"""Some support functions"""


def code0():
    """A trivial code - no change."""
    return {}


def code1():
    """A very simple example (symmetric)."""
    return {"e": "x", "x": "e"}


def code2():
    """A simple example i->s, s->g and g->i."""
    return {"i": "s", "s": "g", "g": "i"}


def code3():
    """A more complicated code."""
    dic = {}
    # lower case letters
    dic["z"] = "a"
    for i in range(ord("a"), ord("z")):
        dic[chr(i)] = chr(i + 1)
    # upper case letters
    dic["Z"] = "A"
    for i in range(ord("A"), ord("Z")):
        dic[chr(i)] = chr(i + 1)
    # space, stop and some other special characters
    dic[" "] = "$"
    dic["."] = "#"
    dic["#"] = "."
    dic["$"] = " "
    dic["?"] = "!"
    dic["!"] = "?"
    return dic


def check_code_is_reversible(dic):
    """Given a dictionary used as a code mapping, this function checks
    whether the set of keys is the same set of values: if the elements
    in the keys are the same unique set as those in the values, then
    this mapping is bijective (the set of values is then actually a
    permutation of the set of input values) and can be inverted.

    If this is not the case, some debug information is printed, and a
    ValueError exception raised.
    """

    unique_keys = set()
    unique_vals = set()
    for key, val in dic.items():
        unique_keys.add(key)
        unique_vals.add(val)
    if unique_vals != unique_keys:
        print("Code is not reversible:")
        print("keys are   %s", sorted(unique_keys))
        print("values are %s", sorted(unique_vals))
        raise ValueError("Code is not reversible - stopping here")
    return True


def test_codes():
    """Check that codes defined above are reversible."""
    for c in (code0(), code1(), code2(), code3()):
        assert check_code_is_reversible(c)


secretmessage \
    = "Zpv$ibwf$tvddfttgvmmz$efdpefe$uijt$tfdsfu$nfttbhf#$Dpohsbuvmbujpot?"


# if this file is executed on it's own, check codes given
if __name__ == "__main__":
    test_codes()