/*
    *  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.commons.compress.harmony.unpack200;
  
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.IdentityHashMap;
  import java.util.List;
  
  /**
   * The SegmentConstantPool spends a lot of time searching through large arrays of Strings looking for matches. This can be sped up by caching the arrays in
   * HashMaps so the String keys are looked up and resolve to positions in the array rather than iterating through the arrays each time.
   *
   * Because the arrays only grow (never shrink or change) we can use the last known size as a way to determine if the array has changed.
   *
   * Note that this cache must be synchronized externally if it is shared.
   */
  public class SegmentConstantPoolArrayCache {
  
      /**
       * CachedArray keeps track of the last known size of an array as well as a HashMap that knows the mapping from element values to the indices of the array
       * which contain that value.
       */
      protected class CachedArray {
          String[] primaryArray;
          int lastKnownSize;
          HashMap<String, List<Integer>> primaryTable;
  
          public CachedArray(final String[] array) {
              this.primaryArray = array;
              this.lastKnownSize = array.length;
              this.primaryTable = new HashMap<>(lastKnownSize);
              cacheIndexes();
          }
  
          /**
           * Given a primaryArray, cache its values in a HashMap to provide a backwards mapping from element values to element indexes. For instance, a
           * primaryArray of: {"Zero", "Foo", "Two", "Foo"} would yield a HashMap of: "Zero" -&gt; 0 "Foo" -&gt; 1, 3 "Two" -&gt; 2 which is then cached.
           */
          protected void cacheIndexes() {
              for (int index = 0; index < primaryArray.length; index++) {
                  final String key = primaryArray[index];
                  primaryTable.computeIfAbsent(key, k -> new ArrayList<>()).add(Integer.valueOf(index));
              }
          }
  
          /**
           * Given a particular key, answer a List of index locations in the array which contain that key.
           *
           * If no elements are found, answer an empty list.
           *
           * @param key String element of the array
           * @return List of indexes containing that key in the array.
           */
          public List<Integer> indexesForKey(final String key) {
              final List<Integer> list = primaryTable.get(key);
              return list != null ? list : Collections.emptyList();
          }
  
          /**
           * Answer the last known size of the array cached. If the last known size is not the same as the current size, the array must have changed.
           *
           * @return int last known size of the cached array
           */
          public int lastKnownSize() {
              return lastKnownSize;
          }
      }
  
      protected IdentityHashMap<String[], CachedArray> knownArrays = new IdentityHashMap<>(1000);
      protected List<Integer> lastIndexes;
      protected String[] lastArray;
  
      protected String lastKey;
  
      /**
       * Given a String array, answer true if the array is correctly cached. Answer false if the array is not cached, or if the array cache is outdated.
       *
       * @param array of String
       * @return boolean true if up-to-date cache, otherwise false.
       */
      protected boolean arrayIsCached(final String[] array) {
          final CachedArray cachedArray = knownArrays.get(array);
          return !(cachedArray == null || cachedArray.lastKnownSize() != array.length);
     }
 
     /**
      * Cache the array passed in as the argument
      *
      * @param array String[] to cache
      */
     protected void cacheArray(final String[] array) {
         if (arrayIsCached(array)) {
             throw new IllegalArgumentException("Trying to cache an array that already exists");
         }
         knownArrays.put(array, new CachedArray(array));
         // Invalidate the cache-within-a-cache
         lastArray = null;
     }
 
     /**
      * Answer the indices for the given key in the given array. If no such key exists in the cached array, answer -1.
      *
      * @param array String[] array to search for the value
      * @param key   String value for which to search
      * @return List collection of index positions in the array
      */
     public List<Integer> indexesForArrayKey(final String[] array, final String key) {
         if (!arrayIsCached(array)) {
             cacheArray(array);
         }
 
         // If the search is one we've just done, don't even
         // bother looking and return the last indices. This
         // is a second cache within the cache. This is
         // efficient because we are usually looking for
         // several secondary elements with the same primary
         // key.
         if (lastArray == array && lastKey == key) {
             return lastIndexes;
         }
 
         // Remember the last thing we found.
         lastArray = array;
         lastKey = key;
         lastIndexes = knownArrays.get(array).indexesForKey(key);
 
         return lastIndexes;
     }
 }