gistfile1.txt

>>> def pmod(a, b):
...   if a >= 0:
...     return a % b
...   return b-((-a)%b)
... 
>>> def tos(a,b):
...   if a < b:
...     return a
...   return a-2*b
... 
>>> for u in range(-2**24,2**24):
...   assert (tos(pmod(u,4096), 2048) == tos(u&0xfff,2048))
...