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//
// Burton Computer Corporation
// http://www.burton-computer.com
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// Copyright (c) 2019, Burton Computer Corporation
// All rights reserved.
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package org.javimmutable.collections;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Supplier;

/**
 * Provides static utility methods for constructing Holder instances.
 */
public class Holders
{
    private static final Empty EMPTY = new Empty();

    /**
     * Creates an empty Holder for the specified type.
     * Shares a single instance for all empty Holders to save memory.
     *
     * @return the Holder
     */
    @SuppressWarnings("unchecked")
    @Nonnull
    public static  Holder of()
    {
        return EMPTY;
    }

    @SuppressWarnings("unchecked")
    public static  Holder holder()
    {
        return EMPTY;
    }

    public static  Holder holder(@Nullable V value)
    {
        return of(value);
    }

    /**
     * Creates a filled Holder for the specified type and (possibly null) value.
     */
    @Nonnull
    public static  Holder of(@Nullable V value)
    {
        return new Filled()
        {
            @Override
            public V getValue()
            {
                return value;
            }

            @Override
            public int hashCode()
            {
                return Holders.hashCode(this);
            }

            @Override
            @SuppressWarnings("unchecked")
            public boolean equals(Object o)
            {
                return (o instanceof Holder) && Holders.areEqual(this, (Holder)o);
            }

            @Override
            public String toString()
            {
                return String.format("[%s]", value);
            }
        };
    }

    /**
     * Creates an empty Holder if value is null or a filled Holder if value is non-null.
     *
     * @return the Holder
     */
    @Nonnull
    public static  Holder fromNullable(@Nullable V value)
    {
        if (value == null) {
            return of();
        } else {
            return of(value);
        }
    }

    private Holders()
    {
        // prevent unrelated derived classes
    }

    private static class Empty
        implements Holder
    {
        public boolean isEmpty()
        {
            return true;
        }

        public boolean isFilled()
        {
            return false;
        }

        public V getValue()
        {
            throw new UnsupportedOperationException("cannot get empty value");
        }

        @Override
        public V getValueOrNull()
        {
            return null;
        }

        @Override
        public V getValueOr(V defaultValue)
        {
            return defaultValue;
        }

        @Override
        public void ifPresent(@Nonnull Consumer consumer)
        {
        }

        @Override
        public  Holder map(@Nonnull Function transforminator)
        {
            return of();
        }

        @Override
        public V orElse(V defaultValue)
        {
            return defaultValue;
        }

        @Override
        public V orElseGet(@Nonnull Supplier supplier)
        {
            return supplier.get();
        }

        @Override
        public  V orElseThrow(@Nonnull Supplier supplier)
            throws X
        {
            throw supplier.get();
        }

        @Override
        public int hashCode()
        {
            return Holders.hashCode(this);
        }

        @Override
        @SuppressWarnings("unchecked")
        public boolean equals(Object o)
        {
            return (o instanceof Holder) && Holders.areEqual(this, (Holder)o);
        }

        @Override
        public String toString()
        {
            return "[]";
        }
    }

    public interface Filled
        extends Holder
    {
        default boolean isEmpty()
        {
            return false;
        }

        default boolean isFilled()
        {
            return true;
        }

        @Override
        default V getValueOrNull()
        {
            return getValue();
        }

        @Override
        default V getValueOr(V defaultValue)
        {
            return getValue();
        }

        @Override
        default void ifPresent(@Nonnull Consumer consumer)
        {
            consumer.accept(getValue());
        }

        @Override
        default  Holder map(@Nonnull Function transforminator)
        {
            return of(transforminator.apply(getValue()));
        }

        @Override
        default V orElse(V defaultValue)
        {
            return getValue();
        }

        @Override
        default V orElseGet(@Nonnull Supplier supplier)
        {
            return getValue();
        }

        @Override
        default  V orElseThrow(@Nonnull Supplier supplier)
            throws X
        {
            return getValue();
        }
    }

    public static  boolean areEqual(Holder a,
                                       Holder b)
    {
        if ((a == null) || (b == null)) {
            return (a == null) && (b == null);
        } else if (a.isEmpty()) {
            return b.isEmpty();
        } else if (b.isEmpty()) {
            return false;
        } else {
            T v1 = a.getValue();
            T v2 = b.getValue();
            if ((v1 == null) || (v2 == null)) {
                return (v1 == null) && (v2 == null);
            } else {
                return v1.equals(v2);
            }
        }
    }

    public static  int hashCode(Holder a)
    {
        if (a.isEmpty()) {
            return -1;
        }
        if (a.getValue() == null) {
            return 1;
        }
        return a.getValue().hashCode();
    }
}