server/protocols/jmap-draft/src/main/java/org/apache/james/jmap/draft/utils/SortingHierarchicalCollections.java

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package org.apache.james.jmap.draft.utils;

import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.function.Function;
import java.util.stream.Collectors;

import org.apache.james.jmap.draft.utils.DependencyGraph.CycleDetectedException;

import com.google.common.collect.Lists;

public class SortingHierarchicalCollections<T, IdT> {

    private final Function<T, IdT> index;
    private final Function<T, Optional<IdT>> parentId;

    public SortingHierarchicalCollections(Function<T, IdT> index,
                                  Function<T, Optional<IdT>> parentId) {
        this.index = index;
        this.parentId = parentId;
    }

    public List<T> sortFromRootToLeaf(Collection<T> elements) throws CycleDetectedException {

        Map<IdT, T> mapOfElementsById = indexElementsById(elements);

        DependencyGraph<T> graph = new DependencyGraph<>(m ->
                parentId.apply(m).map(mapOfElementsById::get));

        elements.forEach(graph::registerItem);

        return graph.getBuildChain().collect(Collectors.toList());
    }

    private Map<IdT, T> indexElementsById(Collection<T> elements) {
        return elements.stream()
                .collect(Collectors.toMap(index, Function.identity()));
    }

    public List<T> sortFromLeafToRoot(Collection<T> elements) throws CycleDetectedException {
        return Lists.reverse(sortFromRootToLeaf(elements));
    }
}