/*
    * Copyright 2014 The Netty Project
    *
    * The Netty Project 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:
    *
    * https://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 io.netty5.util.collection;
  
  import static io.netty5.util.internal.MathUtil.safeFindNextPositivePowerOfTwo;
  
  import java.util.AbstractCollection;
  import java.util.AbstractSet;
  import java.util.Arrays;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.Map;
  import java.util.NoSuchElementException;
  import java.util.Set;
  
  /**
   * A hash map implementation of {@link ShortObjectMap} that uses open addressing for keys.
   * To minimize the memory footprint, this class uses open addressing rather than chaining.
   * Collisions are resolved using linear probing. Deletions implement compaction, so cost of
   * remove can approach O(N) for full maps, which makes a small loadFactor recommended.
   *
   * @param <V> The value type stored in the map.
   */
  public class ShortObjectHashMap<V> implements ShortObjectMap<V> {
  
      /** Default initial capacity. Used if not specified in the constructor */
      public static final int DEFAULT_CAPACITY = 8;
  
      /** Default load factor. Used if not specified in the constructor */
      public static final float DEFAULT_LOAD_FACTOR = 0.5f;
  
      /**
       * Placeholder for null values, so we can use the actual null to mean available.
       * (Better than using a placeholder for available: less references for GC processing.)
       */
      private static final Object NULL_VALUE = new Object();
  
      /** The maximum number of elements allowed without allocating more space. */
      private int maxSize;
  
      /** The load factor for the map. Used to calculate {@link #maxSize}. */
      private final float loadFactor;
  
      private short[] keys;
      private V[] values;
      private int size;
      private int mask;
  
      private final Set<Short> keySet = new KeySet();
      private final Set<Entry<Short, V>> entrySet = new EntrySet();
      private final Iterable<PrimitiveEntry<V>> entries = new Iterable<PrimitiveEntry<V>>() {
          @Override
          public Iterator<PrimitiveEntry<V>> iterator() {
              return new PrimitiveIterator();
          }
      };
  
      public ShortObjectHashMap() {
          this(DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR);
      }
  
      public ShortObjectHashMap(int initialCapacity) {
          this(initialCapacity, DEFAULT_LOAD_FACTOR);
      }
  
      public ShortObjectHashMap(int initialCapacity, float loadFactor) {
          if (loadFactor <= 0.0f || loadFactor > 1.0f) {
              // Cannot exceed 1 because we can never store more than capacity elements;
              // using a bigger loadFactor would trigger rehashing before the desired load is reached.
              throw new IllegalArgumentException("loadFactor must be > 0 and <= 1");
          }
  
          this.loadFactor = loadFactor;
  
          // Adjust the initial capacity if necessary.
          int capacity = safeFindNextPositivePowerOfTwo(initialCapacity);
          mask = capacity - 1;
  
          // Allocate the arrays.
          keys = new short[capacity];
          @SuppressWarnings({ "unchecked", "SuspiciousArrayCast" })
          V[] temp = (V[]) new Object[capacity];
          values = temp;
  
          // Initialize the maximum size value.
          maxSize = calcMaxSize(capacity);
     }
 
     private static <T> T toExternal(T value) {
         assert value != null : "null is not a legitimate internal value. Concurrent Modification?";
         return value == NULL_VALUE ? null : value;
     }
 
     @SuppressWarnings("unchecked")
     private static <T> T toInternal(T value) {
         return value == null ? (T) NULL_VALUE : value;
     }
 
     @Override
     public V get(short key) {
         int index = indexOf(key);
         return index == -1 ? null : toExternal(values[index]);
     }
 
     @Override
     public V put(short key, V value) {
         int startIndex = hashIndex(key);
         int index = startIndex;
 
         for (;;) {
             if (values[index] == null) {
                 // Found empty slot, use it.
                 keys[index] = key;
                 values[index] = toInternal(value);
                 growSize();
                 return null;
             }
             if (keys[index] == key) {
                 // Found existing entry with this key, just replace the value.
                 V previousValue = values[index];
                 values[index] = toInternal(value);
                 return toExternal(previousValue);
             }
 
             // Conflict, keep probing ...
             if ((index = probeNext(index)) == startIndex) {
                 // Can only happen if the map was full at MAX_ARRAY_SIZE and couldn't grow.
                 throw new IllegalStateException("Unable to insert");
             }
         }
     }
 
     @Override
     public void putAll(Map<? extends Short, ? extends V> sourceMap) {
         if (sourceMap instanceof ShortObjectHashMap) {
             // Optimization - iterate through the arrays.
             @SuppressWarnings("unchecked")
             ShortObjectHashMap<V> source = (ShortObjectHashMap<V>) sourceMap;
             for (int i = 0; i < source.values.length; ++i) {
                 V sourceValue = source.values[i];
                 if (sourceValue != null) {
                     put(source.keys[i], sourceValue);
                 }
             }
             return;
         }
 
         // Otherwise, just add each entry.
         for (Entry<? extends Short, ? extends V> entry : sourceMap.entrySet()) {
             put(entry.getKey(), entry.getValue());
         }
     }
 
     @Override
     public V remove(short key) {
         int index = indexOf(key);
         if (index == -1) {
             return null;
         }
 
         V prev = values[index];
         removeAt(index);
         return toExternal(prev);
     }
 
     @Override
     public int size() {
         return size;
     }
 
     @Override
     public boolean isEmpty() {
         return size == 0;
     }
 
     @Override
     public void clear() {
         Arrays.fill(keys, (short) 0);
         Arrays.fill(values, null);
         size = 0;
     }
 
     @Override
     public boolean containsKey(short key) {
         return indexOf(key) >= 0;
     }
 
     @Override
     public boolean containsValue(Object value) {
         @SuppressWarnings("unchecked")
         V v1 = toInternal((V) value);
         for (V v2 : values) {
             // The map supports null values; this will be matched as NULL_VALUE.equals(NULL_VALUE).
             if (v2 != null && v2.equals(v1)) {
                 return true;
             }
         }
         return false;
     }
 
     @Override
     public Iterable<PrimitiveEntry<V>> entries() {
         return entries;
     }
 
     @Override
     public Collection<V> values() {
         return new AbstractCollection<V>() {
             @Override
             public Iterator<V> iterator() {
                 return new Iterator<V>() {
                     final PrimitiveIterator iter = new PrimitiveIterator();
 
                     @Override
                     public boolean hasNext() {
                         return iter.hasNext();
                     }
 
                     @Override
                     public V next() {
                         return iter.next().value();
                     }
 
                     @Override
                     public void remove() {
                         iter.remove();
                     }
                 };
             }
 
             @Override
             public int size() {
                 return size;
             }
         };
     }
 
     @Override
     public int hashCode() {
         // Hashcode is based on all non-zero, valid keys. We have to scan the whole keys
         // array, which may have different lengths for two maps of same size(), so the
         // capacity cannot be used as input for hashing but the size can.
         int hash = size;
         for (short key : keys) {
             // 0 can be a valid key or unused slot, but won't impact the hashcode in either case.
             // This way we can use a cheap loop without conditionals, or hard-to-unroll operations,
             // or the devastatingly bad memory locality of visiting value objects.
             // Also, it's important to use a hash function that does not depend on the ordering
             // of terms, only their values; since the map is an unordered collection and
             // entries can end up in different positions in different maps that have the same
             // elements, but with different history of puts/removes, due to conflicts.
             hash ^= hashCode(key);
         }
         return hash;
     }
 
     @Override
     public boolean equals(Object obj) {
         if (this == obj) {
             return true;
         }
         if (!(obj instanceof ShortObjectMap)) {
             return false;
         }
         @SuppressWarnings("rawtypes")
         ShortObjectMap other = (ShortObjectMap) obj;
         if (size != other.size()) {
             return false;
         }
         for (int i = 0; i < values.length; ++i) {
             V value = values[i];
             if (value != null) {
                 short key = keys[i];
                 Object otherValue = other.get(key);
                 if (value == NULL_VALUE) {
                     if (otherValue != null) {
                         return false;
                     }
                 } else if (!value.equals(otherValue)) {
                     return false;
                 }
             }
         }
         return true;
     }
 
     @Override
     public boolean containsKey(Object key) {
         return containsKey(objectToKey(key));
     }
 
     @Override
     public V get(Object key) {
         return get(objectToKey(key));
     }
 
     @Override
     public V put(Short key, V value) {
         return put(objectToKey(key), value);
     }
 
     @Override
     public V remove(Object key) {
         return remove(objectToKey(key));
     }
 
     @Override
     public Set<Short> keySet() {
         return keySet;
     }
 
     @Override
     public Set<Entry<Short, V>> entrySet() {
         return entrySet;
     }
 
     private short objectToKey(Object key) {
         return (short) ((Short) key).shortValue();
     }
 
     /**
      * Locates the index for the given key. This method probes using double hashing.
      *
      * @param key the key for an entry in the map.
      * @return the index where the key was found, or {@code -1} if no entry is found for that key.
      */
     private int indexOf(short key) {
         int startIndex = hashIndex(key);
         int index = startIndex;
 
         for (;;) {
             if (values[index] == null) {
                 // It's available, so no chance that this value exists anywhere in the map.
                 return -1;
             }
             if (key == keys[index]) {
                 return index;
             }
 
             // Conflict, keep probing ...
             if ((index = probeNext(index)) == startIndex) {
                 return -1;
             }
         }
     }
 
     /**
      * Returns the hashed index for the given key.
      */
     private int hashIndex(short key) {
         // The array lengths are always a power of two, so we can use a bitmask to stay inside the array bounds.
         return hashCode(key) & mask;
     }
 
     /**
      * Returns the hash code for the key.
      */
     private static int hashCode(short key) {
        return (int) key;
     }
 
     /**
      * Get the next sequential index after {@code index} and wraps if necessary.
      */
     private int probeNext(int index) {
         // The array lengths are always a power of two, so we can use a bitmask to stay inside the array bounds.
         return (index + 1) & mask;
     }
 
     /**
      * Grows the map size after an insertion. If necessary, performs a rehash of the map.
      */
     private void growSize() {
         size++;
 
         if (size > maxSize) {
             if(keys.length == Integer.MAX_VALUE) {
                 throw new IllegalStateException("Max capacity reached at size=" + size);
             }
 
             // Double the capacity.
             rehash(keys.length << 1);
         }
     }
 
     /**
      * Removes entry at the given index position. Also performs opportunistic, incremental rehashing
      * if necessary to not break conflict chains.
      *
      * @param index the index position of the element to remove.
      * @return {@code true} if the next item was moved back. {@code false} otherwise.
      */
     private boolean removeAt(final int index) {
         --size;
         // Clearing the key is not strictly necessary (for GC like in a regular collection),
         // but recommended for security. The memory location is still fresh in the cache anyway.
         keys[index] = 0;
         values[index] = null;
 
         // In the interval from index to the next available entry, the arrays may have entries
         // that are displaced from their base position due to prior conflicts. Iterate these
         // entries and move them back if possible, optimizing future lookups.
         // Knuth Section 6.4 Algorithm R, also used by the JDK's IdentityHashMap.
 
         int nextFree = index;
         int i = probeNext(index);
         for (V value = values[i]; value != null; value = values[i = probeNext(i)]) {
             short key = keys[i];
             int bucket = hashIndex(key);
             if (i < bucket && (bucket <= nextFree || nextFree <= i) ||
                 bucket <= nextFree && nextFree <= i) {
                 // Move the displaced entry "back" to the first available position.
                 keys[nextFree] = key;
                 values[nextFree] = value;
                 // Put the first entry after the displaced entry
                 keys[i] = 0;
                 values[i] = null;
                 nextFree = i;
             }
         }
         return nextFree != index;
     }
 
     /**
      * Calculates the maximum size allowed before rehashing.
      */
     private int calcMaxSize(int capacity) {
         // Clip the upper bound so that there will always be at least one available slot.
         int upperBound = capacity - 1;
         return Math.min(upperBound, (int) (capacity * loadFactor));
     }
 
     /**
      * Rehashes the map for the given capacity.
      *
      * @param newCapacity the new capacity for the map.
      */
     private void rehash(int newCapacity) {
         short[] oldKeys = keys;
         V[] oldVals = values;
 
         keys = new short[newCapacity];
         @SuppressWarnings({ "unchecked", "SuspiciousArrayCast" })
         V[] temp = (V[]) new Object[newCapacity];
         values = temp;
 
         maxSize = calcMaxSize(newCapacity);
         mask = newCapacity - 1;
 
         // Insert to the new arrays.
         for (int i = 0; i < oldVals.length; ++i) {
             V oldVal = oldVals[i];
             if (oldVal != null) {
                 // Inlined put(), but much simpler: we don't need to worry about
                 // duplicated keys, growing/rehashing, or failing to insert.
                 short oldKey = oldKeys[i];
                 int index = hashIndex(oldKey);
 
                 for (;;) {
                     if (values[index] == null) {
                         keys[index] = oldKey;
                         values[index] = oldVal;
                         break;
                     }
 
                     // Conflict, keep probing. Can wrap around, but never reaches startIndex again.
                     index = probeNext(index);
                 }
             }
         }
     }
 
     @Override
     public String toString() {
         if (isEmpty()) {
             return "{}";
         }
         StringBuilder sb = new StringBuilder(4 * size);
         sb.append('{');
         boolean first = true;
         for (int i = 0; i < values.length; ++i) {
             V value = values[i];
             if (value != null) {
                 if (!first) {
                     sb.append(", ");
                 }
                 sb.append(keyToString(keys[i])).append('=').append(value == this ? "(this Map)" :
                     toExternal(value));
                 first = false;
             }
         }
         return sb.append('}').toString();
     }
 
     /**
      * Helper method called by {@link #toString()} in order to convert a single map key into a string.
      * This is protected to allow subclasses to override the appearance of a given key.
      */
     protected String keyToString(short key) {
         return Short.toString(key);
     }
 
     /**
      * Set implementation for iterating over the entries of the map.
      */
     private final class EntrySet extends AbstractSet<Entry<Short, V>> {
         @Override
         public Iterator<Entry<Short, V>> iterator() {
             return new MapIterator();
         }
 
         @Override
         public int size() {
             return ShortObjectHashMap.this.size();
         }
     }
 
     /**
      * Set implementation for iterating over the keys.
      */
     private final class KeySet extends AbstractSet<Short> {
         @Override
         public int size() {
             return ShortObjectHashMap.this.size();
         }
 
         @Override
         public boolean contains(Object o) {
             return ShortObjectHashMap.this.containsKey(o);
         }
 
         @Override
         public boolean remove(Object o) {
             return ShortObjectHashMap.this.remove(o) != null;
         }
 
         @Override
         public boolean retainAll(Collection<?> retainedKeys) {
             boolean changed = false;
             for(Iterator<PrimitiveEntry<V>> iter = entries().iterator(); iter.hasNext(); ) {
                 PrimitiveEntry<V> entry = iter.next();
                 if (!retainedKeys.contains(entry.key())) {
                     changed = true;
                     iter.remove();
                 }
             }
             return changed;
         }
 
         @Override
         public void clear() {
             ShortObjectHashMap.this.clear();
         }
 
         @Override
         public Iterator<Short> iterator() {
             return new Iterator<Short>() {
                 private final Iterator<Entry<Short, V>> iter = entrySet.iterator();
 
                 @Override
                 public boolean hasNext() {
                     return iter.hasNext();
                 }
 
                 @Override
                 public Short next() {
                     return iter.next().getKey();
                 }
 
                 @Override
                 public void remove() {
                     iter.remove();
                 }
             };
         }
     }
 
     /**
      * Iterator over primitive entries. Entry key/values are overwritten by each call to {@link #next()}.
      */
     private final class PrimitiveIterator implements Iterator<PrimitiveEntry<V>>, PrimitiveEntry<V> {
         private int prevIndex = -1;
         private int nextIndex = -1;
         private int entryIndex = -1;
 
         private void scanNext() {
             while (++nextIndex != values.length && values[nextIndex] == null) {
             }
         }
 
         @Override
         public boolean hasNext() {
             if (nextIndex == -1) {
                 scanNext();
             }
             return nextIndex != values.length;
         }
 
         @Override
         public PrimitiveEntry<V> next() {
             if (!hasNext()) {
                 throw new NoSuchElementException();
             }
 
             prevIndex = nextIndex;
             scanNext();
 
             // Always return the same Entry object, just change its index each time.
             entryIndex = prevIndex;
             return this;
         }
 
         @Override
         public void remove() {
             if (prevIndex == -1) {
                 throw new IllegalStateException("next must be called before each remove.");
             }
             if (removeAt(prevIndex)) {
                 // removeAt may move elements "back" in the array if they have been displaced because their spot in the
                 // array was occupied when they were inserted. If this occurs then the nextIndex is now invalid and
                 // should instead point to the prevIndex which now holds an element which was "moved back".
                 nextIndex = prevIndex;
             }
             prevIndex = -1;
         }
 
         // Entry implementation. Since this implementation uses a single Entry, we coalesce that
         // into the Iterator object (potentially making loop optimization much easier).
 
         @Override
         public short key() {
             return keys[entryIndex];
         }
 
         @Override
         public V value() {
             return toExternal(values[entryIndex]);
         }
 
         @Override
         public void setValue(V value) {
             values[entryIndex] = toInternal(value);
         }
     }
 
     /**
      * Iterator used by the {@link Map} interface.
      */
     private final class MapIterator implements Iterator<Entry<Short, V>> {
         private final PrimitiveIterator iter = new PrimitiveIterator();
 
         @Override
         public boolean hasNext() {
             return iter.hasNext();
         }
 
         @Override
         public Entry<Short, V> next() {
             if (!hasNext()) {
                 throw new NoSuchElementException();
             }
 
             iter.next();
 
             return new MapEntry(iter.entryIndex);
         }
 
         @Override
         public void remove() {
             iter.remove();
         }
     }
 
     /**
      * A single entry in the map.
      */
     final class MapEntry implements Entry<Short, V> {
         private final int entryIndex;
 
         MapEntry(int entryIndex) {
             this.entryIndex = entryIndex;
         }
 
         @Override
         public Short getKey() {
             verifyExists();
             return keys[entryIndex];
         }
 
         @Override
         public V getValue() {
             verifyExists();
             return toExternal(values[entryIndex]);
         }
 
         @Override
         public V setValue(V value) {
             verifyExists();
             V prevValue = toExternal(values[entryIndex]);
             values[entryIndex] = toInternal(value);
             return prevValue;
         }
 
         private void verifyExists() {
             if (values[entryIndex] == null) {
                 throw new IllegalStateException("The map entry has been removed");
             }
         }
     }
 }