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// 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;
        }
    }
}