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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) 2013, Burton Computer Corporation
// All rights reserved.
//
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//     Redistributions of source code must retain the above copyright
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//     Redistributions in binary form must reproduce the above copyright
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package org.javimmutable.collections.array.bit32;

import org.javimmutable.collections.Cursor;
import org.javimmutable.collections.Holder;
import org.javimmutable.collections.Holders;
import org.javimmutable.collections.Indexed;
import org.javimmutable.collections.JImmutableMap;
import org.javimmutable.collections.MapEntry;
import org.javimmutable.collections.cursors.StandardCursor;

public class StandardBit32Array
        extends Bit32Array
{
    private final int bitmask;
    private final Holder[] entries;

    private StandardBit32Array(int bitmask,
                               Holder[] entries)
    {
        this.bitmask = bitmask;
        this.entries = entries;
    }

    @SuppressWarnings("unchecked")
    StandardBit32Array()
    {
        this(0, new Holder[0]);
    }

    /**
     * Constructor for efficiently creating a Bit32Array with consecutive indexes of up to 32 - startIndex elements
     * from an Indexed collection.  (limit - offset) must be in the range 0 to (32 - startIndex) inclusive.
     *
     * @param source
     * @param offset
     * @param limit
     */
    @SuppressWarnings("unchecked")
    StandardBit32Array(Indexed source,
                       int startIndex,
                       int offset,
                       int limit)
    {
        final int size = limit - offset;
        if (size < 0 || size > (32 - startIndex)) {
            throw new IllegalArgumentException("invalid size " + size);
        } else {
            final Holder[] entries = (Holder[])new Holder[size];
            for (int i = 0; i < size; ++i) {
                entries[i] = Holders.of(source.get(offset + i));
            }
            if (size == 32) {
                this.bitmask = -1;
            } else {
                this.bitmask = ((1 << size) - 1) << startIndex;
            }
            this.entries = entries;
        }
    }

    public Holder get(int index)
    {
        checkIndex(index);
        final int bit = 1 << index;
        final int bitmask = this.bitmask;
        if ((bitmask & bit) == 0) {
            return Holders.of();
        } else {
            return entries[realIndex(bitmask, bit)];
        }
    }

    public Bit32Array assign(int index,
                                T value)
    {
        checkIndex(index);
        final int bit = 1 << index;
        final int bitmask = this.bitmask;
        final int arrayIndex = realIndex(bitmask, bit);
        final Holder[] entries = this.entries;
        if ((bitmask & bit) == 0) {
            final Holder[] newEntries = copyToLargerArray(entries, arrayIndex);
            newEntries[arrayIndex] = Holders.of(value);
            return new StandardBit32Array(bitmask | bit, newEntries);
        } else if (entries[arrayIndex].getValueOrNull() == value) {
            return this;
        } else {
            final Holder[] newEntries = entries.clone();
            newEntries[arrayIndex] = Holders.of(value);
            return new StandardBit32Array(bitmask, newEntries);
        }
    }

    public Bit32Array delete(int index)
    {
        checkIndex(index);
        final int bit = 1 << index;
        final int bitmask = this.bitmask;
        final Holder[] entries = this.entries;
        if ((bitmask & bit) == 0) {
            return this;
        } else {
            switch (entries.length) {
            case 1:
                return Bit32Array.of();
            case 2: {
                final int newBitmask = bitmask & ~bit;
                final int remainingIndex = Integer.numberOfTrailingZeros(newBitmask);
                return new SingleBit32Array(remainingIndex, entries[realIndex(bitmask, 1 << remainingIndex)].getValue());
            }
            default:
                return new StandardBit32Array(bitmask & ~bit, copyToSmallerArray(entries, realIndex(bitmask, bit)));
            }
        }
    }

    public int size()
    {
        return entries.length;
    }

    @Override
    public int firstIndex()
    {
        return Integer.numberOfTrailingZeros(bitmask);
    }

    @Override
    public Cursor> cursor()
    {
        return StandardCursor.of(new CursorSource(bitmask));
    }

    private class CursorSource
            implements StandardCursor.Source>
    {
        private final int remainingMask;
        private final int index;

        private CursorSource(int remainingMask)
        {
            this.remainingMask = remainingMask;
            this.index = Integer.numberOfTrailingZeros(remainingMask);
        }

        @Override
        public boolean atEnd()
        {
            return remainingMask == 0;
        }

        @Override
        public JImmutableMap.Entry currentValue()
        {
            return MapEntry.of(index, get(index).getValue());
        }

        @Override
        public StandardCursor.Source> advance()
        {
            if (remainingMask == 0) {
                return this;
            } else {
                final int bit = 1 << index;
                return new CursorSource(remainingMask & ~bit);
            }
        }
    }

    @SuppressWarnings("unchecked")
    private static  Holder[] copyToSmallerArray(Holder[] oldArray,
                                                      int skipIndex)
    {
        final int newLength = oldArray.length - 1;
        Holder[] newArray = (Holder[])new Holder[newLength];
        System.arraycopy(oldArray, 0, newArray, 0, skipIndex);
        System.arraycopy(oldArray, skipIndex + 1, newArray, skipIndex, newLength - skipIndex);
        return newArray;
    }

    @SuppressWarnings("unchecked")
    private static  Holder[] copyToLargerArray(Holder[] oldArray,
                                                     int insertIndex)
    {
        final int oldLength = oldArray.length;
        Holder[] newArray = (Holder[])new Holder[oldLength + 1];
        if (oldLength > 0) {
            System.arraycopy(oldArray, 0, newArray, 0, insertIndex);
            System.arraycopy(oldArray, insertIndex, newArray, insertIndex + 1, oldLength - insertIndex);
        }
        return newArray;
    }

    private static int realIndex(int bitmask,
                                 int bit)
    {
        return Integer.bitCount(bitmask & (bit - 1));
    }
}