Testing for Dyck words. Inspired by: http://raganwald.com/2018/11/14/dyck-joke.html

dyck.py

#!/usr/bin/env python3


def is_dyck_word(word):
    i = 0
    for ch in word:
        if ch == '[':
            i += 1
        elif ch == ']':
            i -= 1
        # If at any point it goes below 0, there's an unbalanced ']'
        if i < 0:
            return False
    # If there are unbalanced '[', i > 0.
    return i == 0


def main():
    positive_tests = [
        "",
        "[]"
        "[][]",
        "[[]]",
    ]
    negative_tests = [
        "][",
        "[]]",
        "[][][",
        "][][",
    ]
    for test in positive_tests:
        assert is_dyck_word(test), f"Should be a dyck word: {test}"

    for test in negative_tests:
        assert not is_dyck_word(test), f"Should not be a dyck word: {test}"


if __name__ == "__main__":
    main()