how to define hf_register_info in dissector

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I get data in tvbuff_t *tvb, its size is 10 to 50 bit depend on another parameter assume p.
Message structure is as below:

Struct AA  
{    
      Var a : x bit, [0<=range(x)<=32]   
      Var b : y bit, [0<=range(y)<=16]  
};

define static hf_register_info hf[] in dissector, and register it to proto as

proto_register_field_array(proto_xxx, hf, array_length(hf));

Should I define 32+16=48 hf_xxx with different bit_mask and byte_type and select different two hf_xxx for a and b according to p?
Is there any good method to solve this problem?