const allowTraversalCallback = (

    () => {

      const seen = {};

      return ({ nextVertex }) => {//如果没有访问过邻近结点，则返回true

        if (!seen[nextVertex.getKey()]) {

          seen[nextVertex.getKey()] = true;

          return true;

        }

        return false;

      };

    }

  )();

/**

 * @typedef {Object} Callbacks

 *

 * @property {function(vertices: Object): boolean} [allowTraversal] -

 *  Determines whether DFS should traverse from the vertex to its neighbor

 *  (along the edge). By default prohibits visiting the same vertex again.

 *

 * @property {function(vertices: Object)} [enterVertex] - Called when DFS enters the vertex.

 *

 * @property {function(vertices: Object)} [leaveVertex] - Called when DFS leaves the vertex.

 */

/**

 * @param {Callbacks} [callbacks]

 * @returns {Callbacks}

 */

function initCallbacks(callbacks = {}) {//对回调函数进行初始化

  const initiatedCallback = callbacks;

  const stubCallback = () => {};

  const allowTraversalCallback = (

    () => {

      const seen = {};

      return ({ nextVertex }) => {//如果没有访问过邻近结点，则返回true

        if (!seen[nextVertex.getKey()]) {

          seen[nextVertex.getKey()] = true;

          return true;

        }

        return false;

      };

    }

  )();

  initiatedCallback.allowTraversal = callbacks.allowTraversal || allowTraversalCallback;

  initiatedCallback.enterVertex = callbacks.enterVertex || stubCallback;

  initiatedCallback.leaveVertex = callbacks.leaveVertex || stubCallback;

  return initiatedCallback;

}

/**

 * @param {Graph} graph

 * @param {GraphVertex} currentVertex

 * @param {GraphVertex} previousVertex

 * @param {Callbacks} callbacks

 */

function depthFirstSearchRecursive(graph, currentVertex, previousVertex, callbacks) {

  callbacks.enterVertex({ currentVertex, previousVertex });

  graph.getNeighbors(currentVertex).forEach((nextVertex) => {

    if (callbacks.allowTraversal({ previousVertex, currentVertex, nextVertex })) {

      depthFirstSearchRecursive(graph, nextVertex, currentVertex, callbacks);

    }

  });

  callbacks.leaveVertex({ currentVertex, previousVertex });

}

/**

 * @param {Graph} graph

 * @param {GraphVertex} startVertex

 * @param {Callbacks} [callbacks]

 */

export default function depthFirstSearch(graph, startVertex, callbacks) {

  const previousVertex = null;

  depthFirstSearchRecursive(graph, startVertex, previousVertex, initCallbacks(callbacks));

}