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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
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// Copyright (c) 2019, Burton Computer Corporation
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package org.javimmutable.collections;

import javax.annotation.Nonnull;
import javax.annotation.concurrent.Immutable;
import java.util.Collections;
import java.util.EnumSet;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;
import java.util.stream.Collector;

public final class GenericCollector
{
    private static final Set ORDERED = Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.CONCURRENT));
    private static final Set UNORDERED = Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.UNORDERED, Collector.Characteristics.CONCURRENT));

    private GenericCollector()
    {
    }

    @Nonnull
    public static  Collector ordered(@Nonnull C initialValue,
                                                    @Nonnull C empty,
                                                    @Nonnull Predicate isEmpty,
                                                    @Nonnull Func2 adder,
                                                    @Nonnull Func2 combiner)
    {
        return new CollectorImpl<>(ORDERED, initialValue, empty, isEmpty, adder, combiner);
    }

    @Nonnull
    public static  Collector unordered(@Nonnull C initialValue,
                                                      @Nonnull C empty,
                                                      @Nonnull Predicate isEmpty,
                                                      @Nonnull Func2 adder,
                                                      @Nonnull Func2 combiner)
    {
        return new CollectorImpl<>(UNORDERED, initialValue, empty, isEmpty, adder, combiner);
    }


    @Immutable
    private static class CollectorImpl
        implements Collector, C>
    {
        private final Set characteristics;
        private final C initialValues;
        private final C empty;
        private final Func2 adder;
        private final Func2 combiner;

        private CollectorImpl(@Nonnull Set characteristics,
                              @Nonnull C initialValues,
                              @Nonnull C empty,
                              @Nonnull Predicate isEmpty,
                              @Nonnull Func2 adder,
                              @Nonnull Func2 combiner)
        {
            this.characteristics = characteristics;
            this.initialValues = isEmpty.test(initialValues) ? null : initialValues;
            this.empty = empty;
            this.adder = adder;
            this.combiner = combiner;
        }

        @Override
        public Supplier> supplier()
        {
            return () -> new Accumulator<>(adder, combiner, empty);
        }

        @Override
        public BiConsumer, T> accumulator()
        {
            return (a, v) -> a.add(v);
        }

        @Override
        public BinaryOperator> combiner()
        {
            return (left, right) -> left.combine(right);
        }

        @Override
        public Function, C> finisher()
        {
            if (initialValues == null) {
                return a -> a.list;
            } else {
                return a -> combiner.apply(initialValues, a.list);
            }
        }

        @Override
        public Set characteristics()
        {
            return characteristics;
        }
    }

    private static class Accumulator
    {
        private Func2 adder;
        private Func2 combiner;
        private C list;

        private Accumulator(@Nonnull Func2 adder,
                            @Nonnull Func2 combiner,
                            @Nonnull C list)
        {
            this.adder = adder;
            this.combiner = combiner;
            this.list = list;
        }

        private synchronized void add(T value)
        {
            list = adder.apply(list, value);
        }

        private synchronized Accumulator combine(Accumulator other)
        {
            list = combiner.apply(list, other.list);
            return this;
        }
    }
}