aStar.js

const BinaryHeap = require('../binary-heap/binaryHeap');

const pathTo = (node) => {
  const path = [];
  let curr = node;

  while (curr.parent) {
    path.unshift(curr);
    curr = curr.parent;
  }
  return path;
};

const getHeap = () => new BinaryHeap( node => node.f );

const astar = {
  /**
  * Perform an A* Search on a graph given a start and end node.
  * @param {Graph} graph
  * @param {GridNode} start
  * @param {GridNode} end
  * @param {Object} [options]
  * @param {bool} [options.closest] Specifies whether to return the
             path to the closest node if the target is unreachable.
  * @param {Function} [options.heuristic] Heuristic function (see
  *          astar.heuristics).
  */
  search: (graph, start, end, options) => {
    graph.cleanDirty();
    options = options || {};
    const heuristic = options.heuristic || astar.heuristics.manhattan;
    const closest = options.closest || false;

    const openHeap = getHeap();
    let closestNode = start; // set the start node to be the closest if required

    start.h = heuristic(start, end);
    graph.markDirty(start);

    openHeap.push(start);

    while (openHeap.size() > 0) {
      // Grab the lowest f(x) to process next.  Heap keeps this sorted for us.
      const currentNode = openHeap.pop();

      // End case -- result has been found, return the traced path.
      if (currentNode === end) {
        return pathTo(currentNode);
      }

      // Normal case -- move currentNode from open to closed, process each of its neighbors.
      currentNode.closed = true;

      // Find all neighbors for the current node.
      const neighbors = graph.neighbors(currentNode);

      for (let i = 0, il = neighbors.length; i < il; ++i) {
        const neighbor = neighbors[i];

        if (neighbor.closed || neighbor.isWall()) {
          // Not a valid node to process, skip to next neighbor.
          continue;
        }

        // The g score is the shortest distance from start to current node.
        // We need to check if the path we have arrived at
        // this neighbor is the shortest one we have seen yet.
        const gScore = currentNode.g + neighbor.getCost(currentNode);
        const beenVisited = neighbor.visited;

        if (!beenVisited || gScore < neighbor.g) {

          // Found an optimal (so far) path to this node.
          // Take score for node to see how good it is.
          neighbor.visited = true;
          neighbor.parent = currentNode;
          neighbor.h = neighbor.h || heuristic(neighbor, end);
          neighbor.g = gScore;
          neighbor.f = neighbor.g + neighbor.h;
          graph.markDirty(neighbor);
          if (closest) {
            // If the neighbour is closer than the current closestNode
            // or if it's equally close but has a cheaper path than
            // the current closest node then it becomes the closest node
            if (
              neighbor.h < closestNode.h ||
              (neighbor.h === closestNode.h && neighbor.g < closestNode.g)
            ) {
              closestNode = neighbor;
            }
          }

          if (!beenVisited) {
            // Pushing to heap will put it in proper place based on the 'f' value.
            openHeap.push(neighbor);
          } else {
            // Already seen the node, but since it has been
            // rescored we need to reorder it in the heap
            openHeap.rescoreElement(neighbor);
          }
        }
      }
    }

    if (closest) {
      return pathTo(closestNode);
    }

    // No result was found - empty array signifies failure to find path.
    return [];
  },
  // See list of heuristics: http://theory.stanford.edu/~amitp/GameProgramming/Heuristics.html
  heuristics: {
    manhattan: (pos0, pos1) => {
      const d1 = Math.abs(pos1.x - pos0.x);
      const d2 = Math.abs(pos1.y - pos0.y);
      return d1 + d2;
    },
    diagonal: (pos0, pos1) => {
      const D = 1;
      const D2 = Math.sqrt(2);
      const d1 = Math.abs(pos1.x - pos0.x);
      const d2 = Math.abs(pos1.y - pos0.y);
      return (D * (d1 + d2)) + ((D2 - (2 * D)) * Math.min(d1, d2));
    },
  },
  cleanNode: (node) => {
    node.f = 0;
    node.g = 0;
    node.h = 0;
    node.visited = false;
    node.closed = false;
    node.parent = null;
  },
};

module.exports = astar;

// http://theory.stanford.edu/~amitp/GameProgramming/ImplementationNotes.html
// http://www.briangrinstead.com/blog/astar-search-algorithm-in-javascript/
// https://github.com/bgrins/javascript-astar/blob/master/astar.js
// https://en.wikipedia.org/wiki/A*_search_algorithm