/*
    * 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.concurrent;
  
  import io.netty5.util.internal.PlatformDependent;
  
  import java.util.Collections;
  import java.util.IdentityHashMap;
  import java.util.Set;
  
  /**
   * A special variant of {@link ThreadLocal} that yields higher access performance when accessed from a
   * {@link FastThreadLocalThread}.
   * <p>
   * Internally, a {@link FastThreadLocal} uses a constant index in an array, instead of using hash code and hash table,
   * to look for a variable.  Although seemingly very subtle, it yields slight performance advantage over using a hash
   * table, and it is useful when accessed frequently.
   * </p><p>
   * To take advantage of this thread-local variable, your thread must be a {@link FastThreadLocalThread} or its subtype.
   * By default, all threads created by {@link DefaultThreadFactory} are {@link FastThreadLocalThread} due to this reason.
   * </p><p>
   * Note that the fast path is only possible on threads that extend {@link FastThreadLocalThread}, because it requires
   * a special field to store the necessary state.  An access by any other kind of thread falls back to a regular
   * {@link ThreadLocal}.
   * </p>
   *
   * @param <V> the type of the thread-local variable
   * @see ThreadLocal
   */
  public class FastThreadLocal<V> {
  
      private static final int variablesToRemoveIndex = InternalThreadLocalMap.nextVariableIndex();
  
      /**
       * Removes all {@link FastThreadLocal} variables bound to the current thread.  This operation is useful when you
       * are in a container environment, and you don't want to leave the thread local variables in the threads you do not
       * manage.
       */
      public static void removeAll() {
          InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.getIfSet();
          if (threadLocalMap == null) {
              return;
          }
  
          try {
              Object v = threadLocalMap.indexedVariable(variablesToRemoveIndex);
              if (v != null && v != InternalThreadLocalMap.UNSET) {
                  @SuppressWarnings("unchecked")
                  Set<FastThreadLocal<?>> variablesToRemove = (Set<FastThreadLocal<?>>) v;
                  FastThreadLocal<?>[] variablesToRemoveArray =
                          variablesToRemove.toArray(new FastThreadLocal[0]);
                  for (FastThreadLocal<?> tlv: variablesToRemoveArray) {
                      tlv.remove(threadLocalMap);
                  }
              }
          } finally {
              InternalThreadLocalMap.remove();
          }
      }
  
      /**
       * Returns the number of thread local variables bound to the current thread.
       */
      public static int size() {
          InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.getIfSet();
          if (threadLocalMap == null) {
              return 0;
          } else {
              return threadLocalMap.size();
          }
      }
  
      /**
       * Destroys the data structure that keeps all {@link FastThreadLocal} variables accessed from
       * non-{@link FastThreadLocalThread}s.  This operation is useful when you are in a container environment, and you
       * do not want to leave the thread local variables in the threads you do not manage.  Call this method when your
       * application is being unloaded from the container.
       */
      public static void destroy() {
          InternalThreadLocalMap.destroy();
      }
  
      @SuppressWarnings("unchecked")
      private static void addToVariablesToRemove(InternalThreadLocalMap threadLocalMap, FastThreadLocal<?> variable) {
          Object v = threadLocalMap.indexedVariable(variablesToRemoveIndex);
          Set<FastThreadLocal<?>> variablesToRemove;
         if (v == InternalThreadLocalMap.UNSET || v == null) {
             variablesToRemove = Collections.newSetFromMap(new IdentityHashMap<>());
             threadLocalMap.setIndexedVariable(variablesToRemoveIndex, variablesToRemove);
         } else {
             variablesToRemove = (Set<FastThreadLocal<?>>) v;
         }
 
         variablesToRemove.add(variable);
     }
 
     private static void removeFromVariablesToRemove(
             InternalThreadLocalMap threadLocalMap, FastThreadLocal<?> variable) {
 
         Object v = threadLocalMap.indexedVariable(variablesToRemoveIndex);
 
         if (v == InternalThreadLocalMap.UNSET || v == null) {
             return;
         }
 
         @SuppressWarnings("unchecked")
         Set<FastThreadLocal<?>> variablesToRemove = (Set<FastThreadLocal<?>>) v;
         variablesToRemove.remove(variable);
     }
 
     private final int index;
 
     public FastThreadLocal() {
         index = InternalThreadLocalMap.nextVariableIndex();
     }
 
     /**
      * Returns the current value for the current thread
      */
     @SuppressWarnings("unchecked")
     public final V get() {
         InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.get();
         Object v = threadLocalMap.indexedVariable(index);
         if (v != InternalThreadLocalMap.UNSET) {
             return (V) v;
         }
 
         return initialize(threadLocalMap);
     }
 
     /**
      * Returns the current value for the current thread if it exists, {@code null} otherwise.
      */
     @SuppressWarnings("unchecked")
     public final V getIfExists() {
         InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.getIfSet();
         if (threadLocalMap != null) {
             Object v = threadLocalMap.indexedVariable(index);
             if (v != InternalThreadLocalMap.UNSET) {
                 return (V) v;
             }
         }
         return null;
     }
 
     private V initialize(InternalThreadLocalMap threadLocalMap) {
         V v = null;
         try {
             v = initialValue();
         } catch (Exception e) {
             PlatformDependent.throwException(e);
         }
 
         threadLocalMap.setIndexedVariable(index, v);
         addToVariablesToRemove(threadLocalMap, this);
         return v;
     }
 
     /**
      * Set the value for the current thread.
      */
     public final void set(V value) {
         set(InternalThreadLocalMap.get(), value);
     }
 
     /**
      * Set the value for the specified thread local map. The specified thread local map must be for the current thread.
      */
     private void set(InternalThreadLocalMap threadLocalMap, V value) {
         if (value != InternalThreadLocalMap.UNSET) {
             setKnownNotUnset(threadLocalMap, value);
         } else {
             remove(threadLocalMap);
         }
     }
 
     /**
      * @see InternalThreadLocalMap#setIndexedVariable(int, Object).
      */
     private void setKnownNotUnset(InternalThreadLocalMap threadLocalMap, V value) {
         if (threadLocalMap.setIndexedVariable(index, value)) {
             addToVariablesToRemove(threadLocalMap, this);
         }
     }
 
     /**
      * Returns {@code true} if and only if this thread-local variable is set.
      */
     public final boolean isSet() {
         return isSet(InternalThreadLocalMap.getIfSet());
     }
 
     /**
      * Returns {@code true} if and only if this thread-local variable is set.
      * The specified thread local map must be for the current thread.
      */
     private boolean isSet(InternalThreadLocalMap threadLocalMap) {
         return threadLocalMap != null && threadLocalMap.isIndexedVariableSet(index);
     }
     /**
      * Sets the value to uninitialized for the specified thread local map.
      * After this, any subsequent call to get() will trigger a new call to initialValue().
      */
     public final void remove() {
         remove(InternalThreadLocalMap.getIfSet());
     }
 
     /**
      * Sets the value to uninitialized for the specified thread local map.
      * After this, any subsequent call to get() will trigger a new call to initialValue().
      * The specified thread local map must be for the current thread.
      */
     @SuppressWarnings("unchecked")
     private void remove(InternalThreadLocalMap threadLocalMap) {
         if (threadLocalMap == null) {
             return;
         }
 
         Object v = threadLocalMap.removeIndexedVariable(index);
         removeFromVariablesToRemove(threadLocalMap, this);
 
         if (v != InternalThreadLocalMap.UNSET) {
             try {
                 onRemoval((V) v);
             } catch (Exception e) {
                 PlatformDependent.throwException(e);
             }
         }
     }
 
     /**
      * Returns the initial value for this thread-local variable.
      */
     protected V initialValue() throws Exception {
         return null;
     }
 
     /**
      * Invoked when this thread local variable is removed by {@link #remove()}. Be aware that {@link #remove()}
      * is not guaranteed to be called when the `Thread` completes which means you can not depend on this for
      * cleanup of the resources in the case of `Thread` completion.
      */
     protected void onRemoval(@SuppressWarnings("UnusedParameters") V value) throws Exception { }
 }