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    /*
 * Copyright (c) 2018, Haiyang Li.
 *
 * 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 com.landawn.abacus.util;

import java.util.AbstractSet;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.RandomAccess;
import java.util.Set;

import com.landawn.abacus.util.u.Nullable;
import com.landawn.abacus.util.u.OptionalByte;
import com.landawn.abacus.util.u.OptionalChar;
import com.landawn.abacus.util.u.OptionalDouble;
import com.landawn.abacus.util.u.OptionalFloat;
import com.landawn.abacus.util.u.OptionalInt;
import com.landawn.abacus.util.u.OptionalLong;
import com.landawn.abacus.util.u.OptionalShort;
import com.landawn.abacus.util.function.Function;

/**
 * 
    
 * Note: This class includes codes copied from Apache Commons Lang, Google Guava and other open source projects under the Apache License 2.0.
 * The methods copied from other libraries/frameworks/projects may be modified in this class.
 * 
    
 *
 * 
    
 * This is a utility class for iterable data structures, including {@code Collection/Array/Iterator}.
 * 
    
 *
 * 
    
 * The methods in this class should only read the input {@code Collection/Array/Iterator} parameters, not modify them.
 * 
    
 *
 * @see com.landawn.abacus.util.N
 * @see com.landawn.abacus.util.Iterators
 * @see com.landawn.abacus.util.Maps
 * @see com.landawn.abacus.util.StringUtil
 */
public final class Iterables {

    private Iterables() {
        // Utility class.
    }

    /**
     * Returns {@code OptionalChar.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalChar min(final char... a) {
        return a == null || a.length == 0 ? OptionalChar.empty() : OptionalChar.of(N.min(a));
    }

    /**
     * Returns {@code OptionalByte.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalByte min(final byte... a) {
        return a == null || a.length == 0 ? OptionalByte.empty() : OptionalByte.of(N.min(a));
    }

    /**
     * Returns {@code OptionalShort.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalShort min(final short... a) {
        return a == null || a.length == 0 ? OptionalShort.empty() : OptionalShort.of(N.min(a));
    }

    /**
     * Returns {@code OptionalInt.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalInt min(final int... a) {
        return a == null || a.length == 0 ? OptionalInt.empty() : OptionalInt.of(N.min(a));
    }

    /**
     * Returns {@code OptionalLong.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalLong min(final long... a) {
        return a == null || a.length == 0 ? OptionalLong.empty() : OptionalLong.of(N.min(a));
    }

    /**
     * Returns {@code OptionalFloat.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalFloat min(final float... a) {
        return a == null || a.length == 0 ? OptionalFloat.empty() : OptionalFloat.of(N.min(a));
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalDouble min(final double... a) {
        return a == null || a.length == 0 ? OptionalDouble.empty() : OptionalDouble.of(N.min(a));
    }

    /**
     * Returns {@code OptionalChar.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalChar max(final char... a) {
        return a == null || a.length == 0 ? OptionalChar.empty() : OptionalChar.of(N.max(a));
    }

    /**
     * Returns {@code OptionalByte.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalByte max(final byte... a) {
        return a == null || a.length == 0 ? OptionalByte.empty() : OptionalByte.of(N.max(a));
    }

    /**
     * Returns {@code OptionalShort.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalShort max(final short... a) {
        return a == null || a.length == 0 ? OptionalShort.empty() : OptionalShort.of(N.max(a));
    }

    /**
     * Returns {@code OptionalInt.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalInt max(final int... a) {
        return a == null || a.length == 0 ? OptionalInt.empty() : OptionalInt.of(N.max(a));
    }

    /**
     * Returns {@code OptionalLong.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalLong max(final long... a) {
        return a == null || a.length == 0 ? OptionalLong.empty() : OptionalLong.of(N.max(a));
    }

    /**
     * Returns {@code OptionalFloat.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalFloat max(final float... a) {
        return a == null || a.length == 0 ? OptionalFloat.empty() : OptionalFloat.of(N.max(a));
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    @SafeVarargs
    public static OptionalDouble max(final double... a) {
        return a == null || a.length == 0 ? OptionalDouble.empty() : OptionalDouble.of(N.max(a));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    public static > Nullable min(final T[] a) {
        return N.isNullOrEmpty(a) ? Nullable. empty() : Nullable.of(N.min(a));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @param cmp
     * @return
     */
    public static  Nullable min(final T[] a, final Comparator cmp) {
        return N.isNullOrEmpty(a) ? Nullable. empty() : Nullable.of(N.min(a, cmp));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param c
     * @return
     */
    public static > Nullable min(final Collection c) {
        return N.isNullOrEmpty(c) ? Nullable. empty() : Nullable.of(N.min(c));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param c
     * @param cmp
     * @return
     */
    public static  Nullable min(final Collection c, final Comparator cmp) {
        return N.isNullOrEmpty(c) ? Nullable. empty() : Nullable.of(N.min(c, cmp));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param iter
     * @return
     */
    public static > Nullable min(final Iterator iter) {
        return min(iter, N.NULL_MAX_COMPARATOR);
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param iter
     * @param cmp
     * @return
     */
    public static  Nullable min(final Iterator iter, Comparator cmp) {
        cmp = cmp == null ? N.NULL_MAX_COMPARATOR : cmp;

        if (iter == null || iter.hasNext() == false) {
            return Nullable. empty();
        }

        T candidate = null;
        T next = null;

        do {
            next = iter.next();

            if (next == null && cmp == N.NULL_MIN_COMPARATOR) {
                return Nullable.of(next);
            } else if (cmp.compare(next, candidate) < 0) {
                candidate = next;
            }
        } while (iter.hasNext());

        return Nullable.of(candidate);
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @param cmp
     * @return
     */
    @SuppressWarnings("rawtypes")
    public static  Nullable minBy(final T[] a, final Function keyMapper) {
        return min(a, Fn.comparingBy(keyMapper));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param c
     * @param cmp
     * @return
     */
    @SuppressWarnings("rawtypes")
    public static  Nullable minBy(final Collection c, final Function keyMapper) {
        return min(c, Fn.comparingBy(keyMapper));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param iter
     * @param cmp
     * @return
     */
    @SuppressWarnings("rawtypes")
    public static  Nullable minBy(final Iterator iter, final Function keyMapper) {
        return min(iter, Fn.comparingBy(keyMapper));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    public static > Nullable max(final T[] a) {
        return N.isNullOrEmpty(a) ? Nullable. empty() : Nullable.of(N.max(a));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @param cmp
     * @return
     */
    public static  Nullable max(final T[] a, final Comparator cmp) {
        return N.isNullOrEmpty(a) ? Nullable. empty() : Nullable.of(N.max(a, cmp));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param c
     * @return
     */
    public static > Nullable max(final Collection c) {
        return N.isNullOrEmpty(c) ? Nullable. empty() : Nullable.of(N.max(c));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param c
     * @return
     */
    public static  Nullable max(final Collection c, final Comparator cmp) {
        return N.isNullOrEmpty(c) ? Nullable. empty() : Nullable.of(N.max(c, cmp));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param iter
     * @return
     */
    public static > Nullable max(final Iterator iter) {
        return max(iter, N.NULL_MIN_COMPARATOR);
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param iter
     * @param cmp
     * @return
     */
    public static  Nullable max(final Iterator iter, Comparator cmp) {
        cmp = cmp == null ? N.NULL_MIN_COMPARATOR : cmp;

        if (iter == null || iter.hasNext() == false) {
            return Nullable. empty();
        }

        T candidate = null;
        T next = null;

        do {
            next = iter.next();

            if (next == null && cmp == N.NULL_MAX_COMPARATOR) {
                return Nullable.of(next);
            } else if (cmp.compare(next, candidate) > 0) {
                candidate = next;
            }
        } while (iter.hasNext());

        return Nullable.of(candidate);
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @param cmp
     * @return
     */
    @SuppressWarnings("rawtypes")
    public static  Nullable maxBy(final T[] a, final Function keyMapper) {
        return max(a, Fn.comparingBy(keyMapper));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param c
     * @param cmp
     * @return
     */
    @SuppressWarnings("rawtypes")
    public static  Nullable maxBy(final Collection c, final Function keyMapper) {
        return max(c, Fn.comparingBy(keyMapper));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param iter
     * @param cmp
     * @return
     */
    @SuppressWarnings("rawtypes")
    public static  Nullable maxBy(final Iterator iter, final Function keyMapper) {
        return max(iter, Fn.comparingBy(keyMapper));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @return
     */
    public static > Nullable median(final T[] a) {
        return N.isNullOrEmpty(a) ? Nullable. empty() : Nullable.of(N.median(a));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param c
     * @return
     */
    public static > Nullable median(final Collection c) {
        return N.isNullOrEmpty(c) ? Nullable. empty() : Nullable.of(N.median(c));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @param cmp
     * @return
     */
    public static  Nullable median(final T[] a, final Comparator cmp) {
        return N.isNullOrEmpty(a) ? Nullable. empty() : Nullable.of(N.median(a, cmp));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param c
     * @param cmp
     * @return
     */
    public static  Nullable median(final Collection c, final Comparator cmp) {
        return N.isNullOrEmpty(c) ? Nullable. empty() : Nullable.of(N.median(c, cmp));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param a
     * @param keyMapper
     * @return
     */
    @SuppressWarnings("rawtypes")
    public static  Nullable medianBy(final T[] a, final Function keyMapper) {
        return median(a, Fn.comparingBy(keyMapper));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty.
     *
     * @param c
     * @param keyMapper
     * @return
     */
    @SuppressWarnings("rawtypes")
    public static  Nullable medianBy(final Collection c, final Function keyMapper) {
        return median(c, Fn.comparingBy(keyMapper));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or its length/size is less than {@code k}.
     *
     * @param c
     * @param k
     * @return
     */
    public static > Nullable kthLargest(final Collection c, final int k) {
        return N.isNullOrEmpty(c) || c.size() < k ? Nullable. empty() : Nullable.of(N.kthLargest(c, k));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or its length/size is less than {@code k}.
     *
     * @param a
     * @param k
     * @return
     */
    public static > Nullable kthLargest(final T[] a, final int k) {
        return N.isNullOrEmpty(a) || a.length < k ? Nullable. empty() : Nullable.of(N.kthLargest(a, k));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or its length/size is less than {@code k}.
     *
     * @param c
     * @param k
     * @param cmp
     * @return
     */
    public static  Nullable kthLargest(final Collection c, final int k, final Comparator cmp) {
        return N.isNullOrEmpty(c) || c.size() < k ? Nullable. empty() : Nullable.of(N.kthLargest(c, k, cmp));
    }

    /**
     * Returns {@code Nullable.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or its length/size is less than {@code k}.
     *
     * @param a
     * @param k
     * @param cmp
     * @return
     */
    public static  Nullable kthLargest(final T[] a, final int k, final Comparator cmp) {
        return N.isNullOrEmpty(a) || a.length < k ? Nullable. empty() : Nullable.of(N.kthLargest(a, k, cmp));
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param a
     * @return
     */
    public static  OptionalDouble averageInt(final T[] a) {
        return averageInt(a, Fn.numToInt());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param a
     * @param fromIndex
     * @param toIndex
     * @return
     */
    public static  OptionalDouble averageInt(final T[] a, final int fromIndex, final int toIndex) {
        return averageInt(a, fromIndex, toIndex, Fn.numToInt());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param 
     * @param a
     * @param func
     * @return
     * @throws E the e
     */
    public static  OptionalDouble averageInt(final T[] a, final Throwables.ToIntFunction func) throws E {
        if (N.isNullOrEmpty(a)) {
            return OptionalDouble.empty();
        }

        return averageInt(a, 0, a.length, func);
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param 
     * @param a
     * @param fromIndex
     * @param toIndex
     * @param func
     * @return
     * @throws E the e
     */
    public static  OptionalDouble averageInt(final T[] a, final int fromIndex, final int toIndex,
            final Throwables.ToIntFunction func) throws E {
        N.checkFromToIndex(fromIndex, toIndex, N.len(a));

        if (fromIndex == toIndex) {
            return OptionalDouble.empty();
        }

        return OptionalDouble.of(((double) N.sumInt(a, fromIndex, toIndex, func)) / (toIndex - fromIndex));
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param c
     * @return
     */
    public static  OptionalDouble averageInt(final Collection c) {
        return averageInt(c, Fn.numToInt());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param c
     * @param fromIndex
     * @param toIndex
     * @return
     */
    public static  OptionalDouble averageInt(final Collection c, final int fromIndex, final int toIndex) {
        return averageInt(c, fromIndex, toIndex, Fn.numToInt());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param 
     * @param c
     * @param func
     * @return
     * @throws E the e
     */
    public static  OptionalDouble averageInt(final Collection c, final Throwables.ToIntFunction func)
            throws E {
        if (N.isNullOrEmpty(c)) {
            return OptionalDouble.empty();
        }

        return OptionalDouble.of(((double) N.sumInt(c, func)) / c.size());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param 
     * @param c
     * @param fromIndex
     * @param toIndex
     * @param func
     * @return
     * @throws E the e
     */
    public static  OptionalDouble averageInt(final Collection c, final int fromIndex, final int toIndex,
            final Throwables.ToIntFunction func) throws E {
        N.checkFromToIndex(fromIndex, toIndex, N.size(c));

        if (fromIndex == toIndex) {
            return OptionalDouble.empty();
        }

        return OptionalDouble.of(((double) N.sumInt(c, fromIndex, toIndex, func)) / (toIndex - fromIndex));
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param a
     * @return
     */
    public static  OptionalDouble averageLong(final T[] a) {
        return averageLong(a, Fn.numToLong());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param a
     * @param fromIndex
     * @param toIndex
     * @return
     */
    public static  OptionalDouble averageLong(final T[] a, final int fromIndex, final int toIndex) {
        return averageLong(a, fromIndex, toIndex, Fn.numToLong());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param 
     * @param a
     * @param func
     * @return
     * @throws E the e
     */
    public static  OptionalDouble averageLong(final T[] a, final Throwables.ToLongFunction func) throws E {
        if (N.isNullOrEmpty(a)) {
            return OptionalDouble.empty();
        }

        return averageLong(a, 0, a.length, func);
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param 
     * @param a
     * @param fromIndex
     * @param toIndex
     * @param func
     * @return
     * @throws E the e
     */
    public static  OptionalDouble averageLong(final T[] a, final int fromIndex, final int toIndex,
            final Throwables.ToLongFunction func) throws E {
        N.checkFromToIndex(fromIndex, toIndex, N.len(a));

        if (fromIndex == toIndex) {
            return OptionalDouble.empty();
        }

        return OptionalDouble.of(((double) N.sumLong(a, fromIndex, toIndex, func)) / (toIndex - fromIndex));
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param c
     * @return
     */
    public static  OptionalDouble averageLong(final Collection c) {
        return averageLong(c, Fn.numToLong());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param c
     * @param fromIndex
     * @param toIndex
     * @return
     */
    public static  OptionalDouble averageLong(final Collection c, final int fromIndex, final int toIndex) {
        return averageLong(c, fromIndex, toIndex, Fn.numToLong());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param 
     * @param c
     * @param func
     * @return
     * @throws E the e
     */
    public static  OptionalDouble averageLong(final Collection c, final Throwables.ToLongFunction func)
            throws E {
        if (N.isNullOrEmpty(c)) {
            return OptionalDouble.empty();
        }

        return OptionalDouble.of(((double) N.sumLong(c, func)) / c.size());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param 
     * @param c
     * @param fromIndex
     * @param toIndex
     * @param func
     * @return
     * @throws E the e
     */
    public static  OptionalDouble averageLong(final Collection c, final int fromIndex, final int toIndex,
            final Throwables.ToLongFunction func) throws E {
        N.checkFromToIndex(fromIndex, toIndex, N.size(c));

        if (fromIndex == toIndex) {
            return OptionalDouble.empty();
        }

        return OptionalDouble.of(((double) N.sumLong(c, fromIndex, toIndex, func)) / (toIndex - fromIndex));
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param a
     * @return
     */
    public static  OptionalDouble averageDouble(final T[] a) {
        return averageDouble(a, Fn.numToDouble());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param a
     * @param fromIndex
     * @param toIndex
     * @return
     */
    public static  OptionalDouble averageDouble(final T[] a, final int fromIndex, final int toIndex) {
        return averageDouble(a, fromIndex, toIndex, Fn.numToDouble());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param 
     * @param a
     * @param func
     * @return
     * @throws E the e
     */
    public static  OptionalDouble averageDouble(final T[] a, final Throwables.ToDoubleFunction func) throws E {
        if (N.isNullOrEmpty(a)) {
            return OptionalDouble.empty();
        }

        return averageDouble(a, 0, a.length, func);
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param 
     * @param a
     * @param fromIndex
     * @param toIndex
     * @param func
     * @return
     * @throws E the e
     */
    public static  OptionalDouble averageDouble(final T[] a, final int fromIndex, final int toIndex,
            final Throwables.ToDoubleFunction func) throws E {
        N.checkFromToIndex(fromIndex, toIndex, N.len(a));

        if (fromIndex == toIndex) {
            return OptionalDouble.empty();
        }

        final KahanSummation summation = new KahanSummation();

        for (int i = fromIndex; i < toIndex; i++) {
            summation.add(func.applyAsDouble(a[i]));
        }

        return summation.average();
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param c
     * @return
     */
    public static  OptionalDouble averageDouble(final Collection c) {
        return averageDouble(c, Fn.numToDouble());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param c
     * @param fromIndex
     * @param toIndex
     * @return
     */
    public static  OptionalDouble averageDouble(final Collection c, final int fromIndex, final int toIndex) {
        return averageDouble(c, fromIndex, toIndex, Fn.numToDouble());
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param 
     * @param c
     * @param func
     * @return
     * @throws E the e
     */
    public static  OptionalDouble averageDouble(final Collection c, final Throwables.ToDoubleFunction func)
            throws E {
        if (N.isNullOrEmpty(c)) {
            return OptionalDouble.empty();
        }

        final KahanSummation summation = new KahanSummation();

        for (T e : c) {
            summation.add(func.applyAsDouble(e));
        }

        return summation.average();
    }

    /**
     * Returns {@code OptionalDouble.empty()} if the specified {@code Array/Collection} is {@code null} or empty, or {@code fromIndex == toIndex}.
     *
     * @param 
     * @param 
     * @param c
     * @param fromIndex
     * @param toIndex
     * @param func
     * @return
     * @throws E the e
     */
    public static  OptionalDouble averageDouble(final Collection c, final int fromIndex, final int toIndex,
            final Throwables.ToDoubleFunction func) throws E {
        N.checkFromToIndex(fromIndex, toIndex, N.size(c));

        if (fromIndex == toIndex) {
            return OptionalDouble.empty();
        }

        final KahanSummation summation = new KahanSummation();

        if (c instanceof List && c instanceof RandomAccess) {
            final List list = (List) c;

            for (int i = fromIndex; i < toIndex; i++) {
                summation.add(func.applyAsDouble(list.get(i)));
            }
        } else {
            int idx = 0;

            for (T e : c) {
                if (idx++ < fromIndex) {
                    continue;
                }

                summation.add(func.applyAsDouble(e));

                if (idx >= toIndex) {
                    break;
                }
            }
        }

        return summation.average();
    }

    public static OptionalInt indexOf(final Object[] a, final Object objToFind) {
        return Index.of(a, objToFind);
    }

    public static OptionalInt indexOf(final Collection c, final Object objToFind) {
        return Index.of(c, objToFind);
    }

    public static OptionalInt lastIndexOf(final Object[] a, final Object objToFind) {
        return Index.last(a, objToFind);
    }

    public static OptionalInt lastIndexOf(final Collection c, final Object objToFind) {
        return Index.last(c, objToFind);
    }

    /**
     * Find first or last index.
     *
     * @param 
     * @param 
     * @param 
     * @param a
     * @param predicateForFirst
     * @param predicateForLast
     * @return the optional int
     * @throws E the e
     * @throws E2 the e2
     */
    public static  OptionalInt findFirstOrLastIndex(final T[] a,
            final Throwables.Predicate predicateForFirst, final Throwables.Predicate predicateForLast) throws E, E2 {
        if (N.isNullOrEmpty(a)) {
            return OptionalInt.empty();
        }

        final OptionalInt res = N.findFirstIndex(a, predicateForFirst);

        return res.isPresent() ? res : N.findLastIndex(a, predicateForLast);
    }

    public static  OptionalInt findFirstOrLastIndex(final Collection c,
            final Throwables.Predicate predicateForFirst, final Throwables.Predicate predicateForLast) throws E, E2 {
        if (N.isNullOrEmpty(c)) {
            return OptionalInt.empty();
        }

        final OptionalInt res = N.findFirstIndex(c, predicateForFirst);

        return res.isPresent() ? res : N.findLastIndex(c, predicateForLast);
    }

    /**
     * Find first and last index.
     *
     * @param 
     * @param 
     * @param a
     * @param predicate
     * @return the pair
     * @throws E the e
     */
    public static  Pair findFirstAndLastIndex(final T[] a, final Throwables.Predicate predicate)
            throws E {
        return findFirstAndLastIndex(a, predicate, predicate);
    }

    /**
     * Find first and last index.
     *
     * @param 
     * @param 
     * @param 
     * @param a
     * @param predicateForFirst
     * @param predicateForLast
     * @return the pair
     * @throws E the e
     * @throws E2 the e2
     */
    public static  Pair findFirstAndLastIndex(final T[] a,
            final Throwables.Predicate predicateForFirst, final Throwables.Predicate predicateForLast) throws E, E2 {
        if (N.isNullOrEmpty(a)) {
            return Pair.of(OptionalInt.empty(), OptionalInt.empty());
        }

        return Pair.of(N.findFirstIndex(a, predicateForFirst), N.findLastIndex(a, predicateForLast));
    }

    /**
     * Find first and last index.
     *
     * @param 
     * @param 
     * @param c
     * @param predicate
     * @return the pair
     * @throws E the e
     */
    public static  Pair findFirstAndLastIndex(final Collection c,
            final Throwables.Predicate predicate) throws E {
        return findFirstAndLastIndex(c, predicate, predicate);
    }

    /**
     * Find first and last index.
     *
     * @param 
     * @param 
     * @param 
     * @param c
     * @param predicateForFirst
     * @param predicateForLast
     * @return the pair
     * @throws E the e
     * @throws E2 the e2
     */
    public static  Pair findFirstAndLastIndex(final Collection c,
            final Throwables.Predicate predicateForFirst, final Throwables.Predicate predicateForLast) throws E, E2 {
        if (N.isNullOrEmpty(c)) {
            return Pair.of(OptionalInt.empty(), OptionalInt.empty());
        }

        return Pair.of(N.findFirstIndex(c, predicateForFirst), N.findLastIndex(c, predicateForLast));
    }

    /**
     * Find first or last.
     *
     * @param 
     * @param 
     * @param 
     * @param a
     * @param predicateForFirst
     * @param predicateForLast
     * @return the nullable
     * @throws E the e
     * @throws E2 the e2
     */
    public static  Nullable findFirstOrLast(final T[] a,
            final Throwables.Predicate predicateForFirst, final Throwables.Predicate predicateForLast) throws E, E2 {
        if (N.isNullOrEmpty(a)) {
            return Nullable. empty();
        }

        final Nullable res = N.findFirst(a, predicateForFirst);

        return res.isPresent() ? res : N.findLast(a, predicateForLast);
    }

    /**
     * Find first or last.
     *
     * @param 
     * @param 
     * @param 
     * @param c
     * @param predicateForFirst
     * @param predicateForLast
     * @return the nullable
     * @throws E the e
     * @throws E2 the e2
     */
    public static  Nullable findFirstOrLast(final Collection c,
            final Throwables.Predicate predicateForFirst, final Throwables.Predicate predicateForLast) throws E, E2 {
        if (N.isNullOrEmpty(c)) {
            return Nullable. empty();
        }

        final Nullable res = N.findFirst(c, predicateForFirst);

        return res.isPresent() ? res : N.findLast(c, predicateForLast);
    }

    /**
     * Find first and last.
     *
     * @param 
     * @param 
     * @param a
     * @param predicate
     * @return the pair
     * @throws E the e
     */
    public static  Pair, Nullable> findFirstAndLast(final T[] a, final Throwables.Predicate predicate)
            throws E {
        return findFirstAndLast(a, predicate, predicate);
    }

    /**
     * Find first and last.
     *
     * @param 
     * @param 
     * @param 
     * @param a
     * @param predicateForFirst
     * @param predicateForLast
     * @return the pair
     * @throws E the e
     * @throws E2 the e2
     */
    public static  Pair, Nullable> findFirstAndLast(final T[] a,
            final Throwables.Predicate predicateForFirst, final Throwables.Predicate predicateForLast) throws E, E2 {
        if (N.isNullOrEmpty(a)) {
            return Pair.of(Nullable. empty(), Nullable. empty());
        }

        return Pair.of(N.findFirst(a, predicateForFirst), N.findLast(a, predicateForLast));
    }

    /**
     * Find first and last.
     *
     * @param 
     * @param 
     * @param c
     * @param predicate
     * @return the pair
     * @throws E the e
     */
    public static  Pair, Nullable> findFirstAndLast(final Collection c,
            final Throwables.Predicate predicate) throws E {
        return findFirstAndLast(c, predicate, predicate);
    }

    /**
     * Find first and last.
     *
     * @param 
     * @param 
     * @param 
     * @param c
     * @param predicateForFirst
     * @param predicateForLast
     * @return the pair
     * @throws E the e
     * @throws E2 the e2
     */
    public static  Pair, Nullable> findFirstAndLast(final Collection c,
            final Throwables.Predicate predicateForFirst, final Throwables.Predicate predicateForLast) throws E, E2 {
        if (N.isNullOrEmpty(c)) {
            return Pair.of(Nullable. empty(), Nullable. empty());
        }

        return Pair.of(N.findFirst(c, predicateForFirst), N.findLast(c, predicateForLast));
    }

    //    /**
    //     *
    //     * @param 
    //     * @param 
    //     * @param a
    //     * @param b
    //     * @return
    //     * @see N#crossJoin(Collection, Collection)
    //     * @deprecated replaced by {@code N.crossJoin(Collection, Collection)}
    //     */
    //    @Deprecated
    //    public static  List> crossJoin(final Collection a, final Collection b) {
    //        return crossJoin(a, b, Fn. pair());
    //    }
    //
    //    /**
    //     *
    //     * @param 
    //     * @param 
    //     * @param 
    //     * @param 
    //     * @param a
    //     * @param b
    //     * @param func
    //     * @return
    //     * @throws E
    //     * @see N#crossJoin(Collection, Collection, com.landawn.abacus.util.Throwables.BiFunction)
    //     * @deprecated replaced by {@code N.crossJoin(Collection, Collection, com.landawn.abacus.util.Try.BiFunction)}
    //     */
    //    @Deprecated
    //    public static  List crossJoin(final Collection a, final Collection b,
    //            final Throwables.BiFunction func) throws E {
    //        N.checkArgNotNull(func, "func");
    //
    //        final List result = new ArrayList<>(N.size(a) * N.size(b));
    //
    //        if (N.isNullOrEmpty(a) || N.isNullOrEmpty(b)) {
    //            return result;
    //        }
    //
    //        for (T ae : a) {
    //            for (U be : b) {
    //                result.add(func.apply(ae, be));
    //            }
    //        }
    //
    //        return result;
    //    }
    //
    //    /**
    //     * The time complexity is O(n + m) : n is the size of this Seq and m is the size of specified collection b.
    //     *
    //     * @param  the generic type
    //     * @param  the generic type
    //     * @param  the element type
    //     * @param  the generic type
    //     * @param a the a
    //     * @param b the b
    //     * @param leftKeyMapper the left key mapper
    //     * @param rightKeyMapper the right key mapper
    //     * @return the list
    //     * @throws E the e
    //     * @throws E2 the e2
    //     * @see N#innerJoin(Collection, Collection, com.landawn.abacus.util.Throwables.Function, com.landawn.abacus.util.Throwables.Function)
    //     * @see sql join
    //     * @deprecated replaced by {@code N.innerJoin(Collection, Collection, com.landawn.abacus.util.Try.Function, com.landawn.abacus.util.Try.Function)}
    //     */
    //    @Deprecated
    //    public static  List> innerJoin(final Collection a, final Collection b,
    //            final Throwables.Function leftKeyMapper, final Throwables.Function rightKeyMapper)
    //            throws E, E2 {
    //        final List> result = new ArrayList<>(N.min(9, N.size(a), N.size(b)));
    //
    //        if (N.isNullOrEmpty(a) || N.isNullOrEmpty(b)) {
    //            return result;
    //        }
    //
    //        final ListMultimap rightKeyMap = ListMultimap.from(b, rightKeyMapper);
    //
    //        for (T left : a) {
    //            final List rights = rightKeyMap.get(leftKeyMapper.apply(left));
    //
    //            if (N.notNullOrEmpty(rights)) {
    //                for (U right : rights) {
    //                    result.add(Pair.of(left, right));
    //                }
    //            }
    //        }
    //
    //        return result;
    //    }
    //
    //    /**
    //     * The time complexity is O(n * m) : n is the size of this Seq and m is the size of specified collection b.
    //     *
    //     * @param  the generic type
    //     * @param  the generic type
    //     * @param  the element type
    //     * @param a the a
    //     * @param b the b
    //     * @param predicate the predicate
    //     * @return the list
    //     * @throws E the e
    //     * @see N#innerJoin(Collection, Collection, com.landawn.abacus.util.Throwables.BiPredicate)
    //     * @see sql join
    //     * @deprecated replaced by {@code N.innerJoin(Collection, Collection, com.landawn.abacus.util.Try.BiPredicate)}
    //     */
    //    @Deprecated
    //    public static  List> innerJoin(final Collection a, final Collection b,
    //            final Throwables.BiPredicate predicate) throws E {
    //        final List> result = new ArrayList<>(N.min(9, N.size(a), N.size(b)));
    //
    //        if (N.isNullOrEmpty(a) || N.isNullOrEmpty(b)) {
    //            return result;
    //        }
    //
    //        for (T left : a) {
    //            for (U right : b) {
    //                if (predicate.test(left, right)) {
    //                    result.add(Pair.of(left, right));
    //                }
    //            }
    //        }
    //
    //        return result;
    //    }
    //
    //    /**
    //     * The time complexity is O(n + m) : n is the size of this Seq and m is the size of specified collection b.
    //     *
    //     * @param  the generic type
    //     * @param  the generic type
    //     * @param  the element type
    //     * @param  the generic type
    //     * @param a the a
    //     * @param b the b
    //     * @param leftKeyMapper the left key mapper
    //     * @param rightKeyMapper the right key mapper
    //     * @return the list
    //     * @throws E the e
    //     * @throws E2 the e2
    //     * @see N#fullJoin(Collection, Collection, com.landawn.abacus.util.Throwables.Function, com.landawn.abacus.util.Throwables.Function)
    //     * @see sql join
    //     * @deprecated replaced by {@code N.fullJoin(Collection, Collection, com.landawn.abacus.util.Try.Function, com.landawn.abacus.util.Try.Function)}
    //     */
    //    @Deprecated
    //    public static  List> fullJoin(final Collection a, final Collection b,
    //            final Throwables.Function leftKeyMapper, final Throwables.Function rightKeyMapper)
    //            throws E, E2 {
    //        final List> result = new ArrayList<>(N.max(9, N.size(a), N.size(b)));
    //
    //        if (N.isNullOrEmpty(a)) {
    //            for (T left : a) {
    //                result.add(Pair.of(left, (U) null));
    //            }
    //        } else if (N.isNullOrEmpty(b)) {
    //            for (U right : b) {
    //                result.add(Pair.of((T) null, right));
    //            }
    //        } else {
    //            final ListMultimap rightKeyMap = ListMultimap.from(b, rightKeyMapper);
    //            final Map joinedRights = new IdentityHashMap<>();
    //
    //            for (T left : a) {
    //                final List rights = rightKeyMap.get(leftKeyMapper.apply(left));
    //
    //                if (N.notNullOrEmpty(rights)) {
    //                    for (U right : rights) {
    //                        result.add(Pair.of(left, right));
    //                        joinedRights.put(right, right);
    //                    }
    //                } else {
    //                    result.add(Pair.of(left, (U) null));
    //                }
    //            }
    //
    //            for (U right : b) {
    //                if (joinedRights.containsKey(right) == false) {
    //                    result.add(Pair.of((T) null, right));
    //                }
    //            }
    //        }
    //
    //        return result;
    //    }
    //
    //    /**
    //     * The time complexity is O(n * m) : n is the size of this Seq and m is the size of specified collection b.
    //     *
    //     * @param  the generic type
    //     * @param  the generic type
    //     * @param  the element type
    //     * @param a the a
    //     * @param b the b
    //     * @param predicate the predicate
    //     * @return the list
    //     * @throws E the e
    //     * @see N#fullJoin(Collection, Collection, com.landawn.abacus.util.Throwables.BiPredicate)
    //     * @see sql join
    //     * @deprecated replaced by {@code N.fullJoin(Collection, Collection, com.landawn.abacus.util.Try.BiPredicate)}
    //     */
    //    @Deprecated
    //    public static  List> fullJoin(final Collection a, final Collection b,
    //            final Throwables.BiPredicate predicate) throws E {
    //        final List> result = new ArrayList<>(N.max(9, N.size(a), N.size(b)));
    //
    //        if (N.isNullOrEmpty(a)) {
    //            for (T left : a) {
    //                result.add(Pair.of(left, (U) null));
    //            }
    //        } else if (N.isNullOrEmpty(b)) {
    //            for (U right : b) {
    //                result.add(Pair.of((T) null, right));
    //            }
    //        } else {
    //            final Map joinedRights = new IdentityHashMap<>();
    //
    //            for (T left : a) {
    //                boolean joined = false;
    //
    //                for (U right : b) {
    //                    if (predicate.test(left, right)) {
    //                        result.add(Pair.of(left, right));
    //                        joinedRights.put(right, right);
    //                        joined = true;
    //                    }
    //                }
    //
    //                if (joined == false) {
    //                    result.add(Pair.of(left, (U) null));
    //                }
    //            }
    //
    //            for (U right : b) {
    //                if (joinedRights.containsKey(right) == false) {
    //                    result.add(Pair.of((T) null, right));
    //                }
    //            }
    //        }
    //
    //        return result;
    //    }
    //
    //    /**
    //     * The time complexity is O(n + m) : n is the size of this Seq and m is the size of specified collection b.
    //     *
    //     * @param  the generic type
    //     * @param  the generic type
    //     * @param  the element type
    //     * @param  the generic type
    //     * @param a the a
    //     * @param b the b
    //     * @param leftKeyMapper the left key mapper
    //     * @param rightKeyMapper the right key mapper
    //     * @return the list
    //     * @throws E the e
    //     * @throws E2 the e2
    //     * @see N#leftJoin(Collection, Collection, com.landawn.abacus.util.Throwables.Function, com.landawn.abacus.util.Throwables.Function)
    //     * @see sql join
    //     * @deprecated replaced by {@code N.leftJoin(Collection, Collection, com.landawn.abacus.util.Try.Function, com.landawn.abacus.util.Try.Function)}
    //     */
    //    @Deprecated
    //    public static  List> leftJoin(final Collection a, final Collection b,
    //            final Throwables.Function leftKeyMapper, final Throwables.Function rightKeyMapper)
    //            throws E, E2 {
    //        final List> result = new ArrayList<>(N.size(a));
    //
    //        if (N.isNullOrEmpty(a)) {
    //            return result;
    //        } else if (N.isNullOrEmpty(b)) {
    //            for (T left : a) {
    //                result.add(Pair.of(left, (U) null));
    //            }
    //        } else {
    //            final ListMultimap rightKeyMap = ListMultimap.from(b, rightKeyMapper);
    //
    //            for (T left : a) {
    //                final List rights = rightKeyMap.get(leftKeyMapper.apply(left));
    //
    //                if (N.notNullOrEmpty(rights)) {
    //                    for (U right : rights) {
    //                        result.add(Pair.of(left, right));
    //                    }
    //                } else {
    //                    result.add(Pair.of(left, (U) null));
    //                }
    //            }
    //        }
    //
    //        return result;
    //    }
    //
    //    /**
    //     * The time complexity is O(n * m) : n is the size of this Seq and m is the size of specified collection b.
    //     *
    //     * @param  the generic type
    //     * @param  the generic type
    //     * @param  the element type
    //     * @param a the a
    //     * @param b the b
    //     * @param predicate the predicate
    //     * @return the list
    //     * @throws E the e
    //     * @see N#leftJoin(Collection, Collection, com.landawn.abacus.util.Throwables.BiPredicate)
    //     * @see sql join
    //     * @deprecated replaced by {@code N.leftJoin(Collection, Collection, com.landawn.abacus.util.Try.BiPredicate)}
    //     */
    //    @Deprecated
    //    public static  List> leftJoin(final Collection a, final Collection b,
    //            final Throwables.BiPredicate predicate) throws E {
    //        final List> result = new ArrayList<>(N.size(a));
    //
    //        if (N.isNullOrEmpty(a)) {
    //            return result;
    //        } else if (N.isNullOrEmpty(b)) {
    //            for (T left : a) {
    //                result.add(Pair.of(left, (U) null));
    //            }
    //        } else {
    //            for (T left : a) {
    //                boolean joined = false;
    //
    //                for (U right : b) {
    //                    if (predicate.test(left, right)) {
    //                        result.add(Pair.of(left, right));
    //                        joined = true;
    //                    }
    //                }
    //
    //                if (joined == false) {
    //                    result.add(Pair.of(left, (U) null));
    //                }
    //            }
    //        }
    //
    //        return result;
    //    }
    //
    //    /**
    //     * The time complexity is O(n + m) : n is the size of this Seq and m is the size of specified collection b.
    //     *
    //     * @param  the generic type
    //     * @param  the generic type
    //     * @param  the element type
    //     * @param  the generic type
    //     * @param a the a
    //     * @param b the b
    //     * @param leftKeyMapper the left key mapper
    //     * @param rightKeyMapper the right key mapper
    //     * @return the list
    //     * @throws E the e
    //     * @throws E2 the e2
    //     * @see N#rightJoin(Collection, Collection, com.landawn.abacus.util.Throwables.Function, com.landawn.abacus.util.Throwables.Function)
    //     * @see sql join
    //     * @deprecated replaced by {@code N.rightJoin(Collection, Collection, com.landawn.abacus.util.Try.Function, com.landawn.abacus.util.Try.Function)}
    //     */
    //    @Deprecated
    //    public static  List> rightJoin(final Collection a, final Collection b,
    //            final Throwables.Function leftKeyMapper, final Throwables.Function rightKeyMapper)
    //            throws E, E2 {
    //        final List> result = new ArrayList<>(N.size(b));
    //
    //        if (N.isNullOrEmpty(b)) {
    //            return result;
    //        } else if (N.isNullOrEmpty(a)) {
    //            for (U right : b) {
    //                result.add(Pair.of((T) null, right));
    //            }
    //        } else {
    //            final ListMultimap leftKeyMap = ListMultimap.from(a, leftKeyMapper);
    //
    //            for (U right : b) {
    //                final List lefts = leftKeyMap.get(rightKeyMapper.apply(right));
    //
    //                if (N.notNullOrEmpty(lefts)) {
    //                    for (T left : lefts) {
    //                        result.add(Pair.of(left, right));
    //                    }
    //                } else {
    //                    result.add(Pair.of((T) null, right));
    //                }
    //            }
    //        }
    //
    //        return result;
    //    }
    //
    //    /**
    //     * The time complexity is O(n * m) : n is the size of this Seq and m is the size of specified collection b.
    //     *
    //     * @param  the generic type
    //     * @param  the generic type
    //     * @param  the element type
    //     * @param a the a
    //     * @param b the b
    //     * @param predicate the predicate
    //     * @return the list
    //     * @throws E the e
    //     * @see N#rightJoin(Collection, Collection, com.landawn.abacus.util.Throwables.BiPredicate)
    //     * @see sql join
    //     * @deprecated replaced by {@code N.rightJoin(Collection, Collection, com.landawn.abacus.util.Try.BiPredicate)}
    //     */
    //    @Deprecated
    //    public static  List> rightJoin(final Collection a, final Collection b,
    //            final Throwables.BiPredicate predicate) throws E {
    //        final List> result = new ArrayList<>(N.size(b));
    //
    //        if (N.isNullOrEmpty(b)) {
    //            return result;
    //        } else if (N.isNullOrEmpty(a)) {
    //            for (U right : b) {
    //                result.add(Pair.of((T) null, right));
    //            }
    //        } else {
    //            for (U right : b) {
    //                boolean joined = false;
    //
    //                for (T left : a) {
    //                    if (predicate.test(left, right)) {
    //                        result.add(Pair.of(left, right));
    //                        joined = true;
    //                    }
    //                }
    //
    //                if (joined == false) {
    //                    result.add(Pair.of((T) null, right));
    //                }
    //            }
    //        }
    //
    //        return result;
    //    }

    @SuppressWarnings("rawtypes")
    public static  boolean padLeft(final List list, final int minLen, final T objToAdd) {
        N.checkArgNotNegative(minLen, "minLen");

        final int size = N.size(list);

        if (size < minLen) {
            final int elementCountToAdd = minLen - size;
            final Object[] a = new Object[elementCountToAdd];

            if (objToAdd != null) {
                N.fill(a, objToAdd);

                list.addAll(0, (List) Arrays.asList(a));
            }

            return true;
        }

        return false;
    }

    @SuppressWarnings("rawtypes")
    public static  boolean padRight(final Collection c, final int minLen, final T objToAdd) {
        N.checkArgNotNegative(minLen, "minLen");

        final int size = N.size(c);

        if (size < minLen) {
            final int elementCountToAdd = minLen - size;
            final Object[] a = new Object[elementCountToAdd];

            if (objToAdd != null) {
                N.fill(a, objToAdd);

                c.addAll((Collection) Arrays.asList(a));
            }

            return true;
        }

        return false;
    }

    public static abstract class SetView extends ImmutableSet {
        SetView(final Set set) {
            super(set);
        }

        public > S copyInto(S set) {
            set.addAll(this);
            return set;
        }
    }

    /**
     * Returns an unmodifiable view of the union of two sets. The returned set contains all
     * elements that are contained in either backing set. Iterating over the returned set iterates
     * first over all the elements of {@code set1}, then over each element of {@code set2}, in order,
     * that is not contained in {@code set1}.
     *
     * 
    Results are undefined if {@code set1} and {@code set2} are sets based on different
     * equivalence relations (as {@link HashSet}, {@link TreeSet}, and the {@link Map#keySet} of an
     * {@code IdentityHashMap} all are).
     */
    public static  SetView union(final Set set1, final Set set2) {
        // N.checkArgNotNull(set1, "set1");
        // N.checkArgNotNull(set2, "set2");

        Set tmp = null;

        if (set1 == null) {
            tmp = set2 == null ? N. emptySet() : set2;
        } else if (set2 == null) {
            tmp = set1;
        } else {
            tmp = new AbstractSet() {
                @Override
                public ObjIterator iterator() {
                    return new ObjIterator() {
                        private final Iterator iter1 = set1.iterator();
                        private final Iterator iter2 = set2.iterator();
                        private final E NONE = (E) N.NULL_MASK;
                        private E next = NONE;
                        private E tmp = null;

                        @Override
                        public boolean hasNext() {
                            if (iter1.hasNext() || next != NONE) {
                                return true;
                            }

                            while (iter2.hasNext()) {
                                next = iter2.next();

                                if (!set1.contains(next)) {
                                    return true;
                                }
                            }

                            next = NONE;

                            return false;
                        }

                        @Override
                        public E next() {
                            if (hasNext() == false) {
                                throw new NoSuchElementException();
                            }

                            if (iter1.hasNext()) {
                                return iter1.next();
                            } else {
                                tmp = next;
                                next = NONE;
                                return tmp;
                            }
                        }
                    };
                }

                @Override
                public boolean contains(Object object) {
                    return set1.contains(object) || set2.contains(object);
                }

                @Override
                public int size() {
                    int size = set1.size();

                    for (E e : set2) {
                        if (!set1.contains(e)) {
                            size++;
                        }
                    }

                    return size;
                }

                @Override
                public boolean isEmpty() {
                    return set1.isEmpty() && set2.isEmpty();
                }
            };
        }

        return new SetView(tmp) {
            @Override
            public > S copyInto(S set) {
                set.addAll(set1);
                set.addAll(set2);
                return set;
            }
        };
    }

    /**
     * Returns an unmodifiable view of the intersection of two sets. The returned set contains
     * all elements that are contained by both backing sets. The iteration order of the returned set
     * matches that of {@code set1}.
     *
     * 
    Results are undefined if {@code set1} and {@code set2} are sets based on different
     * equivalence relations (as {@code HashSet}, {@code TreeSet}, and the keySet of an {@code
     * IdentityHashMap} all are).
     *
     * 
    Note: The returned view performs slightly better when {@code set1} is the smaller of
     * the two sets. If you have reason to believe one of your sets will generally be smaller than the
     * other, pass it first. Unfortunately, since this method sets the generic type of the returned
     * set based on the type of the first set passed, this could in rare cases force you to make a
     * cast, for example:
     *
     * 
    {@code
     * Set aFewBadObjects = ...
     * Set manyBadStrings = ...
     *
     * // impossible for a non-String to be in the intersection
     * SuppressWarnings("unchecked")
     * Set badStrings = (Set) Sets.intersection(
     *     aFewBadObjects, manyBadStrings);
     * }
     *
     * 
    This is unfortunate, but should come up only very rarely.
     */
    public static  SetView intersection(final Set set1, final Set set2) {
        // N.checkArgNotNull(set1, "set1");
        // N.checkArgNotNull(set2, "set2");

        Set tmp = null;

        if (set1 == null || set2 == null) {
            tmp = N. emptySet();
        } else {
            tmp = new AbstractSet() {
                @Override
                public ObjIterator iterator() {
                    return new ObjIterator() {
                        private final Iterator iter1 = set1.iterator();
                        private final E NONE = (E) N.NULL_MASK;
                        private E next = NONE;
                        private E tmp = null;

                        @Override
                        public boolean hasNext() {
                            if (next != NONE) {
                                return true;
                            }

                            while (iter1.hasNext()) {
                                next = iter1.next();

                                if (set2.contains(next)) {
                                    return true;
                                }
                            }

                            next = NONE;

                            return false;
                        }

                        @Override
                        public E next() {
                            if (hasNext() == false) {
                                throw new NoSuchElementException();
                            }

                            tmp = next;
                            next = NONE;
                            return tmp;
                        }
                    };
                }

                @Override
                public boolean contains(Object object) {
                    return set1.contains(object) && set2.contains(object);
                }

                @Override
                public boolean containsAll(Collection collection) {
                    return set1.containsAll(collection) && set2.containsAll(collection);
                }

                @Override
                public int size() {
                    int size = 0;

                    for (E e : set1) {
                        if (set2.contains(e)) {
                            size++;
                        }
                    }

                    return size;
                }

                @Override
                public boolean isEmpty() {
                    return Collections.disjoint(set1, set2);
                }
            };
        }

        return new SetView(tmp) {
        };
    }

    /**
     * Returns an unmodifiable view of the difference of two sets. The returned set contains
     * all elements that are contained by {@code set1} and not contained by {@code set2}. {@code set2}
     * may also contain elements not present in {@code set1}; these are simply ignored. The iteration
     * order of the returned set matches that of {@code set1}.
     *
     * 
    Results are undefined if {@code set1} and {@code set2} are sets based on different
     * equivalence relations (as {@code HashSet}, {@code TreeSet}, and the keySet of an {@code
     * IdentityHashMap} all are).
     */
    public static  SetView difference(final Set set1, final Set set2) {
        // N.checkArgNotNull(set1, "set1");
        // N.checkArgNotNull(set2, "set2");

        Set tmp = null;

        if (set2 == null) {
            tmp = set1 == null ? N. emptySet() : set1;
        } else {
            tmp = new AbstractSet() {
                @Override
                public ObjIterator iterator() {
                    return new ObjIterator() {
                        private final Iterator iter1 = set1.iterator();
                        private final E NONE = (E) N.NULL_MASK;
                        private E next = NONE;
                        private E tmp = null;

                        @Override
                        public boolean hasNext() {
                            if (next != NONE) {
                                return true;
                            }

                            while (iter1.hasNext()) {
                                next = iter1.next();

                                if (!set2.contains(next)) {
                                    return true;
                                }
                            }

                            next = NONE;

                            return false;
                        }

                        @Override
                        public E next() {
                            if (hasNext() == false) {
                                throw new NoSuchElementException();
                            }

                            tmp = next;
                            next = NONE;
                            return tmp;
                        }
                    };
                }

                @Override
                public boolean contains(Object object) {
                    return set1.contains(object) && !set2.contains(object);
                }

                @Override
                public int size() {
                    int size = 0;

                    for (E e : set1) {
                        if (!set2.contains(e)) {
                            size++;
                        }
                    }

                    return size;
                }

                @Override
                public boolean isEmpty() {
                    return set2.containsAll(set1);
                }
            };
        }

        return new SetView(tmp) {
        };
    }

    /**
     * Returns an unmodifiable view of the symmetric difference of two sets. The returned set
     * contains all elements that are contained in either {@code set1} or {@code set2} but not in
     * both. The iteration order of the returned set is undefined.
     *
     * 
    Results are undefined if {@code set1} and {@code set2} are sets based on different
     * equivalence relations (as {@code HashSet}, {@code TreeSet}, and the keySet of an {@code
     * IdentityHashMap} all are).
     *
     * @since 3.0
     */
    public static  SetView symmetricDifference(final Set set1, final Set set2) {
        // N.checkArgNotNull(set1, "set1");
        // N.checkArgNotNull(set2, "set2");

        Set tmp = null;

        if (set1 == null) {
            tmp = set2 == null ? N. emptySet() : set2;
        } else if (set2 == null) {
            tmp = set1;
        } else {
            tmp = new AbstractSet() {
                @Override
                public ObjIterator iterator() {
                    return new ObjIterator() {
                        private final Iterator iter1 = set1.iterator();
                        private final Iterator iter2 = set2.iterator();
                        private final E NONE = (E) N.NULL_MASK;
                        private E next = NONE;
                        private E tmp = null;

                        @Override
                        public boolean hasNext() {
                            if (next != NONE) {
                                return true;
                            }

                            while (iter1.hasNext()) {
                                next = iter1.next();

                                if (!set2.contains(next)) {
                                    return true;
                                }
                            }

                            while (iter2.hasNext()) {
                                next = iter2.next();

                                if (!set1.contains(next)) {
                                    return true;
                                }
                            }

                            next = NONE;

                            return false;
                        }

                        @Override
                        public E next() {
                            if (hasNext() == false) {
                                throw new NoSuchElementException();
                            }

                            tmp = next;
                            next = NONE;
                            return tmp;
                        }
                    };
                }

                @Override
                public boolean contains(Object object) {
                    return set1.contains(object) ^ set2.contains(object);
                }

                @Override
                public int size() {
                    int size = 0;

                    for (E e : set1) {
                        if (!set2.contains(e)) {
                            size++;
                        }
                    }

                    for (E e : set2) {
                        if (!set1.contains(e)) {
                            size++;
                        }
                    }

                    return size;
                }

                @Override
                public boolean isEmpty() {
                    return set1.equals(set2);
                }
            };
        }

        return new SetView(tmp) {
        };
    }

    /**
     * The Class Slice.
     *
     * @param 
     */
    static final class Slice extends ImmutableCollection {

        /** The from index. */
        private final int fromIndex;

        /** The to index. */
        private final int toIndex;

        /**
         * Instantiates a new sub collection.
         *
         * @param a
         * @param fromIndex
         * @param toIndex
         */
        Slice(final T[] a, final int fromIndex, final int toIndex) {
            this(Array.asList(a), fromIndex, toIndex);
        }

        /**
         * Instantiates a new sub collection.
         *
         * @param c
         * @param fromIndex
         * @param toIndex
         */
        Slice(final Collection c, final int fromIndex, final int toIndex) {
            super(c);
            this.fromIndex = fromIndex;
            this.toIndex = toIndex;
        }

        /**
         *
         * @param o
         * @return
         */
        @Override
        public boolean contains(Object o) {
            final Iterator iter = this.iterator();

            while (iter.hasNext()) {
                if (N.equals(iter.next(), o)) {
                    return true;
                }
            }

            return false;
        }

        /**
         *
         * @param c
         * @return
         */
        @Override
        public boolean containsAll(final Collection c) {
            for (Object e : c) {
                if (contains(e) == false) {
                    return false;
                }
            }

            return true;
        }

        /**
         * Checks if is empty.
         *
         * @return true, if is empty
         */
        @Override
        public boolean isEmpty() {
            return size() == 0;
        }

        /**
         *
         * @return the int
         */
        @Override
        public int size() {
            return toIndex - fromIndex;
        }

        /**
         *
         * @return the iterator
         */
        @Override
        public ObjIterator iterator() {
            if (coll == null || fromIndex == toIndex) {
                return ObjIterator. empty();
            }

            final Iterator iter = coll.iterator();

            if (fromIndex == 0 && toIndex == coll.size()) {
                return ObjIterator.of(iter);
            } else if (fromIndex == 0) {
                return Iterators.limit(iter, toIndex - fromIndex);
            } else if (toIndex == coll.size()) {
                return Iterators.skip(iter, fromIndex);
            } else {
                return Iterators.skipAndLimit(iter, fromIndex, toIndex - fromIndex);
            }
        }

        /**
         *
         * @return the object[]
         */
        @Override
        public Object[] toArray() {
            final Iterator iter = this.iterator();
            final Object[] a = new Object[size()];

            for (int i = 0, len = a.length; i < len; i++) {
                a[i] = iter.next();
            }

            return a;
        }

        /**
         *
         * @param 
         * @param a
         * @return the a[]
         */
        @Override
        public  A[] toArray(A[] a) {
            if (a.length < size()) {
                a = N.copyOf(a, size());
            }

            final Iterator iter = this.iterator();

            for (int i = 0, len = a.length; i < len; i++) {
                a[i] = (A) iter.next();
            }

            return a;
        }
    }
}
    
    
    
    

    




    
    
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