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#!/usr/bin/env python3
"""Implementation of RFC 1982 Serial Number Arithmetic.
Run as a program, this takes start and finish serial values and tells
you what intermediate steps (if any) are needed to make the specified
change.
Used as a module, this implements the Serial class and an iterator
which returns the intermediate steps (if any) needed between two
Serial values.
"""
class Serial:
"""Implementation of RFC 1982 Serial Number Arithmetic.
Per the RFC, only two operations are defined on Serial objects:
addition and comparision, both within a restricted range specified
by the RFC.
The default modulus is 2**32, you can change this by subclassing
the Serial class and overriding the class's modulus variable. The
modulus must be a power of two.
See RFC 1982 for discussion of the ways in which Serial numbers do
not work like normal integers. In particular, note that there's a
corner case in which one can have a pair of Serial numbers I1 and
I2 where I1 is neither equal to, less than, nor greater than I2.
This is deliberate and is not a bug in the code. See the RFC.
"""
modulus = 2 ** 32
def __init__(self, val):
self._val = int(val)
if self._val < 0 or self._val >= self.modulus:
raise ValueError
def __add__(self, val):
val = int(val)
if val < 0 or val > (self.modulus - 1) >> 1:
raise ValueError
return type(self)((self._val + val) & (self.modulus - 1))
def __le__(self, other):
return (self.modulus - int(self) + int(other)) & (self.modulus >> 1) == 0
def __ge__(self, other):
return (self.modulus + int(self) - int(other)) & (self.modulus >> 1) == 0
def __eq__(self, other):
return int(self) == int(other)
def __ne__(self, other):
return int(self) != int(other)
def __lt__(self, other):
return self != other and self <= other
def __gt__(self, other):
return self != other and self >= other
def __int__(self):
return self._val
def __str__(self):
return f"{int(self):{len(str(self.modulus))}d}"
def find_intermediate(start, finish):
"""
Find the sequence of intermediate values (if any) needed to step
from start to finish
"""
while not (start < finish): # sic: serial numbers are not (quite) integers
start += ((Serial.modulus >> 1) - 1)
yield start
def main():
from argparse import ArgumentParser
ap = ArgumentParser(description = __doc__)
ap.add_argument("start", type = Serial)
ap.add_argument("finish", type = Serial)
args = ap.parse_args()
print(f"Start at {args.start!s}")
if args.start < args.finish:
print("No step needed")
else:
for wrap in find_intermediate(args.start, args.finish):
print(f"Step via {wrap!s}")
print(f"End at {args.finish!s}")
if __name__ == "__main__":
main()
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