gp7

  function permute1(x) { 
      if (x.length == 1) return x;
      let even = [];
      let odd = []; 
      for (let i = 0; i < x.length; i += 2) { 
           even[i / 2] = x[i];
           odd[i / 2] = x[i + 1];
      }
      return [].concat(permute1(even), permute1(odd));
  }

  function permute2(x, offset, stride) {
      if (!offset) offset = 0;
      if (!stride) stride = 1;
      if (stride >= x.length) return [x[offset]];
      return [].concat(permute2(x, offset, stride * 2), permute2(x, offset + stride, stride * 2));
  }
  
  function permute3(x) {
      let result = [];
      for (let i = 0; i < x.length; i++) {
          let k = i;
          // pretend 32-bit ints
          k = ((k >> 1) & 0x55555555) | ((k & 0x55555555) << 1);
          k = ((k >> 2) & 0x33333333) | ((k & 0x33333333) << 2);
          k = ((k >> 4) & 0x0F0F0F0F) | ((k & 0x0F0F0F0F) << 4);
          k = ((k >> 8) & 0x00FF00FF) | ((k & 0x00FF00FF) << 8);
          k = ( k >> 16             ) | ( k               << 16); 
          k = k >> (64 - Math.log2(x.length));
          if (k < 0) k += x.length; // fix up due to signed ints
          result[i] = x[k];
      }
      return result;
  }