n queens in ramda

ramda-n-queens.js

  describe("n queens", function() {
    var isSafe = R.curry((col, placedQueens) => {
      var row = R.length(placedQueens);
      var notThreatened = ([r, c]) => col !== c && Math.abs(col - c) !== row - r;
      var rowsWithQueen = R.zip(R.reverse(R.range(0, row)), placedQueens);
      return R.all(notThreatened, rowsWithQueen);
    });

    var queens = (n) => {
      var columns = R.range(0, n);
      var placeQueens = (k) => {
        if (k === 0) {
          return R.of([]);
        }

        var findSafePlacements = (placedQueens) => {
          var safePlacementsFor = R.pipe(
            R.filter(isSafe(R.__, placedQueens)),
            R.map(R.prepend(R.__, placedQueens))
          );
          return safePlacementsFor(columns);
        };

        return R.chain(findSafePlacements, placeQueens(k - 1));
      };

      return placeQueens(n);
    };

    it("no solutions for 2x2 or 3x3", function() {
      queens(2).should.eql([]);
      queens(3).should.eql([]);
    });

    it("two solutions for 4x4", function() {
      queens(4).should.eql([
        [2, 0, 3, 1],
        [1, 3, 0, 2]
      ]);
    });

    it("ten solutions for 5x5", function() {
      queens(5).should.eql([
        [3, 1, 4, 2, 0],
        [2, 4, 1, 3, 0],
        [4, 2, 0, 3, 1],
        [3, 0, 2, 4, 1],
        [4, 1, 3, 0, 2],
        [0, 3, 1, 4, 2],
        [1, 4, 2, 0, 3],
        [0, 2, 4, 1, 3],
        [2, 0, 3, 1, 4],
        [1, 3, 0, 2, 4]
      ]);
    });

    it("four solutions for 6x6", function() {
      queens(6).should.eql([
        [4, 2, 0, 5, 3, 1],
        [3, 0, 4, 1, 5, 2],
        [2, 5, 1, 4, 0, 3],
        [1, 3, 5, 0, 2, 4]
      ]);
    });
  });