/*
 * Copyright (C) 2018-2021 Lightbend Inc. <https://www.lightbend.com>
 */

// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// http://code.google.com/p/protobuf/
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
//     * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

package akka.protobuf;

import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.TreeMap;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.SortedMap;

/**
 * A custom map implementation from FieldDescriptor to Object optimized to
 * minimize the number of memory allocations for instances with a small number
 * of mappings. The implementation stores the first {@code k} mappings in an
 * array for a configurable value of {@code k}, allowing direct access to the
 * corresponding {@code Entry}s without the need to create an Iterator. The
 * remaining entries are stored in an overflow map. Iteration over the entries
 * in the map should be done as follows:
 *
 * <pre>   {@code
 * for (int i = 0; i < fieldMap.getNumArrayEntries(); i++) {
 *   process(fieldMap.getArrayEntryAt(i));
 * }
 * for (Map.Entry<K, V> entry : fieldMap.getOverflowEntries()) {
 *   process(entry);
 * }
 * }</pre>
 *
 * The resulting iteration is in order of ascending field tag number. The
 * object returned by {@link #entrySet()} adheres to the same contract but is
 * less efficient as it necessarily involves creating an object for iteration.
 * <p>
 * The tradeoff for this memory efficiency is that the worst case running time
 * of the {@code put()} operation is {@code O(k + lg n)}, which happens when
 * entries are added in descending order. {@code k} should be chosen such that
 * it covers enough common cases without adversely affecting larger maps. In
 * practice, the worst case scenario does not happen for extensions because
 * extension fields are serialized and deserialized in order of ascending tag
 * number, but the worst case scenario can happen for DynamicMessages.
 * <p>
 * The running time for all other operations is similar to that of
 * {@code TreeMap}.
 * <p>
 * Instances are not thread-safe until {@link #makeImmutable()} is called,
 * after which any modifying operation will result in an
 * {@link UnsupportedOperationException}.
 *
 * @author darick@google.com Darick Tong
 */
// This class is final for all intents and purposes because the constructor is
// private. However, the FieldDescriptor-specific logic is encapsulated in
// a subclass to aid testability of the core logic.
class SmallSortedMap<K extends Comparable<K>, V> extends AbstractMap<K, V> {

  /**
   * Creates a new instance for mapping FieldDescriptors to their values.
   * The {@link #makeImmutable()} implementation will convert the List values
   * of any repeated fields to unmodifiable lists.
   *
   * @param arraySize The size of the entry array containing the
   *        lexicographically smallest mappings.
   */
  static <FieldDescriptorType extends
      FieldSet.FieldDescriptorLite<FieldDescriptorType>>
      SmallSortedMap<FieldDescriptorType, Object> newFieldMap(int arraySize) {
    return new SmallSortedMap<FieldDescriptorType, Object>(arraySize) {
      @Override
      @SuppressWarnings("unchecked")
      public void makeImmutable() {
        if (!isImmutable()) {
          for (int i = 0; i < getNumArrayEntries(); i++) {
            final Map.Entry<FieldDescriptorType, Object> entry =
                getArrayEntryAt(i);
            if (entry.getKey().isRepeated()) {
              final List value = (List) entry.getValue();
              entry.setValue(Collections.unmodifiableList(value));
            }
          }
          for (Map.Entry<FieldDescriptorType, Object> entry :
                   getOverflowEntries()) {
            if (entry.getKey().isRepeated()) {
              final List value = (List) entry.getValue();
              entry.setValue(Collections.unmodifiableList(value));
            }
          }
        }
        super.makeImmutable();
      }
    };
  }

  /**
   * Creates a new instance for testing.
   *
   * @param arraySize The size of the entry array containing the
   *        lexicographically smallest mappings.
   */
  static <K extends Comparable<K>, V> SmallSortedMap<K, V> newInstanceForTest(
      int arraySize) {
    return new SmallSortedMap<K, V>(arraySize);
  }

  private final int maxArraySize;
  // The "entry array" is actually a List because generic arrays are not
  // allowed. ArrayList also nicely handles the entry shifting on inserts and
  // removes.
  private List<Entry> entryList;
  private Map<K, V> overflowEntries;
  private boolean isImmutable;
  // The EntrySet is a stateless view of the Map. It's initialized the first
  // time it is requested and reused henceforth.
  private volatile EntrySet lazyEntrySet;

  /**
   * @code arraySize Size of the array in which the lexicographically smallest
   *       mappings are stored. (i.e. the {@code k} referred to in the class
   *       documentation).
   */
  private SmallSortedMap(int arraySize) {
    this.maxArraySize = arraySize;
    this.entryList = Collections.emptyList();
    this.overflowEntries = Collections.emptyMap();
  }

  /** Make this map immutable from this point forward. */
  public void makeImmutable() {
    if (!isImmutable) {
      // Note: There's no need to wrap the entryList in an unmodifiableList
      // because none of the list's accessors are exposed. The iterator() of
      // overflowEntries, on the other hand, is exposed so it must be made
      // unmodifiable.
      overflowEntries = overflowEntries.isEmpty() ?
          Collections.<K, V>emptyMap() :
          Collections.unmodifiableMap(overflowEntries);
      isImmutable = true;
    }
  }

  /** @return Whether {@link #makeImmutable()} has been called. */
  public boolean isImmutable() {
    return isImmutable;
  }

  /** @return The number of entries in the entry array. */
  public int getNumArrayEntries() {
    return entryList.size();
  }

  /** @return The array entry at the given {@code index}. */
  public Map.Entry<K, V> getArrayEntryAt(int index) {
    return entryList.get(index);
  }

  /** @return There number of overflow entries. */
  public int getNumOverflowEntries() {
    return overflowEntries.size();
  }

  /** @return An iterable over the overflow entries. */
  public Iterable<Map.Entry<K, V>> getOverflowEntries() {
    return overflowEntries.isEmpty() ?
        EmptySet.<Map.Entry<K, V>>iterable() :
        overflowEntries.entrySet();
  }

  @Override
  public int size() {
    return entryList.size() + overflowEntries.size();
  }

  /**
   * The implementation throws a {@code ClassCastException} if o is not an
   * object of type {@code K}.
   *
   * {@inheritDoc}
   */
  @Override
  public boolean containsKey(Object o) {
    @SuppressWarnings("unchecked")
    final K key = (K) o;
    return binarySearchInArray(key) >= 0 || overflowEntries.containsKey(key);
  }

  /**
   * The implementation throws a {@code ClassCastException} if o is not an
   * object of type {@code K}.
   *
   * {@inheritDoc}
   */
  @Override
  public V get(Object o) {
    @SuppressWarnings("unchecked")
    final K key = (K) o;
    final int index = binarySearchInArray(key);
    if (index >= 0) {
      return entryList.get(index).getValue();
    }
    return overflowEntries.get(key);
  }

  @Override
  public V put(K key, V value) {
    checkMutable();
    final int index = binarySearchInArray(key);
    if (index >= 0) {
      // Replace existing array entry.
      return entryList.get(index).setValue(value);
    }
    ensureEntryArrayMutable();
    final int insertionPoint = -(index + 1);
    if (insertionPoint >= maxArraySize) {
      // Put directly in overflow.
      return getOverflowEntriesMutable().put(key, value);
    }
    // Insert new Entry in array.
    if (entryList.size() == maxArraySize) {
      // Shift the last array entry into overflow.
      final Entry lastEntryInArray = entryList.remove(maxArraySize - 1);
      getOverflowEntriesMutable().put(lastEntryInArray.getKey(),
                                      lastEntryInArray.getValue());
    }
    entryList.add(insertionPoint, new Entry(key, value));
    return null;
  }

  @Override
  public void clear() {
    checkMutable();
    if (!entryList.isEmpty()) {
      entryList.clear();
    }
    if (!overflowEntries.isEmpty()) {
      overflowEntries.clear();
    }
  }

  /**
   * The implementation throws a {@code ClassCastException} if o is not an
   * object of type {@code K}.
   *
   * {@inheritDoc}
   */
  @Override
  public V remove(Object o) {
    checkMutable();
    @SuppressWarnings("unchecked")
    final K key = (K) o;
    final int index = binarySearchInArray(key);
    if (index >= 0) {
      return removeArrayEntryAt(index);
    }
    // overflowEntries might be Collections.unmodifiableMap(), so only
    // call remove() if it is non-empty.
    if (overflowEntries.isEmpty()) {
      return null;
    } else {
      return overflowEntries.remove(key);
    }
  }

  private V removeArrayEntryAt(int index) {
    checkMutable();
    final V removed = entryList.remove(index).getValue();
    if (!overflowEntries.isEmpty()) {
      // Shift the first entry in the overflow to be the last entry in the
      // array.
      final Iterator<Map.Entry<K, V>> iterator =
          getOverflowEntriesMutable().entrySet().iterator();
      entryList.add(new Entry(iterator.next()));
      iterator.remove();
    }
    return removed;
  }

  /**
   * @param key The key to find in the entry array.
   * @return The returned integer position follows the same semantics as the
   *     value returned by {@link java.util.Arrays#binarySearch()}.
   */
  private int binarySearchInArray(K key) {
    int left = 0;
    int right = entryList.size() - 1;

    // Optimization: For the common case in which entries are added in
    // ascending tag order, check the largest element in the array before
    // doing a full binary search.
    if (right >= 0) {
      int cmp = key.compareTo(entryList.get(right).getKey());
      if (cmp > 0) {
        return -(right + 2);  // Insert point is after "right".
      } else if (cmp == 0) {
        return right;
      }
    }

    while (left <= right) {
      int mid = (left + right) / 2;
      int cmp = key.compareTo(entryList.get(mid).getKey());
      if (cmp < 0) {
        right = mid - 1;
      } else if (cmp > 0) {
        left = mid + 1;
      } else {
        return mid;
      }
    }
    return -(left + 1);
  }

  /**
   * Similar to the AbstractMap implementation of {@code keySet()} and
   * {@code values()}, the entry set is created the first time this method is
   * called, and returned in response to all subsequent calls.
   *
   * {@inheritDoc}
   */
  @Override
  public Set<Map.Entry<K, V>> entrySet() {
    if (lazyEntrySet == null) {
      lazyEntrySet = new EntrySet();
    }
    return lazyEntrySet;
  }

  /**
   * @throws UnsupportedOperationException if {@link #makeImmutable()} has
   *         has been called.
   */
  private void checkMutable() {
    if (isImmutable) {
      throw new UnsupportedOperationException();
    }
  }

  /**
   * @return a {@link SortedMap} to which overflow entries mappings can be
   *         added or removed.
   * @throws UnsupportedOperationException if {@link #makeImmutable()} has been
   *         called.
   */
  @SuppressWarnings("unchecked")
  private SortedMap<K, V> getOverflowEntriesMutable() {
    checkMutable();
    if (overflowEntries.isEmpty() && !(overflowEntries instanceof TreeMap)) {
      overflowEntries = new TreeMap<K, V>();
    }
    return (SortedMap<K, V>) overflowEntries;
  }

  /**
   * Lazily creates the entry list. Any code that adds to the list must first
   * call this method.
   */
  private void ensureEntryArrayMutable() {
    checkMutable();
    if (entryList.isEmpty() && !(entryList instanceof ArrayList)) {
      entryList = new ArrayList<Entry>(maxArraySize);
    }
  }

  /**
   * Entry implementation that implements Comparable in order to support
   * binary search within the entry array. Also checks mutability in
   * {@link #setValue()}.
   */
  private class Entry implements Map.Entry<K, V>, Comparable<Entry> {

    private final K key;
    private V value;

    Entry(Map.Entry<K, V> copy) {
      this(copy.getKey(), copy.getValue());
    }

    Entry(K key, V value) {
      this.key = key;
      this.value = value;
    }

    //@Override (Java 1.6 override semantics, but we must support 1.5)
    public K getKey() {
      return key;
    }

    //@Override (Java 1.6 override semantics, but we must support 1.5)
    public V getValue() {
      return value;
    }

    //@Override (Java 1.6 override semantics, but we must support 1.5)
    public int compareTo(Entry other) {
      return getKey().compareTo(other.getKey());
    }

    //@Override (Java 1.6 override semantics, but we must support 1.5)
    public V setValue(V newValue) {
      checkMutable();
      final V oldValue = this.value;
      this.value = newValue;
      return oldValue;
    }

    @Override
    public boolean equals(Object o) {
      if (o == this) {
        return true;
      }
      if (!(o instanceof Map.Entry)) {
        return false;
      }
      @SuppressWarnings("unchecked")
      Map.Entry<?, ?> other = (Map.Entry<?, ?>) o;
      return equals(key, other.getKey()) && equals(value, other.getValue());
    }

    @Override
    public int hashCode() {
      return (key == null ? 0 : key.hashCode()) ^
          (value == null ? 0 : value.hashCode());
    }

    @Override
    public String toString() {
      return key + "=" + value;
    }

    /** equals() that handles null values. */
    private boolean equals(Object o1, Object o2) {
      return o1 == null ? o2 == null : o1.equals(o2);
    }
  }

  /**
   * Stateless view of the entries in the field map.
   */
  private class EntrySet extends AbstractSet<Map.Entry<K, V>> {

    @Override
    public Iterator<Map.Entry<K, V>> iterator() {
      return new EntryIterator();
    }

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

    /**
     * Throws a {@link ClassCastException} if o is not of the expected type.
     *
     * {@inheritDoc}
     */
    @Override
    public boolean contains(Object o) {
      @SuppressWarnings("unchecked")
      final Map.Entry<K, V> entry = (Map.Entry<K, V>) o;
      final V existing = get(entry.getKey());
      final V value = entry.getValue();
      return existing == value ||
          (existing != null && existing.equals(value));
    }

    @Override
    public boolean add(Map.Entry<K, V> entry) {
      if (!contains(entry)) {
        put(entry.getKey(), entry.getValue());
        return true;
      }
      return false;
    }

    /**
     * Throws a {@link ClassCastException} if o is not of the expected type.
     *
     * {@inheritDoc}
     */
    @Override
    public boolean remove(Object o) {
      @SuppressWarnings("unchecked")
      final Map.Entry<K, V> entry = (Map.Entry<K, V>) o;
      if (contains(entry)) {
        SmallSortedMap.this.remove(entry.getKey());
        return true;
      }
      return false;
    }

    @Override
    public void clear() {
      SmallSortedMap.this.clear();
    }
  }

  /**
   * Iterator implementation that switches from the entry array to the overflow
   * entries appropriately.
   */
  private class EntryIterator implements Iterator<Map.Entry<K, V>> {

    private int pos = -1;
    private boolean nextCalledBeforeRemove;
    private Iterator<Map.Entry<K, V>> lazyOverflowIterator;

    //@Override (Java 1.6 override semantics, but we must support 1.5)
    public boolean hasNext() {
      return (pos + 1) < entryList.size() ||
          getOverflowIterator().hasNext();
    }

    //@Override (Java 1.6 override semantics, but we must support 1.5)
    public Map.Entry<K, V> next() {
      nextCalledBeforeRemove = true;
      // Always increment pos so that we know whether the last returned value
      // was from the array or from overflow.
      if (++pos < entryList.size()) {
        return entryList.get(pos);
      }
      return getOverflowIterator().next();
    }

    //@Override (Java 1.6 override semantics, but we must support 1.5)
    public void remove() {
      if (!nextCalledBeforeRemove) {
        throw new IllegalStateException("remove() was called before next()");
      }
      nextCalledBeforeRemove = false;
      checkMutable();

      if (pos < entryList.size()) {
        removeArrayEntryAt(pos--);
      } else {
        getOverflowIterator().remove();
      }
    }

    /**
     * It is important to create the overflow iterator only after the array
     * entries have been iterated over because the overflow entry set changes
     * when the client calls remove() on the array entries, which invalidates
     * any existing iterators.
     */
    private Iterator<Map.Entry<K, V>> getOverflowIterator() {
      if (lazyOverflowIterator == null) {
        lazyOverflowIterator = overflowEntries.entrySet().iterator();
      }
      return lazyOverflowIterator;
    }
  }

  /**
   * Helper class that holds immutable instances of an Iterable/Iterator that
   * we return when the overflow entries is empty. This eliminates the creation
   * of an Iterator object when there is nothing to iterate over.
   */
  private static class EmptySet {

    private static final Iterator<Object> ITERATOR = new Iterator<Object>() {
      //@Override (Java 1.6 override semantics, but we must support 1.5)
      public boolean hasNext() {
        return false;
      }
      //@Override (Java 1.6 override semantics, but we must support 1.5)
      public Object next() {
        throw new NoSuchElementException();
      }
      //@Override (Java 1.6 override semantics, but we must support 1.5)
      public void remove() {
        throw new UnsupportedOperationException();
      }
    };

    private static final Iterable<Object> ITERABLE = new Iterable<Object>() {
      //@Override (Java 1.6 override semantics, but we must support 1.5)
      public Iterator<Object> iterator() {
        return ITERATOR;
      }
    };

    @SuppressWarnings("unchecked")
    static <T> Iterable<T> iterable() {
      return (Iterable<T>) ITERABLE;
    }
  }
}
