package com.atlassian.vcache.internal.core.metrics;
   
   import java.util.Map;
   import java.util.Optional;
   import java.util.Set;
   import java.util.function.BiConsumer;
   import java.util.function.Function;
   
   import static java.util.Objects.requireNonNull;
  
  /**
   * Wrapper for a function used to create missing values. Calls the supplied handler with
   * the elapsed time taken in nanoseconds, and {@link Optional#empty()} means it was was not called.
   * All passes the number of keys passed to the factory.
   *
   * @param <K> the key type
   * @param <V> the value type
   * @since 1.0.0
   */
  class TimedFactory<K, V> implements Function<Set<K>, Map<K, V>>, AutoCloseable {
      private final Function<Set<K>, Map<K, V>> delegate;
      private final BiConsumer<Optional<Long>, Long> handler;
  
      private Optional<Long> elapsedDuration = Optional.empty();
      private long numberOfKeys;
  
      TimedFactory(Function<Set<K>, Map<K, V>> delegate, BiConsumer<Optional<Long>, Long> handler) {
          this.delegate = requireNonNull(delegate);
          this.handler = requireNonNull(handler);
      }
  
      @Override
      public Map<K, V> apply(Set<K> keys) {
          try (ElapsedTimer ignored = new ElapsedTimer(t -> {
              elapsedDuration = Optional.of(t);
              numberOfKeys = (long) keys.size();
          })) {
              return delegate.apply(keys);
          }
      }
  
      @Override
      public void close() {
          handler.accept(elapsedDuration, numberOfKeys);
      }
  
      long getNumberOfKeys() {
          return numberOfKeys;
      }
  }