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Library providing immutable/persistent collection classes for Java. While collections are immutable they provide methods for adding and removing values by creating new modified copies of themselves. Each copy shares almost all of its structure with other copies to minimize memory consumption.
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//
// Burton Computer Corporation
// http://www.burton-computer.com
//
// Copyright (c) 2021, Burton Computer Corporation
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
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//     Redistributions of source code must retain the above copyright
//     notice, this list of conditions and the following disclaimer.
//
//     Redistributions in binary form must reproduce the above copyright
//     notice, this list of conditions and the following disclaimer in
//     the documentation and/or other materials provided with the
//     distribution.
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//     of its contributors may be used to endorse or promote products
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package org.javimmutable.collections.list;

import org.javimmutable.collections.JImmutableStack;
import org.javimmutable.collections.SplitableIterator;
import org.javimmutable.collections.common.StreamConstants;
import org.javimmutable.collections.iterators.IteratorHelper;
import org.javimmutable.collections.iterators.SequenceIterator;
import org.javimmutable.collections.serialization.JImmutableStackProxy;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import javax.annotation.concurrent.Immutable;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

/**
 * Singly linked list implementation of JImmutableStack that stores and retrieves values
 * in the reverse order of the corresponding add() method calls.  If forward or random
 * access to stored values is required use JImmutableList instead, but this class may be
 * significantly faster when its limitations are acceptable.
 */
@Immutable
public class JImmutableLinkedStack
    implements JImmutableStack,
               Serializable
{
    @SuppressWarnings("unchecked")
    private static final JImmutableLinkedStack EMPTY = new JImmutableLinkedStack(null, null);
    private static final long serialVersionUID = -121805;

    private final V value;
    private final JImmutableLinkedStack next;

    private JImmutableLinkedStack(V value,
                                  JImmutableLinkedStack next)
    {
        this.value = value;
        this.next = next;
    }

    @SuppressWarnings("unchecked")
    public static  JImmutableStack of()
    {
        return EMPTY;
    }

    @SuppressWarnings("unchecked")
    public static  JImmutableStack of(T value)
    {
        return new JImmutableLinkedStack<>(value, EMPTY);
    }

    public static  JImmutableStack of(List values)
    {
        JImmutableStack list = of();
        for (T value : values) {
            list = list.insert(value);
        }
        return list;
    }

    @SafeVarargs
    public static  JImmutableStack of(T... values)
    {
        JImmutableStack list = of();
        for (T value : values) {
            list = list.insert(value);
        }
        return list;
    }

    public boolean isEmpty()
    {
        return next == null;
    }

    @Override
    public boolean isNonEmpty()
    {
        return next != null;
    }

    public V getHead()
    {
        if (next == null) {
            throw new UnsupportedOperationException();
        }
        return value;
    }

    @Nonnull
    @Override
    public JImmutableStack getTail()
    {
        if (next == null) {
            return this;
        } else {
            return next;
        }
    }

    @Nonnull
    @Override
    public JImmutableStack insert(@Nullable V value)
    {
        return new JImmutableLinkedStack<>(value, this);
    }

    @Nonnull
    @Override
    public JImmutableStack remove()
    {
        return getTail();
    }

    @Override
    public int getSpliteratorCharacteristics()
    {
        return StreamConstants.SPLITERATOR_ORDERED;
    }

    public List makeList()
    {
        final List answer = new ArrayList<>();
        JImmutableStack next = this;
        while (!next.isEmpty()) {
            answer.add(next.getHead());
            next = next.getTail();
        }
        return answer;
    }

    @Override
    public boolean equals(Object o)
    {
        return (o instanceof JImmutableStack) && IteratorHelper.iteratorEquals(iterator(), ((JImmutableStack)o).iterator());
    }

    @Override
    public int hashCode()
    {
        return IteratorHelper.iteratorHashCode(iterator());
    }

    @Override
    public String toString()
    {
        return IteratorHelper.iteratorToString(iterator());
    }

    @Nonnull
    @Override
    public JImmutableStack getInsertableSelf()
    {
        return this;
    }

    @Nonnull
    @Override
    public SplitableIterator iterator()
    {
        return SequenceIterator.iterator(this);
    }

    @Override
    public void checkInvariants()
    {
        if ((next == null) && !(this == EMPTY)) {
            throw new IllegalStateException();
        }
    }

    private Object writeReplace()
    {
        return new JImmutableStackProxy(this);
    }
}