This kata is a continuation of [Part 1](http://www.codewars.com/kata/the-fibfusc-function-part-1). The `fibfusc` function is defined recursively as follows:

    fibfusc(0) = (1, 0)
    fibfusc(1) = (0, 1)
    fibfusc(2n) = ((x + y)(x - y), y(2x + 3y)), where (x, y) = fibfusc(n)
    fibfusc(2n + 1) = (-y(2x + 3y), (x + 2y)(x + 4y)), where (x, y) = fibfusc(n)

Your job is to produce the code for the `fibfusc` function. In this kata, your function will be tested with large values of n (up to 2000 bits), so you should be concerned about stack overflow and timeouts. Moreover, since the `fibfusc` function grows exponentially, your function should take an extra argument `num_digits` with a default value of `None`. When not `None`, `num_digits` takes a positive `int` value and the returned values shall be truncated to the last `num_digits` digits. 

For example, let `n = 101`. With `num_digits` not set or set to `None`, `fibfusc` should return: 
`(-280571172992510140037611932413038677189525L, 734544867157818093234908902110449296423351L)`. If `num_digits = 12`, the function should return `(-38677189525L, 449296423351L)`. Notice in particular, that for any value of `n` greater than 1, `x` is negative, and the truncated value of `x` should also be negative.

Hint 1: Use modulo `10 ** num_digits` arithmetic to keep all intermediary results small.

Hint 2: Consider using an iterative ["number climber"](http://www.codewars.com/kata/number-climber) to avoid stack overflows.

```python

USER_FILENAME = "<user_code>"
import sys, json, builtins
import math, heapq, bisect, itertools, functools, collections, statistics, fractions, decimal, operator, array, random, re, string, os, pathlib
try:
    import resource as _res
except Exception:
    _res = None
from collections import Counter, defaultdict, deque, OrderedDict
from functools import lru_cache, reduce
from itertools import accumulate, permutations, combinations, product
from math import gcd, ceil, floor, sqrt
from decimal import Decimal
try:
    from typing import *
except Exception:
    pass
try:
    sys.setrecursionlimit(1_000_000)
except Exception:
    pass
try:
    import site as _site
    try:
        _sys_sites = _site.getsitepackages() if hasattr(_site, "getsitepackages") else []
        for _p in _sys_sites:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
    try:
        _usp = _site.getusersitepackages()
        if isinstance(_usp, str):
            _usp = [_usp]
        for _p in _usp:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
except Exception:
    pass

def _disabled_input(*args, **kwargs):
    raise RuntimeError("input() is disabled in callable mode")
builtins.input = _disabled_input

ns = {"__name__": "__judge__"}  # prevent if __name__ == "__main__" blocks
for _k, _v in list(globals().items()):
    if not _k.startswith("_"):
        ns[_k] = _v
_CODE = 'def fibfusc(n, num_digits=None):\n    m = 10 ** num_digits if num_digits is not None else None\n    x, y = 1, 0\n    for i in reversed(range(n.bit_length())):\n        if n & 1 << i == 0:\n            x, y = ((x + y) * (x - y), y * (2*x + 3*y))\n        else:\n            x, y = (-y * (2*x + 3*y), (x + 2*y) * (x + 4*y))\n        if m is not None:\n            x %= m\n            y %= m\n    if m is not None and n >= 2:\n        x -= m\n    return x, y'
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'fibfusc'
if _fn_name:
    if "Solution" in ns:
        try:
            _obj = ns["Solution"]()
            if hasattr(_obj, _fn_name):
                _target = getattr(_obj, _fn_name)
        except Exception:
            _target = None
    if _target is None and _fn_name in ns and callable(ns[_fn_name]):
        _target = ns[_fn_name]
# Fallback: if there's exactly one user-defined function, use it
if _target is None:
    _cands = [v for k, v in ns.items() if callable(v) and getattr(v, "__module__", "") in ("__judge__", "__main__")]
    _user_funcs = [c for c in _cands if getattr(c, "__name__", "").startswith("_") is False]
    if len(_user_funcs) == 1:
        _target = _user_funcs[0]
if _target is None:
    raise RuntimeError(f"Could not resolve callable '{_fn_name}'")

_raw = sys.stdin.read().strip()
if _raw:
    _parsed = json.loads(_raw)
    if isinstance(_parsed, dict):
        _kwargs = _parsed.get("kwargs", {})
        _args = _parsed.get("args", [])
    else:
        _args, _kwargs = _parsed, {}
else:
    _args, _kwargs = [], {}
if not isinstance(_args, (list, tuple)):
    _args = [_args]
_res = _target(*_args, **_kwargs)
try:
    sys.stdout.write(repr(_res) + "\n")
except Exception:
    pass

```

Measured memory_bytes: 5586