docs: function(x, y, z, w) {
        var perm = this.perm;
        var grad4 = this.grad4;
  
        var n0, n1, n2, n3, n4; // Noise contributions from the five corners
        // Skew the (x,y,z,w) space to determine which cell of 24 simplices we're in
        var s = (x + y + z + w) * F4; // Factor for 4D skewing
        var i = Math.floor(x + s);
        var j = Math.floor(y + s);
        var k = Math.floor(z + s);
        var l = Math.floor(w + s);
        var t = (i + j + k + l) * G4; // Factor for 4D unskewing
        var X0 = i - t; // Unskew the cell origin back to (x,y,z,w) space
        var Y0 = j - t;
        var Z0 = k - t;
        var W0 = l - t;
        var x0 = x - X0; // The x,y,z,w distances from the cell origin
        var y0 = y - Y0;
        var z0 = z - Z0;
        var w0 = w - W0;
        // For the 4D case, the simplex is a 4D shape I won't even try to describe.
        // To find out which of the 24 possible simplices we're in, we need to
        // determine the magnitude ordering of x0, y0, z0 and w0.
        // Six pair-wise comparisons are performed between each possible pair
        // of the four coordinates, and the results are used to rank the numbers.
        var rankx = 0;
        var ranky = 0;
        var rankz = 0;
        var rankw = 0;
        if (x0 > y0) rankx++;
        else ranky++;
        if (x0 > z0) rankx++;
        else rankz++;
        if (x0 > w0) rankx++;
        else rankw++;
        if (y0 > z0) ranky++;
        else rankz++;
        if (y0 > w0) ranky++;
        else rankw++;
        if (z0 > w0) rankz++;
        else rankw++;
        var i1, j1, k1, l1; // The integer offsets for the second simplex corner
        var i2, j2, k2, l2; // The integer offsets for the third simplex corner
        var i3, j3, k3, l3; // The integer offsets for the fourth simplex corner
        // simplex[c] is a 4-vector with the numbers 0, 1, 2 and 3 in some order.
        // Many values of c will never occur, since e.g. x>y>z>w makes x<z, y<w and x<w
        // impossible. Only the 24 indices which have non-zero entries make any sense.
        // We use a thresholding to set the coordinates in turn from the largest magnitude.
        // Rank 3 denotes the largest coordinate.
        i1 = rankx >= 3 ? 1 : 0;
        j1 = ranky >= 3 ? 1 : 0;
        k1 = rankz >= 3 ? 1 : 0;
        l1 = rankw >= 3 ? 1 : 0;
        // Rank 2 denotes the second largest coordinate.
        i2 = rankx >= 2 ? 1 : 0;
        j2 = ranky >= 2 ? 1 : 0;
        k2 = rankz >= 2 ? 1 : 0;
        l2 = rankw >= 2 ? 1 : 0;
        // Rank 1 denotes the second smallest coordinate.
        i3 = rankx >= 1 ? 1 : 0;
        j3 = ranky >= 1 ? 1 : 0;
        k3 = rankz >= 1 ? 1 : 0;
        l3 = rankw >= 1 ? 1 : 0;
        // The fifth corner has all coordinate offsets = 1, so no need to compute that.
        var x1 = x0 - i1 + G4; // Offsets for second corner in (x,y,z,w) coords
        var y1 = y0 - j1 + G4;
        var z1 = z0 - k1 + G4;
        var w1 = w0 - l1 + G4;
        var x2 = x0 - i2 + 2.0 * G4; // Offsets for third corner in (x,y,z,w) coords
        var y2 = y0 - j2 + 2.0 * G4;
        var z2 = z0 - k2 + 2.0 * G4;
        var w2 = w0 - l2 + 2.0 * G4;
        var x3 = x0 - i3 + 3.0 * G4; // Offsets for fourth corner in (x,y,z,w) coords
        var y3 = y0 - j3 + 3.0 * G4;
        var z3 = z0 - k3 + 3.0 * G4;
        var w3 = w0 - l3 + 3.0 * G4;
        var x4 = x0 - 1.0 + 4.0 * G4; // Offsets for last corner in (x,y,z,w) coords
        var y4 = y0 - 1.0 + 4.0 * G4;
        var z4 = z0 - 1.0 + 4.0 * G4;
        var w4 = w0 - 1.0 + 4.0 * G4;
        // Work out the hashed gradient indices of the five simplex corners
        var ii = i & 255;
        var jj = j & 255;
        var kk = k & 255;
        var ll = l & 255;
        // Calculate the contribution from the five corners
        var t0 = 0.6 - x0 * x0 - y0 * y0 - z0 * z0 - w0 * w0;
        if (t0 < 0) n0 = 0.0;
        else {
          var gi0 = (perm[ii + perm[jj + perm[kk + perm[ll]]]] % 32) * 4;
          t0 *= t0;
          n0 = t0 * t0 * (grad4[gi0] * x0 + grad4[gi0 + 1] * y0 + grad4[gi0 + 2] * z0 + grad4[gi0 + 3] * w0);
        }
        var t1 = 0.6 - x1 * x1 - y1 * y1 - z1 * z1 - w1 * w1;
        if (t1 < 0) n1 = 0.0;
        else {
          var gi1 = (perm[ii + i1 + perm[jj + j1 + perm[kk + k1 + perm[ll + l1]]]] % 32) * 4;
          t1 *= t1;
          n1 = t1 * t1 * (grad4[gi1] * x1 + grad4[gi1 + 1] * y1 + grad4[gi1 + 2] * z1 + grad4[gi1 + 3] * w1);
        }
        var t2 = 0.6 - x2 * x2 - y2 * y2 - z2 * z2 - w2 * w2;
        if (t2 < 0) n2 = 0.0;
        else {
          var gi2 = (perm[ii + i2 + perm[jj + j2 + perm[kk + k2 + perm[ll + l2]]]] % 32) * 4;
          t2 *= t2;
          n2 = t2 * t2 * (grad4[gi2] * x2 + grad4[gi2 + 1] * y2 + grad4[gi2 + 2] * z2 + grad4[gi2 + 3] * w2);
        }
        var t3 = 0.6 - x3 * x3 - y3 * y3 - z3 * z3 - w3 * w3;
        if (t3 < 0) n3 = 0.0;
        else {
          var gi3 = (perm[ii + i3 + perm[jj + j3 + perm[kk + k3 + perm[ll + l3]]]] % 32) * 4;
          t3 *= t3;
          n3 = t3 * t3 * (grad4[gi3] * x3 + grad4[gi3 + 1] * y3 + grad4[gi3 + 2] * z3 + grad4[gi3 + 3] * w3);
        }
        var t4 = 0.6 - x4 * x4 - y4 * y4 - z4 * z4 - w4 * w4;
        if (t4 < 0) n4 = 0.0;
        else {
          var gi4 = (perm[ii + 1 + perm[jj + 1 + perm[kk + 1 + perm[ll + 1]]]] % 32) * 4;
          t4 *= t4;
          n4 = t4 * t4 * (grad4[gi4] * x4 + grad4[gi4 + 1] * y4 + grad4[gi4 + 2] * z4 + grad4[gi4 + 3] * w4);
        }
        // Sum up and scale the result to cover the range [-1,1]
        return 27.0 * (n0 + n1 + n2 + n3 + n4);
      }