Create nested hash from params with plain js?

I have this clever little piece of vanilla Javascript that converts url params into a hash:

"a=a&b=b".split("&").reduce((function(params, param) {
  var param_split = param.split("=").map(function(value) {
    return decodeURIComponent(value.replace("+", " "));
  });
  params[param_split[0]] = param_split[1];
  return params;
}), {});

Which outputs this hash:

{
  a: "a",
  b: "b"
}

But sometimes I get url params that are nested like this:

"a=a&b%5Bc%5D=bc&b%5Bd%5D=bd"

My script would output:

{
  a: "a",
  b[c]: "bc",
  b[d]: "bd"
}

What should I do in order to be able to output a nested hash like this?:

{
  a: "a",
  b: {
    c: "bc",
    d: "bd"
  }
}