content: /*
   * 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.apache.arrow.vector.util;
  
  import java.util.Collection;
  import java.util.HashSet;
  import java.util.LinkedHashMap;
  import java.util.Map;
  import java.util.Optional;
  import java.util.Set;
  import java.util.stream.Collectors;
  
  /**
   * An implementation of a multimap that supports constant time look-up by a generic key or an
   * ordinal.
   *
   * <p>This class extends the functionality a regular {@link Map} with ordinal lookup support. Upon
   * insertion an unused ordinal is assigned to the inserted (key, value) tuple. Upon update the same
   * ordinal id is re-used while value is replaced. Upon deletion of an existing item, its
   * corresponding ordinal is recycled and could be used by another item.
   *
   * <p>For any instance with N items, this implementation guarantees that ordinals are in the range
   * of [0, N). However, the ordinal assignment is dynamic and may change after an insertion or
   * deletion. Consumers of this class are responsible for explicitly checking the ordinal
   * corresponding to a key via {@link MultiMapWithOrdinal#getOrdinal(Object)} before attempting to
   * execute a lookup with an ordinal.
   *
   * @param <K> key type
   * @param <V> value type
   */
  public class MultiMapWithOrdinal<K, V> implements MapWithOrdinal<K, V> {
  
    private final Map<K, Set<Integer>> keyToOrdinal = new LinkedHashMap<>();
    private final IntObjectHashMap<V> ordinalToValue = new IntObjectHashMap<>();
  
    /**
     * Returns the value corresponding to the given ordinal.
     *
     * @param id ordinal value for lookup
     * @return an instance of V
     */
    @Override
    public V getByOrdinal(int id) {
      return ordinalToValue.get(id);
    }
  
    /**
     * Returns the ordinal corresponding to the given key.
     *
     * @param key key for ordinal lookup
     * @return ordinal value corresponding to key if it exists or -1
     */
    @Override
    public int getOrdinal(K key) {
      Set<Integer> pair = getOrdinals(key);
      if (!pair.isEmpty()) {
        return pair.iterator().next();
      }
      return -1;
    }
  
    private Set<Integer> getOrdinals(K key) {
      return keyToOrdinal.getOrDefault(key, new HashSet<>());
    }
  
    @Override
    public int size() {
      return ordinalToValue.size();
    }
  
    @Override
    public boolean isEmpty() {
      return ordinalToValue.isEmpty();
    }
  
    /** get set of values for key. */
    @Override
    public V get(K key) {
      Set<Integer> ordinals = keyToOrdinal.get(key);
      if (ordinals == null) {
        return null;
      }
      return ordinals.stream().map(ordinalToValue::get).collect(Collectors.toList()).get(0);
    }
  
    /** get set of values for key. */
    @Override
    public Collection<V> getAll(K key) {
      Set<Integer> ordinals = keyToOrdinal.get(key);
      if (ordinals == null) {
        return null;
      }
      return ordinals.stream().map(ordinalToValue::get).collect(Collectors.toList());
    }
  
    /**
     * Inserts the tuple (key, value) into the multimap with automatic ordinal assignment.
     *
     * <p>A new ordinal is assigned if key/value pair does not exists.
     *
     * <p>If overwrite is true the existing key will be overwritten with value else value will be
     * appended to the multimap.
     */
    @Override
    public boolean put(K key, V value, boolean overwrite) {
      if (overwrite) {
        removeAll(key);
      }
      Set<Integer> ordinalSet = getOrdinals(key);
      int nextOrdinal = ordinalToValue.size();
      ordinalToValue.put(nextOrdinal, value);
      boolean changed = ordinalSet.add(nextOrdinal);
      keyToOrdinal.put(key, ordinalSet);
      return changed;
    }
  
    @Override
    public Collection<V> values() {
      return ordinalToValue.values();
    }
  
    @Override
    public boolean containsKey(K key) {
      return keyToOrdinal.containsKey(key);
    }
  
    /**
     * Removes the element corresponding to the key/value if exists with ordinal re-cycling.
     *
     * <p>The ordinal corresponding to the given key may be re-assigned to another tuple. It is
     * important that consumer checks the ordinal value via {@link
     * MultiMapWithOrdinal#getOrdinal(Object)} before attempting to look-up by ordinal.
     *
     * <p>If the multimap is changed return true.
     */
    @Override
    public synchronized boolean remove(K key, V value) {
      Set<Integer> removalSet = getOrdinals(key);
      if (removalSet.isEmpty()) {
        return false;
      }
      Optional<V> removeValue =
          removalSet.stream().map(ordinalToValue::get).filter(value::equals).findFirst();
      if (!removeValue.isPresent()) {
        return false;
      }
      int removalOrdinal = removeKv(removalSet, key, value);
      int lastOrdinal = ordinalToValue.size();
      if (lastOrdinal != removalOrdinal) { // we didn't remove the last ordinal
        swapOrdinal(lastOrdinal, removalOrdinal);
      }
      return true;
    }
  
    private void swapOrdinal(int lastOrdinal, int removalOrdinal) {
      V swapOrdinalValue = ordinalToValue.remove(lastOrdinal);
      ordinalToValue.put(removalOrdinal, swapOrdinalValue);
      K swapOrdinalKey =
          keyToOrdinal.entrySet().stream()
              .filter(kv -> kv.getValue().stream().anyMatch(o -> o == lastOrdinal))
              .map(Map.Entry::getKey)
              .findFirst()
              .orElseThrow(() -> new IllegalStateException("MultimapWithOrdinal in bad state"));
      ordinalToValue.put(removalOrdinal, swapOrdinalValue);
      Set<Integer> swapSet = getOrdinals(swapOrdinalKey);
      swapSet.remove(lastOrdinal);
      swapSet.add(removalOrdinal);
      keyToOrdinal.put(swapOrdinalKey, swapSet);
    }
  
    private int removeKv(Set<Integer> removalSet, K key, V value) {
      Integer removalOrdinal =
          removalSet.stream()
              .filter(i -> ordinalToValue.get(i).equals(value))
              .findFirst()
              .orElseThrow(() -> new IllegalStateException("MultimapWithOrdinal in bad state"));
      ordinalToValue.remove(removalOrdinal);
      removalSet.remove(removalOrdinal);
      if (removalSet.isEmpty()) {
        keyToOrdinal.remove(key);
      } else {
        keyToOrdinal.put(key, removalSet);
      }
      return removalOrdinal;
    }
  
    /** remove all entries of key. */
    @Override
    public synchronized boolean removeAll(K key) {
      Collection<V> values = this.getAll(key);
      if (values == null) {
        return false;
      }
      for (V v : values) {
        this.remove(key, v);
      }
      return true;
    }
  
    @Override
    public void clear() {
      ordinalToValue.clear();
      keyToOrdinal.clear();
    }
  
    @Override
    public Set<K> keys() {
      return keyToOrdinal.keySet();
    }
  }
  