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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) 2019, 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:
//
//     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.
//
//     Neither the name of the Burton Computer Corporation nor the names
//     of its contributors may be used to endorse or promote products
//     derived from this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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package org.javimmutable.collections.tree;

import org.javimmutable.collections.JImmutableMap;

import javax.annotation.Nonnull;
import javax.annotation.concurrent.ThreadSafe;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.TreeMap;

@ThreadSafe
class TreeMapBuilder
    implements JImmutableMap.Builder
{
    private final Comparator comparator;
    private final Map values;

    TreeMapBuilder(@Nonnull Comparator comparator)
    {
        this.comparator = comparator;
        values = new TreeMap<>(comparator);
    }

    @Nonnull
    @Override
    public synchronized JImmutableMap build()
    {
        if (values.isEmpty()) {
            return JImmutableTreeMap.of(comparator);
        } else {
            final List> sorted = new ArrayList<>(values.entrySet());
            final AbstractNode root = buildTree(sorted, 0, sorted.size());
            return new JImmutableTreeMap<>(comparator, root);
        }
    }

    @Nonnull
    @Override
    public synchronized JImmutableMap.Builder add(@Nonnull K key,
                                                        V value)
    {
        values.put(key, value);
        return this;
    }

    public synchronized int size()
    {
        return values.size();
    }

    private AbstractNode buildTree(@Nonnull List> values,
                                         int offset,
                                         int limit)
    {
        assert limit > offset;
        int count = limit - offset;
        if (count == 1) {
            final Entry e = values.get(offset);
            return ValueNode.instance(e.getKey(), e.getValue());
        } else if (count == 2) {
            final Entry a = values.get(offset);
            final Entry b = values.get(offset + 1);
            final AbstractNode right = ValueNode.instance(b.getKey(), b.getValue());
            return new ValueNode<>(a.getKey(), a.getValue(), FringeNode.instance(), right);
        } else {
            final int middle = offset + count / 2;
            final Entry e = values.get(middle);
            final AbstractNode left = buildTree(values, offset, middle);
            final AbstractNode right = buildTree(values, middle + 1, limit);
            return new ValueNode<>(e.getKey(), e.getValue(), left, right);
        }
    }
}