docs: private byte[] bufX( long bias, int scale, int off, int log ) {
      byte[] bs = new byte[(_len<<log)+off];
      int j = 0;
      for( int i=0; i<_len; i++ ) {
        long le = -bias;
        if(_id == null || _id.length == 0 || (j < _id.length && _id[j] == i)){
          if( isNA2(j) ) {
            le = NAS[log];
          } else {
            int x = (_xs[j]==Integer.MIN_VALUE+1 ? 0 : _xs[j])-scale;
            le += x >= 0
                ? _ls[j]*PrettyPrint.pow10i( x)
                : _ls[j]/PrettyPrint.pow10i(-x);
          }
          ++j;
        }
        switch( log ) {
          case 0:          bs [i    +off] = (byte)le ; break;
          case 1: UDP.set2(bs,(i<<1)+off,  (short)le); break;
          case 2: UDP.set4(bs,(i<<2)+off,    (int)le); break;
          case 3: UDP.set8(bs,(i<<3)+off,         le); break;
          default: throw H2O.fail();
        }
      }
      assert j == _sparseLen:"j = " + j + ", len = " + _sparseLen + ", len2 = " + _len + ", id[j] = " + _id[j];
      return bs;
    }