solution_17.py

def coding_problem_17(path_str):
    r"""
    Suppose we represent our file system by a string in the following manner:
    The string "dir\n\tsubdir1\n\tsubdir2\n\t\tfile.ext" represents:

        dir
            subdir1
            subdir2
                file.ext

    The directory dir contains an empty sub-directory subdir1 and a sub-directory subdir2 containing a file file.ext.
    The string "dir\n\tsubdir1\n\t\tfile1.ext\n\t\tsubsubdir1\n\tsubdir2\n\t\tsubsubdir2\n\t\t\tfile2.ext" represents:

        dir
            subdir1
                file1.ext
                subsubdir1
            subdir2
                subsubdir2
                    file2.ext

    The directory dir contains two sub-directories subdir1 and subdir2. subdir1 contains a file file1.ext and an empty
    second-level sub-directory subsubdir1. subdir2 contains a second-level sub-directory subsubdir2 containing a
    file file2.ext.

    We are interested in finding the longest (number of characters) absolute path to a file within our file system.
    For example, in the second example above, the longest absolute path is "dir/subdir2/subsubdir2/file2.ext", and its
    length is 32 (not including the double quotes).

    Given a string representing the file system in the above format, return the length of the longest absolute path to
    a file in the abstracted file system. If there is no file in the system, return 0.

    The name of a file contains at least a period and an extension.
    The name of a directory or sub-directory will not contain a period.
    Examples:
    
    >>> coding_problem_17('file1.ext')
    9

    >>> coding_problem_17('dir\n\tfile1.ext')
    13

    >>> coding_problem_17('0\n\t23.567\n12345.789')
    9

    >>> coding_problem_17('dir\n\t\tfile1.ext')
    Traceback (most recent call last):
    ...
    RuntimeError: Malformed path string: nesting more than one level at a time.

    >>> coding_problem_17('dir\n\tfile1.ext\n\t\tchild_of_a_file.ext')
    Traceback (most recent call last):
    ...
    RuntimeError: Malformed path string: a file cannot contain something else.

    >>> coding_problem_17('dir\n\tfile1.ext\n\tsubdir\n\t\tsubsubdir\n\t\t\ttsubsubsubdir')
    13

    >>> coding_problem_17('dir\n\tsubdir1\n\t\tfile1.ext\n\t\tsubsubdir1\n\tsubdir2\n\t\tsubsubdir2\n\t\t\tfile2.ext')
    32
    
    >>> coding_problem_17('dir\n\tsubdir1\n\t\tfile1.ext\n\t\tsubsubdir1\n\tsubdir2\n\t\tsubsubdir2\n\t\t\tfile2.ext' +
    ...                   '\ndir2\n\tsubdir1\n\tsubdir2\n\t\tsubsubdir1\n\t\t\tsubsubsubdir3\n\t\t\t\tfile3.ext')
    47
    """
    if not path_str:
        return 0

    dirs, max_len = [None], 0
    for token in path_str.split('\n'):

        tabs = 0
        while token[tabs] == '\t':
            tabs += 1

        if tabs > len(dirs):
            raise RuntimeError('Malformed path string: nesting more than one level at a time.')

        if tabs == len(dirs):  # go one level deeper

            if '.' in dirs[-1]:  # container is a file
                raise RuntimeError('Malformed path string: a file cannot contain something else.')

            dirs.append(None)  # make room for the new path item

        else:  # tabs < level, ascend

            dirs = dirs[:tabs + 1]

        dirs[-1] = str.strip(token)
        if '.' in dirs[-1]:  # path ends with a file
            max_len = max(max_len, len('/'.join(dirs)))

    return max_len


if __name__ == '__main__':

    import doctest
    doctest.testmod(verbose=True)