88 lines
2.6 KiB
Python
88 lines
2.6 KiB
Python
"""
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An implementation of Damm check digits.
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"""
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import re
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from typing import NamedTuple
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class DammException(Exception):
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"""Base class for all exceptions in the module."""
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class CheckDigitException(DammException):
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"""Thrown when a string did not check when decoding a supposedly checked string."""
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class NoMatchException(DammException):
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"""Thrown when a string did not match."""
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class Damm(NamedTuple):
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"""A Damm coded number."""
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MATRIX = (
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(0, 3, 1, 7, 5, 9, 8, 6, 4, 2),
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(7, 0, 9, 2, 1, 5, 4, 8, 6, 3),
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(4, 2, 0, 6, 8, 7, 1, 3, 5, 9),
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(1, 7, 5, 0, 9, 8, 3, 4, 2, 6),
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(6, 1, 2, 3, 0, 4, 5, 9, 7, 8),
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(3, 6, 7, 4, 2, 0, 9, 5, 8, 1),
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(5, 8, 6, 9, 7, 2, 0, 1, 3, 4),
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(8, 9, 4, 5, 3, 6, 2, 0, 1, 7),
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(9, 4, 3, 8, 6, 1, 7, 2, 0, 5),
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(2, 5, 8, 1, 4, 3, 6, 7, 9, 0),
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)
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DAMM_PATTERN = re.compile(r"(?P<value>\d+)-?(?P<check>\d)")
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value: int
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def __str__(self):
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return f"{self.value}-{self.encode(self.value)}"
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def __repr__(self):
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return f"{self.__class__.__name__}({self.value}, {self.encode(self.value)})"
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@classmethod
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def checkdigit(cls, value: int) -> int:
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"""Compute a check digit for a given value."""
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state = 0
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for digit in str(value):
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state = cls.MATRIX[state][int(digit)]
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return state
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@classmethod
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def encode(cls, value: int) -> str:
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"""'encode' a value with a check digit."""
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return f"{value}-{cls.checkdigit(value)}"
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@classmethod
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def verify(cls, value: str) -> bool:
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"""Verify a Damm encoded number, returning its value if valid.
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The core Damm encoding property is that the LAST digit is ALWAYS the check. When the rest of
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the string is correctly Damm encoded, the Damm of the entire string will be 0.
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"""
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return cls.checkdigit(int(value.replace("-", ""))) == 0
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@classmethod
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def from_str(cls, value: str) -> "Damm":
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"""Verify a Damm coded string, and return its decoding."""
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if match := re.fullmatch(cls.DAMM_PATTERN, value):
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given_value = match.group("value")
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computed_code = cls.encode(given_value)
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given_code = int(match.group("check"))
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if computed_code == given_code:
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return Damm(int(given_value))
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else:
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raise CheckDigitException(
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f"Value {value!r} had check digit {given_code!r}, but computed check was {computed_code!r}"
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)
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else:
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raise NoMatchException(f"Input {value!r} was not a valid decimal number")
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