/*
    * SPDX-FileCopyrightText: none
    * SPDX-License-Identifier: CC0-1.0
    */
   
   package dev.metaschema.core.qname;
   
   import java.net.URI;
   import java.util.Map;
  import java.util.Optional;
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.atomic.AtomicInteger;
  
  import dev.metaschema.core.util.ObjectUtils;
  import edu.umd.cs.findbugs.annotations.NonNull;
  import nl.talsmasoftware.lazy4j.Lazy;
  
  /**
   * An integer-based cache of namespaces to reduce the memory footprint of
   * namespaces used by reusing instances with the same namespace.
   */
  public final class NamespaceCache {
    @NonNull
    private static final Lazy<NamespaceCache> INSTANCE = ObjectUtils.notNull(Lazy.of(NamespaceCache::new));
  
    private final Map<String, Integer> nsToIndex = new ConcurrentHashMap<>();
    private final Map<Integer, String> indexToNs = new ConcurrentHashMap<>();
    private final Map<Integer, URI> indexToNsUri = new ConcurrentHashMap<>();
    /**
     * The next available namespace index position.
     * <p>
     * This value starts at 1, since the "" no namspace has the zero position.
     */
    private final AtomicInteger indexCounter = new AtomicInteger();
  
    /**
     * Get the singleton instance.
     *
     * @return the singleton instance
     */
    @NonNull
    public static NamespaceCache instance() {
      return ObjectUtils.notNull(INSTANCE.get());
    }
  
    private NamespaceCache() {
      // claim the "0" position
      int noNamespaceIndex = indexOf("");
      assert noNamespaceIndex == 0;
    }
  
    /**
     * Get the index value of the provided namespace.
     *
     * @param namespace
     *          the namespace
     * @return the index value
     */
    public int indexOf(@NonNull String namespace) {
      return nsToIndex.computeIfAbsent(namespace, key -> {
        int nextIndex = indexCounter.getAndIncrement();
        indexToNs.put(nextIndex, namespace);
        return nextIndex;
      });
    }
  
    /**
     * Lookup the index value for an existing namespace.
     *
     * @param namespace
     *          the namespace to lookup
     * @return an optional containing the index value, if it exists
     */
    @NonNull
    public Optional<Integer> get(@NonNull String namespace) {
      return ObjectUtils.notNull(Optional.ofNullable(nsToIndex.get(namespace)));
    }
  
    /**
     * Lookup the namespace using the index value for an existing namespace.
     *
     * @param index
     *          the index value to lookup
     * @return an optional containing the namespace, if the index value exists
     */
    @NonNull
    public Optional<String> get(int index) {
      return ObjectUtils.notNull(Optional.ofNullable(indexToNs.get(index)));
    }
  
    /**
     * Lookup the namespace using the index value for an existing namespace.
     *
     * @param index
     *          the index value to lookup
     * @return an optional containing the namespace as a URI, if the index value
     *         exists
     */
    @NonNull
   public Optional<URI> getAsURI(int index) {
     return ObjectUtils.notNull(Optional.ofNullable(indexToNsUri.computeIfAbsent(index, key -> {
       Optional<String> namespace = get(key);
       URI nsUri = null;
       if (namespace.isPresent()) {
         nsUri = URI.create(namespace.get());
         indexToNsUri.put(key, nsUri);
       }
       return nsUri;
     })));
   }
 }