node-dijkstra/libs/Graph.js

const Queue = require('./PriorityQueue');
const removeDeepFromMap = require('./removeDeepFromMap');
const toDeepMap = require('./toDeepMap');

/** Creates and manages a graph */
class Graph {

  /**
   * Creates a new Graph, optionally initializing it a nodes graph representation.
   *
   * A graph representation is an object that has as keys the name of the point and as values
   * the points reacheable from that node, with the cost to get there:
   *
   *     {
   *       node (Number|String): {
   *         neighbor (Number|String): cost (Number),
   *         ...,
   *       },
   *     }
   *
   * @param {Object} [graph] - Initial graph definition
   * @example
   *
   * const route = new Graph();
   *
   * // Pre-populated graph
   * const route = new Graph({
   *   A: { B: 1 },
   *   B: { A: 1, C: 2, D: 4 },
   * });
   */
  constructor(graph) {
    this.graph = graph ? toDeepMap(graph) : new Map();
  }

  /**
   * Adds a node to the graph
   *
   * @param {string} name      - Name of the node
   * @param {Object} neighbors - Neighbouring nodes and cost to reach them
   * @return {this}
   * @example
   *
   * const route = new Graph();
   *
   * route.addNode('A', { B: 1 });
   *
   * // It's possible to chain the calls
   * route
   *   .addNode('B', { A: 1 })
   *   .addNode('C', { A: 3 });
   */
  addNode(name, neighbors) {
    this.graph.set(name, toDeepMap(neighbors));

    return this;
  }

  /**
   * @deprecated since version 2.0, use `Graph#addNode` isntead
   */
  addVertex(name, neighbors) {
    return this.addNode(name, neighbors);
  }

  /**
   * Removes a node and all it's referencies from the graph
   *
   * @param {string|number} key - Key of the node to remove from the graph
   * @return {this}
   * @example
   *
   * const route = new Graph({
   *   A: { B: 1, C: 5 },
   *   B: { A: 3 },
   *   C: { B: 2, A: 2 },
   * });
   *
   * route.removeNode('C');
   * // The graph now is:
   * // { A: { B: 1 }, B: { A: 3 } }
   */
  removeNode(key) {
    this.graph = removeDeepFromMap(this.graph, key);

    return this;
  }

  /**
   * Compute the shortest path between the specified nodes
   *
   * @param {string}  start     - Starting node
   * @param {string}  goal      - Node we want to reach
   * @param {object}  [options] - Options
   *
   * @param {boolean} [options.trim]    - Exclude the origin and destination nodes from the result
   * @param {boolean} [options.reverse] - Return the path in reversed order
   * @param {boolean} [options.cost]    - Also return the cost of the path when set to true
   *
   * @return {array|object} Computed path between the nodes.
   *
   *  When `option.cost` is set to true, the returned value will be an object with shape:
   *    - `path` *(Array)*: Computed path between the nodes
   *    - `cost` *(Number)*: Cost of the path
   *
   * @example
   *
   * const route = new Graph()
   *
   * route.addNode('A', { B: 1 })
   * route.addNode('B', { A: 1, C: 2, D: 4 })
   * route.addNode('C', { B: 2, D: 1 })
   * route.addNode('D', { C: 1, B: 4 })
   *
   * route.path('A', 'D') // => ['A', 'B', 'C', 'D']
   *
   * // trimmed
   * route.path('A', 'D', { trim: true }) // => [B', 'C']
   *
   * // reversed
   * route.path('A', 'D', { reverse: true }) // => ['D', 'C', 'B', 'A']
   *
   * // include the cost
   * route.path('A', 'D', { cost: true })
   * // => {
   * //       path: [ 'A', 'B', 'C', 'D' ],
   * //       cost: 4
   * //    }
   */
  path(start, goal, options = {}) {
    // Don't run when we don't have nodes set
    if (!this.graph.size) {
      if (options.cost) return { path: null, cost: 0 };

      return null;
    }

    const explored = new Set();
    const frontier = new Queue();
    const previous = new Map();

    let path = [];
    let totalCost = 0;

    // Add the starting point to the frontier, it will be the first node visited
    frontier.set(start, 0);

    // Run until we have visited every node in the frontier
    while (!frontier.isEmpty()) {
      // Get the node in the frontier with the lowest cost (`priority`)
      const node = frontier.next();

      // When the node with the lowest cost in the frontier in our goal node,
      // we can compute the path and exit the loop
      if (node.key === goal) {
        // Set the total cost to the current value
        totalCost = node.priority;

        let nodeKey = node.key;
        while (previous.has(nodeKey)) {
          path.push(nodeKey);
          nodeKey = previous.get(nodeKey);
        }

        break;
      }

      // Add the current node to the explored set
      explored.add(node.key);

      // Loop all the neighboring nodes
      const neighbors = this.graph.get(node.key) || new Map();
      neighbors.forEach((nCost, nNode) => {
        // If we already explored the node, skip it
        if (explored.has(nNode)) return null;

        // If the neighboring node is not yet in the frontier, we add it with
        // the correct cost
        if (!frontier.has(nNode)) {
          previous.set(nNode, node.key);
          return frontier.set(nNode, node.priority + nCost);
        }

        const frontierPriority = frontier.get(nNode).priority;
        const nodeCost = node.priority + nCost;

        // Othewhise we only update the cost of this node in the frontier when
        // it's below what's currently set
        if (nodeCost < frontierPriority) {
          previous.set(nNode, node.key);
          return frontier.set(nNode, nodeCost);
        }

        return null;
      });
    }

    // Return null when no path can be found
    if (!path.length) {
      if (options.cost) return { path: null, cost: 0 };

      return null;
    }

    // From now on, keep in mind that `path` is populated in reverse order,
    // from destination to origin

    // Remove the first value (the goal node) if we want a trimmed result
    if (options.trim) {
      path.shift();
    } else {
      // Add the origin waypoint at the end of the array
      path = path.concat([start]);
    }

    // Reverse the path if we don't want it reversed, so the result will be
    // from `start` to `goal`
    if (!options.reverse) {
      path = path.reverse();
    }

    // Return an object if we also want the cost
    if (options.cost) {
      return {
        path,
        cost: totalCost,
      };
    }

    return path;
  }

  /**
   * @deprecated since version 2.0, use `Graph#path` isntead
   */
  shortestPath(...args) {
    return this.path(...args);
  }

}

module.exports = Graph;