/*
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * Licensed 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 com.facebook.buck.util;

import com.facebook.buck.util.function.ThrowingSupplier;
import com.facebook.buck.util.types.Either;
import com.google.common.base.Preconditions;
import com.google.common.base.Throwables;
import javax.annotation.Nullable;

/** A Generic memoizer capable of memoizing Exceptions */
class AbstractMemoizer<T, E extends Exception> {

  @Nullable private volatile Either<T, E> value = null;

  /**
   * Get the value and memoize the result. Constructs the value if it hasn't been memoized before,
   * or if the previously memoized value has been collected.
   *
   * @param delegate delegate for constructing the value
   * @return the value
   */
  public T get(ThrowingSupplier<T, E> delegate, Class<E> exceptionClass) throws E {
    if (value == null) {
      synchronized (this) {
        if (value == null) {
          try {
            T t = Preconditions.checkNotNull(delegate.get());
            value = Either.ofLeft(t);
            return t;
          } catch (Exception e) {
            if (exceptionClass.isInstance(e)) {
              value = Either.ofRight(exceptionClass.cast(e));
              Throwables.throwIfInstanceOf(e, exceptionClass);
            }
            Throwables.throwIfUnchecked(e);
            throw new IllegalStateException(e);
          }
        }
      }
    }
    if (value.isLeft()) {
      return value.getLeft();
    }
    throw value.getRight();
  }
}
