/*
 * Copyright 2019-2022 The Polypheny Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * This file incorporates code covered by the following terms:
 *
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to you under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.polypheny.db.util;


import java.io.Serializable;
import java.util.AbstractList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import javax.annotation.Nonnull;


/**
 * Pair of objects.
 *
 * Because a pair implements {@link #equals(Object)}, {@link #hashCode()} and {@link #compareTo(Pair)}, it can be used in any kind of {@link java.util.Collection}.
 *
 * @param <T1> Left-hand type
 * @param <T2> Right-hand type
 */
public class Pair<T1, T2> implements Comparable<Pair<T1, T2>>, Map.Entry<T1, T2>, Serializable {

    public final T1 left;
    public final T2 right;


    /**
     * Creates a Pair.
     *
     * @param left left value
     * @param right right value
     */
    public Pair( T1 left, T2 right ) {
        this.left = left;
        this.right = right;
    }


    /**
     * Creates a Pair of appropriate type.
     *
     * This is a shorthand that allows you to omit implicit types. For example, you can write:
     * <blockquote>return Pair.of(s, n);</blockquote>
     * instead of
     * <blockquote>return new Pair&lt;String, Integer&gt;(s, n);</blockquote>
     *
     * @param left left value
     * @param right right value
     * @return A Pair
     */
    public static <T1, T2> Pair<T1, T2> of( T1 left, T2 right ) {
        return new Pair<>( left, right );
    }


    /**
     * Creates a {@code Pair} from a {@link java.util.Map.Entry}.
     */
    public static <K, V> Pair<K, V> of( Map.Entry<K, V> entry ) {
        return of( entry.getKey(), entry.getValue() );
    }


    public boolean equals( Object obj ) {
        return this == obj
                || (obj instanceof Pair)
                && Objects.equals( this.left, ((Pair) obj).left )
                && Objects.equals( this.right, ((Pair) obj).right );
    }


    /**
     * {@inheritDoc}
     *
     * Computes hash code consistent with {@link java.util.Map.Entry#hashCode()}.
     */
    @Override
    public int hashCode() {
        int keyHash = left == null ? 0 : left.hashCode();
        int valueHash = right == null ? 0 : right.hashCode();
        return keyHash ^ valueHash;
    }


    @Override
    public int compareTo( @Nonnull Pair<T1, T2> that ) {
        //noinspection unchecked
        int c = compare( (Comparable) this.left, (Comparable) that.left );
        if ( c == 0 ) {
            //noinspection unchecked
            c = compare( (Comparable) this.right, (Comparable) that.right );
        }
        return c;
    }


    public String toString() {
        return "<" + left + ", " + right + ">";
    }


    @Override
    public T1 getKey() {
        return left;
    }


    @Override
    public T2 getValue() {
        return right;
    }


    @Override
    public T2 setValue( T2 value ) {
        throw new UnsupportedOperationException();
    }


    /**
     * Compares a pair of comparable values of the same type. Null collates less than everything else, but equal to itself.
     *
     * @param c1 First value
     * @param c2 Second value
     * @return a negative integer, zero, or a positive integer if c1 is less than, equal to, or greater than c2.
     */
    private static <C extends Comparable<C>> int compare( C c1, C c2 ) {
        if ( c1 == null ) {
            if ( c2 == null ) {
                return 0;
            } else {
                return -1;
            }
        } else if ( c2 == null ) {
            return 1;
        } else {
            return c1.compareTo( c2 );
        }
    }


    /**
     * Converts a collection of Pairs into a Map.
     *
     * This is an obvious thing to do because Pair is similar in structure to {@link java.util.Map.Entry}.
     *
     * The map contains a copy of the collection of Pairs; if you change the collection, the map does not change.
     *
     * @param pairs Collection of Pair objects
     * @return map with the same contents as the collection
     */
    public static <K, V> Map<K, V> toMap( Iterable<Pair<K, V>> pairs ) {
        final Map<K, V> map = new HashMap<>();
        for ( Pair<K, V> pair : pairs ) {
            map.put( pair.left, pair.right );
        }
        return map;
    }


    /**
     * Converts two lists into a list of {@link Pair}s, whose length is the lesser of the lengths of the source lists.
     *
     * @param ks Left list
     * @param vs Right list
     * @return List of pairs
     * @see org.apache.calcite.linq4j.Ord#zip(java.util.List)
     */
    public static <K, V> List<Pair<K, V>> zip( List<K> ks, List<V> vs ) {
        return zip( ks, vs, false );
    }


    /**
     * Converts two lists into a list of {@link Pair}s.
     *
     * The length of the combined list is the lesser of the lengths of the source lists. But typically the source lists will be the same length.
     *
     * @param ks Left list
     * @param vs Right list
     * @param strict Whether to fail if lists have different size
     * @return List of pairs
     * @see org.apache.calcite.linq4j.Ord#zip(java.util.List)
     */
    public static <K, V> List<Pair<K, V>> zip( final List<K> ks, final List<V> vs, boolean strict ) {
        final int size;
        if ( strict ) {
            if ( ks.size() != vs.size() ) {
                throw new AssertionError();
            }
            size = ks.size();
        } else {
            size = Math.min( ks.size(), vs.size() );
        }
        return new AbstractList<>() {
            @Override
            public Pair<K, V> get( int index ) {
                return Pair.of( ks.get( index ), vs.get( index ) );
            }


            @Override
            public int size() {
                return size;
            }
        };
    }


    /**
     * Converts two iterables into an iterable of {@link Pair}s.
     *
     * The resulting iterator ends whenever the first of the input iterators ends. But typically the source iterators will be the same length.
     *
     * @param ks Left iterable
     * @param vs Right iterable
     * @return Iterable over pairs
     */
    public static <K, V> Iterable<Pair<K, V>> zip( final Iterable<? extends K> ks, final Iterable<? extends V> vs ) {
        return () -> {
            final Iterator<? extends K> kIterator = ks.iterator();
            final Iterator<? extends V> vIterator = vs.iterator();

            return new ZipIterator<>( kIterator, vIterator );
        };
    }


    /**
     * Converts two arrays into a list of {@link Pair}s.
     *
     * The length of the combined list is the lesser of the lengths of the source arrays. But typically the source arrays will be the same length.
     *
     * @param ks Left array
     * @param vs Right array
     * @return List of pairs
     */
    public static <K, V> List<Pair<K, V>> zip( final K[] ks, final V[] vs ) {
        return new AbstractList<Pair<K, V>>() {
            @Override
            public Pair<K, V> get( int index ) {
                return Pair.of( ks[index], vs[index] );
            }


            @Override
            public int size() {
                return Math.min( ks.length, vs.length );
            }
        };
    }


    /**
     * Returns an iterable over the left slice of an iterable.
     *
     * @param iterable Iterable over pairs
     * @param <L> Left type
     * @param <R> Right type
     * @return Iterable over the left elements
     */
    public static <L, R> Iterable<L> left( final Iterable<? extends Map.Entry<L, R>> iterable ) {
        return () -> new LeftIterator<>( iterable.iterator() );
    }


    /**
     * Returns an iterable over the right slice of an iterable.
     *
     * @param iterable Iterable over pairs
     * @param <L> right type
     * @param <R> Right type
     * @return Iterable over the right elements
     */
    public static <L, R> Iterable<R> right( final Iterable<? extends Map.Entry<L, R>> iterable ) {
        return () -> new RightIterator<>( iterable.iterator() );
    }


    public static <K, V> List<K> left( final List<? extends Map.Entry<K, V>> pairs ) {
        return new AbstractList<K>() {
            @Override
            public K get( int index ) {
                return pairs.get( index ).getKey();
            }


            @Override
            public int size() {
                return pairs.size();
            }
        };
    }


    public static <K, V> List<V> right( final List<? extends Map.Entry<K, V>> pairs ) {
        return new AbstractList<V>() {
            @Override
            public V get( int index ) {
                return pairs.get( index ).getValue();
            }


            @Override
            public int size() {
                return pairs.size();
            }
        };
    }


    /**
     * Returns an iterator that iterates over (i, i + 1) pairs in an iterable.
     *
     * For example, {@code adjacents([3, 5, 7])} returns [(3, 5), (5, 7)].</p>
     *
     * @param iterable Source collection
     * @param <T> Element type
     * @return Iterable over adjacent element pairs
     */
    public static <T> Iterable<Pair<T, T>> adjacents( final Iterable<T> iterable ) {
        return () -> {
            final Iterator<T> iterator = iterable.iterator();
            if ( !iterator.hasNext() ) {
                return Collections.emptyIterator();
            }
            return new AdjacentIterator<>( iterator );
        };
    }


    /**
     * Returns an iterator that iterates over (0, i) pairs in an iterable for i &gt; 0.
     *
     * For example, {@code firstAnd([3, 5, 7])} returns [(3, 5), (3, 7)].
     *
     * @param iterable Source collection
     * @param <T> Element type
     * @return Iterable over pairs of the first element and all other elements
     */
    public static <T> Iterable<Pair<T, T>> firstAnd( final Iterable<T> iterable ) {
        return () -> {
            final Iterator<T> iterator = iterable.iterator();
            if ( !iterator.hasNext() ) {
                return Collections.emptyIterator();
            }
            final T first = iterator.next();
            return new FirstAndIterator<>( iterator, first );
        };
    }


    /**
     * Iterator that returns the left field of each pair.
     *
     * @param <L> Left-hand type
     * @param <R> Right-hand type
     */
    private static class LeftIterator<L, R> implements Iterator<L> {

        private final Iterator<? extends Map.Entry<L, R>> iterator;


        LeftIterator( Iterator<? extends Map.Entry<L, R>> iterator ) {
            this.iterator = Objects.requireNonNull( iterator );
        }


        @Override
        public boolean hasNext() {
            return iterator.hasNext();
        }


        @Override
        public L next() {
            return iterator.next().getKey();
        }


        @Override
        public void remove() {
            iterator.remove();
        }

    }


    /**
     * Iterator that returns the right field of each pair.
     *
     * @param <L> Left-hand type
     * @param <R> Right-hand type
     */
    private static class RightIterator<L, R> implements Iterator<R> {

        private final Iterator<? extends Map.Entry<L, R>> iterator;


        RightIterator( Iterator<? extends Map.Entry<L, R>> iterator ) {
            this.iterator = Objects.requireNonNull( iterator );
        }


        @Override
        public boolean hasNext() {
            return iterator.hasNext();
        }


        @Override
        public R next() {
            return iterator.next().getValue();
        }


        @Override
        public void remove() {
            iterator.remove();
        }

    }


    /**
     * Iterator that returns the first element of a collection paired with every other element.
     *
     * @param <E> Element type
     */
    private static class FirstAndIterator<E> implements Iterator<Pair<E, E>> {

        private final Iterator<E> iterator;
        private final E first;


        FirstAndIterator( Iterator<E> iterator, E first ) {
            this.iterator = Objects.requireNonNull( iterator );
            this.first = first;
        }


        @Override
        public boolean hasNext() {
            return iterator.hasNext();
        }


        @Override
        public Pair<E, E> next() {
            return of( first, iterator.next() );
        }


        @Override
        public void remove() {
            throw new UnsupportedOperationException( "remove" );
        }

    }


    /**
     * Iterator that pairs elements from two iterators.
     *
     * @param <L> Left-hand type
     * @param <R> Right-hand type
     */
    private static class ZipIterator<L, R> implements Iterator<Pair<L, R>> {

        private final Iterator<? extends L> leftIterator;
        private final Iterator<? extends R> rightIterator;


        ZipIterator( Iterator<? extends L> leftIterator, Iterator<? extends R> rightIterator ) {
            this.leftIterator = Objects.requireNonNull( leftIterator );
            this.rightIterator = Objects.requireNonNull( rightIterator );
        }


        @Override
        public boolean hasNext() {
            return leftIterator.hasNext() && rightIterator.hasNext();
        }


        @Override
        public Pair<L, R> next() {
            return Pair.of( leftIterator.next(), rightIterator.next() );
        }


        @Override
        public void remove() {
            leftIterator.remove();
            rightIterator.remove();
        }

    }


    /**
     * Iterator that returns consecutive pairs of elements from an underlying iterator.
     *
     * @param <E> Element type
     */
    private static class AdjacentIterator<E> implements Iterator<Pair<E, E>> {

        private final E first;
        private final Iterator<E> iterator;
        E previous;


        AdjacentIterator( Iterator<E> iterator ) {
            this.iterator = Objects.requireNonNull( iterator );
            this.first = iterator.next();
            previous = first;
        }


        @Override
        public boolean hasNext() {
            return iterator.hasNext();
        }


        @Override
        public Pair<E, E> next() {
            final E current = iterator.next();
            final Pair<E, E> pair = of( previous, current );
            previous = current;
            return pair;
        }


        @Override
        public void remove() {
            throw new UnsupportedOperationException( "remove" );
        }

    }

}

