code: def num_to_var_int(x):
      
      x = int(x)
      if x < 253:
          return from_int_to_byte(x)
  
      elif x < 65536:
          return from_int_to_byte(253) + encode(x, 256, 2)[::-1]
  
      elif x < 4294967296:
          return from_int_to_byte(254) + encode(x, 256, 4)[::-1]
  
      else:
          return from_int_to_byte(255) + encode(x, 256, 8)[::-1]
docs: (bitcoin-specific): convert an integer into a variable-length integer