package io.atlassian.fugue.optic;
   
   import io.atlassian.fugue.Either;
   import io.atlassian.fugue.Monoid;
   import io.atlassian.fugue.Pair;
   
   import java.util.function.Function;
   
   /**
   * A {@link Getter} can be seen as a glorified get method between a type S and a
   * type A.
   * <p>
   * A {@link Getter} is also a valid {@link Fold}
   *
   * @param <S> the source of a {@link Getter}
   * @param <A> the target of a {@link Getter}
   */
  public abstract class Getter<S, A> {
  
    Getter() {
      super();
    }
  
    /**
     * get the target of a {@link Getter}
     */
    public abstract A get(S s);
  
    /**
     * join two {@link Getter} with the same target
     */
    public final <S1> Getter<Either<S, S1>, A> sum(final Getter<S1, A> other) {
      return getter(e -> e.fold(this::get, other::get));
    }
  
    /**
     * pair two disjoint {@link Getter}
     */
    public final <S1, A1> Getter<Pair<S, S1>, Pair<A, A1>> product(final Getter<S1, A1> other) {
      return getter(p2 -> Pair.pair(this.get(p2.left()), other.get(p2.right())));
    }
  
    public final <B> Getter<Pair<S, B>, Pair<A, B>> first() {
      return getter(p -> Pair.pair(this.get(p.left()), p.right()));
    }
  
    public final <B> Getter<Pair<B, S>, Pair<B, A>> second() {
      return getter(p -> Pair.pair(p.left(), this.get(p.right())));
    }
  
    /*************************************************************/
    /** Compose methods between a {@link Getter} and another Optics */
    /*************************************************************/
  
    /**
     * compose a {@link Getter} with a {@link Fold}
     */
    public final <B> Fold<S, B> composeFold(final Fold<A, B> other) {
      return asFold().composeFold(other);
    }
  
    /**
     * compose a {@link Getter} with a {@link Getter}
     */
    public final <B> Getter<S, B> composeGetter(final Getter<A, B> other) {
      return getter(s -> other.get(get(s)));
    }
  
    /**
     * compose a {@link Getter} with a {@link POptional}
     */
    public final <B, C, D> Fold<S, C> composeOptional(final POptional<A, B, C, D> other) {
      return asFold().composeOptional(other);
    }
  
    /**
     * compose a {@link Getter} with a {@link PPrism}
     */
    public final <B, C, D> Fold<S, C> composePrism(final PPrism<A, B, C, D> other) {
      return asFold().composePrism(other);
    }
  
    /**
     * compose a {@link Getter} with a {@link PLens}
     */
    public final <B, C, D> Getter<S, C> composeLens(final PLens<A, B, C, D> other) {
      return composeGetter(other.asGetter());
    }
  
    /**
     * compose a {@link Getter} with a {@link PIso}
     */
    public final <B, C, D> Getter<S, C> composeIso(final PIso<A, B, C, D> other) {
      return composeGetter(other.asGetter());
    }
  
    /******************************************************************/
    /** Transformation methods to view a {@link Getter} as another Optics */
    /******************************************************************/
 
   /**
    * view a {@link Getter} with a {@link Fold}
    */
   public final Fold<S, A> asFold() {
     return new Fold<S, A>() {
       @Override public <B> Function<S, B> foldMap(final Monoid<B> monoid, final Function<A, B> f) {
         return s -> f.apply(get(s));
       }
     };
   }
 
   public static <A> Getter<A, A> id() {
     return PIso.<A, A> pId().asGetter();
   }
 
   public static <A> Getter<Either<A, A>, A> codiagonal() {
     return getter(e -> e.fold(Function.identity(), Function.identity()));
   }
 
   public static <S, A> Getter<S, A> getter(final Function<S, A> get) {
     return new Getter<S, A>() {
 
       @Override public A get(final S s) {
         return get.apply(s);
       }
     };
   }
 }