code>null</code>.

     *

     * @param <T> type of list.

     * @param l the list.

     * @param index the wanted index.

     * @return the corresponding item of <code>l</code> or <code>null</code>.

     */

    public static <T> T getNoExn(List<T> l, int index) {

        return index >= 0 && index < l.size() ? l.get(index) : null;

    }

    @SuppressWarnings("rawtypes")

    private static final Iterator EMPTY_ITERATOR = new Iterator() {

        @Override

        public boolean hasNext() {

            return false;

        }

        @Override

        public Object next() {

            throw new NoSuchElementException();

        }

        @Override

        public void remove() {

            throw new UnsupportedOperationException();

        }

    };

    @SuppressWarnings("unchecked")

    public static <T> Iterator<T> emptyIterator() {

        return EMPTY_ITERATOR;

    }

    public static <T> LinkedList<T> toLinkedList(final Iterator<T> iter) {

        return addTo(iter, new LinkedList<T>());

    }

    public static <T> ArrayList<T> toArrayList(final Iterator<T> iter, final int estimatedSize) {

        return addTo(iter, new ArrayList<T>(estimatedSize));

    }

    public static <T, C extends Collection<? super T>> C addTo(final Iterator<T> iter, final C c) {

        while (iter.hasNext())

            c.add(iter.next());

        return c;

    }

    public static <T> ListIterator<T> getListIterator(final List<T> l, final boolean reversed) {

        if (!reversed)

            return l.listIterator();

        return reverseListIterator(l.listIterator(l.size()));

    }

    public static <T> ListIterator<T> reverseListIterator(final ListIterator<T> listIter) {

        if (listIter instanceof ReverseListIter)

            return ((ReverseListIter<T>) listIter).listIter;

        else

            return new ReverseListIter<T>(listIter);

    }

    private static final class ReverseListIter<T> implements ListIterator<T> {

        private final ListIterator<T> listIter;

        private ReverseListIter(ListIterator<T> listIter) {

            this.listIter = listIter;

        }

        @Override

        public boolean hasNext() {

            return this.listIter.hasPrevious();

        }

        @Override

        public T next() {

            return this.listIter.previous();

        }

        @Override

        public boolean hasPrevious() {

            return this.listIter.hasNext();

        }

        @Override

        public T previous() {

            return this.listIter.next();

        }

        @Override

        public int nextIndex() {

            return this.listIter.previousIndex();

        }

        @Override

        public int previousIndex() {

            return this.listIter.nextIndex();

        }

        @Override

        public void remove() {

            this.listIter.remove();

        }

        @Override

        public void set(T e) {

            this.listIter.set(e);

        }

        @Override

        public void add(T e) {

            throw new UnsupportedOperationException();

        }

    }

    public static <T> List<T> createList(T item1, T item2) {

        final List<T> res = new ArrayList<T>();

        res.add(item1);

        res.add(item2);

        return res;

    }

    // workaround for lack of @SafeVarargs in Java 6, TODO use Arrays.asList() in Java 7

    public static <T> List<T> createList(T item1, T item2, T item3) {

        final List<T> res = createList(item1, item2);

        res.add(item3);

        return res;

    }

    public static <T> Set<T> createSet(T... items) {

        return new HashSet<T>(Arrays.asList(items));

    }

    public static <T> IdentitySet<T> createIdentitySet(T... items) {

        return new IdentityHashSet<T>(Arrays.asList(items));

    }

    /**

     * Return an {@link IdentitySet} consisting of <code>items</code>.

     *

     * @param items the collection whose elements are to be in the result.

     * @return a set, possibly <code>items</code> if it's already an identity set.

     */

    public static <T> Set<T> toIdentitySet(Collection<T> items) {

        if (items instanceof IdentitySet)

            return (Set<T>) items;

        else

            return new IdentityHashSet<T>(items);

    }

    @SuppressWarnings("rawtypes")

    private static final IdentitySet EMPTY_SET = new EmptyIdentitySet();