/*    Copyright 2012 10gen Inc.
 *
 *    This program is free software: you can redistribute it and/or  modify
 *    it under the terms of the GNU Affero General Public License, version 3,
 *    as published by the Free Software Foundation.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    GNU Affero General Public License for more details.
 *
 *    You should have received a copy of the GNU Affero General Public License
 *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 *    As a special exception, the copyright holders give permission to link the
 *    code of portions of this program with the OpenSSL library under certain
 *    conditions as described in each individual source file and distribute
 *    linked combinations including the program with the OpenSSL library. You
 *    must comply with the GNU Affero General Public License in all respects
 *    for all of the code used other than as permitted herein. If you modify
 *    file(s) with this exception, you may extend this exception to your
 *    version of the file(s), but you are not obligated to do so. If you do not
 *    wish to do so, delete this exception statement from your version. If you
 *    delete this exception statement from all source files in the program,
 *    then also delete it in the license file.
 */

#pragma once

#include <string>
#include <vector>
#include <utility>

#include "mongo/base/disallow_copying.h"
#include "mongo/base/initializer_function.h"
#include "mongo/base/status.h"
#include "mongo/platform/unordered_map.h"
#include "mongo/platform/unordered_set.h"

namespace mongo {

    /**
     * Representation of a dependency graph of "initialization operations."
     *
     * Each operation has a unique name, a function object implementing the operation's behavior,
     * and a set of prerequisite operations, which may be empty.  A legal graph contains no cycles.
     *
     * Instances of this class are used in two phases.  In the first phase, the graph is constructed
     * by repeated calls to addInitializer().  In the second phase, a user calls the topSort()
     * method to produce an initialization order that respects the dependencies among operations, and
     * then uses the getInitializerFunction() to get the behavior function for each operation, in
     * turn.
     *
     * Concurrency Notes: The user is responsible for synchronization.  Multiple threads may
     * simultaneously call the const functions, getInitializerFunction and topSort, on the same
     * instance of InitializerDependencyGraph.  However, no thread may call addInitializer while any
     * thread is executing those functions or addInitializer on the same instance.
     */
    class InitializerDependencyGraph {
        MONGO_DISALLOW_COPYING(InitializerDependencyGraph);

    public:
        InitializerDependencyGraph();
        ~InitializerDependencyGraph();

        /**
         * Add a new initializer node, named "name", to the dependency graph, with the given
         * behavior, "fn", and the given "prerequisites" (input dependencies) and "dependents"
         * (output dependencies).
         *
         * If "!fn" (fn is NULL in function pointer parlance), returns status with code
         * ErrorCodes::badValue.  If "name" is a duplicate of a name already present in the graph,
         * returns "ErrorCodes::duplicateKey".  Otherwise, returns Status::OK() and adds the new node
         * to the graph.  Note that cycles in the dependency graph are not discovered in this phase.
         * Rather, they're discovered by topSort, below.
         */
        Status addInitializer(const std::string& name,
                              const InitializerFunction& fn,
                              const std::vector<std::string>& prerequisites,
                              const std::vector<std::string>& dependents);

        /**
         * Given a dependency operation node named "name", return its behavior function.  Returns
         * a value that evaluates to "false" in boolean context, otherwise.
         */
        InitializerFunction getInitializerFunction(const std::string& name) const;

        /**
         * Construct a topological sort of the dependency graph, and store that order into
         * "sortedNames".  Returns Status::OK() on success.
         *
         * If the graph contains a cycle, returns ErrorCodes::graphContainsCycle, and "sortedNames"
         * is an ordered sequence of nodes involved in a cycle.  In this case, the first and last
         * element of "sortedNames" will be equal.
         *
         * If any node in the graph names a prerequisite that was never added to the graph via
         * addInitializer, this function will return ErrorCodes::badValue.
         *
         * Any other return value indicates an internal error, and should not occur.
         */
        Status topSort(std::vector<std::string>* sortedNames) const;

    private:
        struct NodeData {
            InitializerFunction fn;
            unordered_set<std::string> prerequisites;
        };

        typedef unordered_map<std::string, NodeData> NodeMap;
        typedef NodeMap::value_type Node;

        /**
         * Helper function to recursively top-sort a graph.  Used by topSort().
         */
        static Status recursiveTopSort(
                const NodeMap& nodeMap,
                const Node& currentNode,
                std::vector<std::string>* inProgressNodeNames,
                unordered_set<std::string>* visitedNodeNames,
                std::vector<std::string>* sortedNames);

        /**
         * Map of all named nodes.  Nodes named as prerequisites or dependents but not explicitly
         * added via addInitializer will either be absent from this map or be present with
         * NodeData::fn set to a false-ish value.
         */
        NodeMap _nodes;
    };

}  // namespace mongo