CS50 PS2 solution with tests

README.md

A couple of solutions to CS50 problem 2 ("vanity plates") along with a simple test suite.

plates.py

def is_valid_slicing(plate) -> bool:
    '''Use string slicing and iteration to validate
    license plates.'''

    # Too short or too long
    if len(plate) < 2 or len(plate) > 6:
        return False

    # Must start with two letters
    if not plate[:2].isalpha():
        return False

    # No punctuation, etc
    if not plate.isalnum():
        return False

    # First number cannot be 0, numbers must be
    # at the end.
    for i, c in enumerate(plate[2:], 2):
        if c.isdecimal():
            if c == "0":
                return False
            if not plate[i:].isdecimal():
                return False
            break

    return True


def is_valid_statemachine(plate) -> bool:
    '''Use a state machine to valid license plates.'''

    # vanity plates may contain a maximum of 6 characters
    # and a minimum of 2 characters
    if len(plate) < 2 or len(plate) > 6:
        return False

    state = 0
    for i, c in enumerate(plate):
        # vanity plates may contain...letters or numbers
        # No periods, spaces, or punctuation marks are allowed
        if not (c.isdecimal() or c.isalpha()):
            return False

        if state == 0:
            # All vanity plates must start with at least two letters
            if c.isdecimal():
                return False
            if i == 1:
                state = 1
        elif state == 1:
            # If we find a number, everything else needs to be numbers
            if c.isdigit():
                # First number used cannot be '0'
                if c == "0":
                    return False
                state = 2
        elif state == 2:
            # Letters after numbers is not allowed
            if c.isalpha():
                return False

    return True

test_plates.py

import pytest

from plates import is_valid_slicing, is_valid_statemachine


test_cases = (
        ("A", False),  # Too short
        ("111111", False),  # Does not start with letters
        ("22AAAA", False),  # Does not start with letters
        ("AA..AA", False),  # Contains non-alphanumerics
        ("AA0000", False),  # The first number used cannot be 0
        ("A22222", False),  # Must start with two letters
        ("AA22AA", False),  # Numbers cannot be in middle
        ("AAAAAAA", False),  # Too long
        ("aaaaaaa", False),  # Letters must be uppercase
        ("AA", True),
        ("AA2222", True),
        ("AAAAAA", True),

        # Test cases from https://cs50.harvard.edu/python/2022/psets/2/plates/
        ("CS50", True),
        ("CS05", False),
        ("CS50P", False),
        ("PI3.14", False),
        ("H", False),
        ("OUTATIME", False),

        # What about non-English characters? The RMV isn't actually
        # explicit about what constitutes a valid character set, so
        # these are technically valid.
        ("NIÑO", True),
        ("김세정", True),
    )

@pytest.mark.parametrize(
    "plate,valid", test_cases,
)
def test_is_valid_slicing(plate, valid):
    assert is_valid_slicing(plate) == valid

@pytest.mark.parametrize(
    "plate,valid", test_cases,
)
def test_is_valid_statemachine(plate, valid):
    assert is_valid_statemachine(plate) == valid