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package java.util.functions;

/**
 * Reduce an input object by combining with a base object.
 *
 * <p>All reducer implementations are expected to:
 * <ul>
 *  <li>Provide stable results such that for any {@code a} and {@code b} the
 * result of two {@code reduce} operations are always equivalent. ie.<pre>
 * Foo one = reducer.reduce(a,b);
 * Foo two = reducer.reduce(a,b);
 *
 * assert one.equals(two) && two.equals(one);
 * </pre></li>
 * <li>The reducer should not modify the input object in any way that would
 * change the result.</li>
 * <li>When used for aggregate operations upon many elements reducers
 * should not assume that the {@code reduce} operation will be called upon
 * elements in any specific orderie.<pre>
 * Foo one = reducer.reduce(a,reducer.reduce(b,c));
 * Foo two = reducer.reduce(b,reducer.reduce(a,c));
 *
 * assert one.equals(two) && two.equals(one);
 * </pre>/li>
 * </ul>

 * @param <T> the type of input objects provided to {@code reduce}.
 * @param <U> the type of the base value and output objects from {@code reduce}.
 * This may be the same type as {@code <T>}.
 */
public interface Reducer<T, U> {

    /**
     * Produce a reduced output using the provided input and base objects.
     *
     * @param base the reduction base.
     * @param t the input object.
     * @return reduced output.
     */
    U reduce(U base, T t);

    /**
     * Returns a reducer which conditionally performs reduce on the input from
     * {@code <U>} to {@code <T>}. The reduce is only performed if the provided
     * predicate returns {@code true} for the input {@code <U>}.
     * @param predicate A Predicate for {@code <U>} values.
     * @return A reducer which conditionally performing reduction from {@code <U>} to
     * {@code <T>} if the {@code <U>} input value satisfies a predicate.
     */
    Reducer<T, U> compose(Predicate<? super T> predicate) default Reducers.compose;

    /**
     * Returns a reducer which performs a mapping of inputs from {@code <V>} to
     * {@code <T>} before reducing to the result.
     * @param <V> Type of input objects.
     * @param mapper A mapper from {@code <V>} to {@code <U>}.
     * @return A reducer which performing a mapping from {@code <U>} to
     * {@code <V>}prior to reducing from {@code <U>} to {@code <T>}.
     */
     <V> Reducer<V, U> compose(Mapper<? super V, ? extends T> mapper) default Reducers.compose;
}