Two's Complement in Python

README.md

Two's Complement in Python

Command line interface

• enter a twos-complement number with the 0b prefix to convert to decimal
• enter a decimal value and the width (in bits) to convert to two's complement

test_twoscomp.py

import unittest
import twoscomp

class ToStringTestCase(unittest.TestCase):

    def _run_cases(self, test_cases):
        for value, width, expected in test_cases:
            with self.subTest():
                actual = twoscomp.to_string(value, width)
                self.assertEqual(expected, actual, f"on input {value}")

    def test_positives(self):
        def to_test_case(x):
            return x, 8, "{:010b}".format(x)[2:]
        self._run_cases(map(to_test_case, range(128)))

    def test_negatives(self):
        self._run_cases([
            (-43, 8, '11010101'),
            (-123, 8, '10000101'),
            (-63, 8, '11000001'),
            (-7, 8, '11111001'),
            (-1, 8, '11111111'),
        ])

class FromStringTest(unittest.TestCase):

    def _run_cases(self, test_cases):
        for value, width, expected in test_cases:
            with self.subTest():
                actual = twoscomp.from_string(value, width)
                self.assertEqual(expected, actual, f"on input {value}")

    def test_positives(self):
        def to_test_case(x):
            return "{:010b}".format(x)[2:], 8, x
        self._run_cases(map(to_test_case, range(0, 128)))

    def test_negatives(self):
        self._run_cases([
            ('11010101', 8, -43),
            ('10000101', 8, -123),
            ('11000001', 8, -63),
            ('11111001', 8, -7)
        ])


if __name__ == '__main__':
    unittest.main()

twoscomp.py

#!/usr/bin/env python3


def to_string(value: int, width: int=None) -> str:
    if width is None:
        width = 8  # TODO choose width based on magnitude of value
    if value < 0:
        value = 2 ** width + value
    template = "{:0" + str(width + 2) + "b}"
    return template.format(value)[2:]


def from_string(twoscomp_str: str, width: int=None) -> int:
    if len(twoscomp_str) == 0:
        raise ValueError("input string must be nonempty")
    if twoscomp_str.startswith('0b'):
        twoscomp_str = twoscomp_str[2:]
    if twoscomp_str.startswith('0'):
        return int(twoscomp_str, 2)
    if width is None:
        width = len(twoscomp_str)
    raw_value = int(twoscomp_str, 2)
    return -(2 ** width - raw_value)


def main():
    import sys
    if len(sys.argv) >= 2:
        arg = sys.argv[1]
        width = None if len(sys.argv) == 2 else int(sys.argv[2])
        if arg.startswith('0b'):
            print(from_string(arg, width))
        else:
            value = int(arg)
            print(to_string(value, width))
    return 0


if __name__ == '__main__':
    exit(main())