code: function _makeCosSinTable() {
      var n2 = _n >> 1,
          n4 = _n >> 2,
          n8 = _n >> 3,
          n2p4 = n2 + n4,
          t = Math.sin(Math.PI/_n),
          dc = 2*t*t,
          ds = Math.sqrt(dc*(2 - dc)),
          c = _cstb[n4] = 1,
          s = _cstb[0] = 0;
      t = 2*dc;
      for(var i=1; i<n8; i++) {
        c -= dc;
        dc += t*c;
        s += ds;
        ds -= t*s;
        _cstb[i] = s;
        _cstb[n4 - i] = c;
      }
      if(n8 !== 0) {
        _cstb[n8] = Math.sqrt(0.5);
      }
      for(var j=0; j<n4; j++) {
        _cstb[n2 - j]  = _cstb[j];
      }
      for(var k=0; k<n2p4; k++) {
        _cstb[k + n2] = -_cstb[k];
      }
    }
docs: make trigonometric function table