import sys
import os.path
import subprocess


def datapath(path):
    return os.path.join(os.path.dirname(__file__), 'data', path)


def cutpath(path):
    return os.path.join(os.path.dirname(__file__), 'cut', path)


class FilesDifferent(Exception):
    pass


def assert_files_equal(path1, path2, ignore_trailing_space: bool = False):
    cmd = ["diff", "-u"]
    if sys.platform == "win32":
        cmd.append("--strip-trailing-cr")
    if ignore_trailing_space:
        if sys.platform == "darwin":
            # Ignores too much, but macOS doesn’t have the option below
            cmd.append("-b")
        else:
            cmd.append("--ignore-trailing-space")
    try:
        subprocess.check_output(cmd + [path1, path2], stderr=subprocess.STDOUT)
    except subprocess.CalledProcessError as e:
        raise FilesDifferent('\n' + e.output.decode()) from None


def binomial(n, k):
    """
    Return binomial coefficient ('n choose k').
    This implementation does not use factorials.
    """
    k = min(k, n - k)
    if k < 0:
        return 0
    r = 1
    for j in range(k):
        r *= n - j
        r //= j + 1
    return r