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/**
 * Licensed to The Apereo Foundation under one or more contributor license
 * agreements. See the NOTICE file distributed with this work for additional
 * information regarding copyright ownership.
 *
 *
 * The Apereo Foundation licenses this file to you under the Educational
 * Community 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://opensource.org/licenses/ecl2.txt
 *
 * 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.opencastproject.util.data;

import static java.util.Collections.emptyList;
import static java.util.Collections.singletonList;
import static org.opencastproject.util.data.Option.some;

import java.util.Iterator;
import java.util.List;

/**
 * An algebraic data type representing a disjoint union. By convention left is considered to represent an error while
 * right represents a value.
 *
 * This implementation of Either is much simpler than implementations you may know from other languages or
 * libraries.
 */
public abstract class Either {
  private Either() {
  }

  public abstract LeftProjection left();

  public abstract RightProjection right();

  public abstract  X fold(Match visitor);

  public abstract  X fold(Function left, Function right);

  public abstract boolean isLeft();

  public abstract boolean isRight();

  public interface Match {
    X left(A a);

    X right(B b);
  }

  /**
   * Left projection of either.
   */
  public abstract class LeftProjection implements Iterable {
    private LeftProjection() {
    }

    public abstract A value();

    public abstract A getOrElse(A right);

  }

  /**
   * Right projection of either.
   */
  public abstract class RightProjection implements Iterable {

    private RightProjection() {
    }

    public abstract Either either();

    public abstract  Either bind(Function> f);

    public abstract B value();

    public abstract B getOrElse(B left);

    public abstract Option toOption();

  }

  /**
   * Create a left.
   */
  public static  Either left(final A left) {
    return new Either() {
      @Override
      public  C fold(Match visitor) {
        return visitor.left(left);
      }

      @Override
      public LeftProjection left() {
        return new LeftProjection() {
          @Override
          public A value() {
            return left;
          }

          private List toList() {
            return singletonList(left);
          }

          @Override
          public Iterator iterator() {
            return toList().iterator();
          }

          @Override
          public A getOrElse(A right) {
            return left;
          }
        };
      }

      @Override
      public RightProjection right() {
        final Either self = this;
        return new RightProjection() {
          @Override
          public Either either() {
            return self;
          }

          @Override
          public  Either bind(Function> f) {
            return left(left);
          }

          @Override
          public B value() {
            throw new Error("right projection on left does not have a value");
          }

          @Override
          public B getOrElse(B left) {
            return left;
          }

          @Override
          public Option toOption() {
            return Option.none();
          }

          private List toList() {
            return emptyList();
          }

          @Override
          public Iterator iterator() {
            return toList().iterator();
          }
        };
      }

      @Override
      public  C fold(Function leftf, Function rightf) {
        return leftf.apply(left);
      }

      @Override
      public boolean isLeft() {
        return true;
      }

      @Override
      public boolean isRight() {
        return false;
      }
    };
  }

  /**
   * Create a right.
   */
  public static  Either right(final B right) {
    return new Either() {
      @Override
      public  C fold(Match visitor) {
        return visitor.right(right);
      }

      @Override
      public LeftProjection left() {
        return new LeftProjection() {
          @Override
          public A value() {
            throw new Error("left projection on right does not have a value");
          }

          private List toList() {
            return emptyList();
          }

          @Override
          public Iterator iterator() {
            return toList().iterator();
          }

          @Override
          public A getOrElse(A right) {
            return right;
          }
        };
      }

      @Override
      public RightProjection right() {
        final Either self = this;
        return new RightProjection() {
          @Override
          public Either either() {
            return self;
          }

          @Override
          public  Either bind(Function> f) {
            return f.apply(right);
          }

          @Override
          public B value() {
            return right;
          }

          @Override
          public B getOrElse(B left) {
            return right;
          }

          @Override
          public Option toOption() {
            return some(right);
          }

          private List toList() {
            return singletonList(right);
          }

          @Override
          public Iterator iterator() {
            return toList().iterator();
          }
        };
      }

      @Override
      public  X fold(Function leftf, Function rightf) {
        return rightf.apply(right);
      }

      @Override
      public boolean isLeft() {
        return false;
      }

      @Override
      public boolean isRight() {
        return true;
      }
    };
  }
}