Dartでオフラインリアルタイムどう書く第四回の参考問題

第四回 オフラインリアルタイムどう書くの参考問題をDartで解いてみました。
http://qiita.com/items/9c514267214d3917edf2

import "dart:math";

final List<int> xs = [0, 1, 0, -1];
final List<int> ys = [1, 0, -1, 0];
final int WIDTH = 5;
final int HEIGHT = 5;
final int shiftSize = (log(WIDTH * HEIGHT) / LN2).ceil().toInt();

main() {
  test("", 8512);
  test("af", 4256);
  test("xy", 4256);
  test("pq qr rs st di in ns sx", 184);
  test("af pq qr rs st di in ns sx", 92);
  test("bg ch di ij no st", 185);
  test("bc af ch di no kp mr ns ot pu rs", 16);
  test("ab af", 0);
  test("ty xy", 0);
  test("bg ch ej gh lm lq mr ot rs sx", 11);
  test("ty ch hi mn kp mr rs sx", 18);
  test("xy ch hi mn kp mr rs sx", 32);
  test("ch hi mn kp mr rs sx", 50);
  test("ab cd uv wx", 621);
  test("gh mn st lq qr", 685);
  test("fg gl lm mr rs", 171);
  print("ok");
}

test(String input, int expect) {
  assert(countPath(input) == expect);
}

int str2node(String chr) => chr.codeUnitAt(0) - "a".codeUnitAt(0);

int pos2node(int x, int y) => y * WIDTH + x;

int node2path(int from, int to) {
  return (from < to) ? (from << shiftSize) + to : (to << shiftSize) + from;
}

int parsePath(String path) => node2path(str2node(path[0]), str2node(path[1]));

int countPath(String input) {
  Set<int> visited = new Set.from([0]);
  Set<int> stop = new Set();
  if(input != "") {
    stop = new Set.from(input.split(" ").map(parsePath));
  }
  return count(0, 0, visited, stop);
}

int count(int x, int y, Set visited, Set stop) {
  if(x == WIDTH - 1 && y == HEIGHT - 1) return 1;
  int sum = 0;

  for(var i = 0; i < 4; i++) {
    int nx = x + xs[i];
    int ny = y + ys[i];
    if(0 <= nx && 0 <= ny && nx < WIDTH && ny < HEIGHT) {
      int curNode = pos2node(x, y);
      int nextNode = pos2node(nx, ny);
      int path = node2path(curNode, nextNode);

      if(!visited.contains(nextNode) && !stop.contains(path)) {
        visited.add(nextNode);
        sum += count(nx, ny, visited, stop);
        visited.remove(nextNode);
      }
    }
  }
  return sum;
}