/*
 * Copyright (c) 2020, Fernando Miguel Carvalho, mcarvalho@cc.isel.ipl.pt
 *
 * 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 org.jayield.ops;

import org.jayield.Advancer;
import org.jayield.Query;
import org.jayield.Traverser;
import org.jayield.Yield;

import java.util.function.Function;

public class FlatMap<T, R> implements Advancer<R>, Traverser<R> {
    private final Query<T> upstream;
    private final Function<? super T, ? extends Query<? extends R>> mapper;
    private Query<? extends R> src;

    public FlatMap(Query<T> query, Function<? super T, ? extends Query<? extends R>> mapper) {
        this.upstream = query;
        this.mapper = mapper;
        src = new Query<>(Advancer.empty(), Traverser.empty());
    }

    @Override
    public void traverse(Yield<? super R> yield) {
        upstream.traverse(elem ->
                mapper.apply(elem).traverse(yield));

    }

    @Override
    public boolean tryAdvance(Yield<? super R> yield) {
        while (!src.tryAdvance(yield)) {
            if(!upstream.tryAdvance(t -> src = mapper.apply(t)))
                return false;
        }
        return true;
    }
}
