<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def encode_inputs(self):
 """Return a compact encoding for input variables."""

        litmap = dict()
        nvars = 0
        for i, v in enumerate(self.inputs, start=1):
            litmap[v] = i
            litmap[~v] = -i
            litmap[i] = v
            litmap[-i] = ~v
            nvars += 1
        return litmap, nvars