import re


SINGULARS = (
    (r'ives', 'ife'),
    (r'ves', 'f'),
    (r'zes', ''),
    (r'men', 'man'),
    (r'matrices', 'matrix'),
    (r'([aeiou])ys', '\\1y'),
    (r'([^aeiou])ies', '\\1y'),
    (r'([^aeiou])ices$','\\1ice'),
    (r'ices', 'ex'),
    (r'([sxz]|[cs]h)es$', '\\1'),
    (r'eet', 'oot'),
    (r'ies', 'y'),
    (r'i', 'us'),
    (r's', ''),
)
PLURALS = (
    (r'man$', 'men'),
    (r'([aeiou]y)', '\\1s'),
    (r'([^aeiou])y', '\\1ies'),
    (r'([sxz]|[cs]h)', '\\1es'),
    (r'(cello|disco|hairdo|logo|patio|photo|piano|radio|chef|cliff|ref|roof)',
     '\\1s'),
    (r'o', 'oes'),
    (r'fe', 'ves'),
    (r'eaf', 'eaves'),
    (r'[ei]x', 'ices'),
    (r'us', 'i'),
    (r'', 's'),
)
EXCEPTIONS_SINGULAR = {
    'person': 'people',
    'child': 'children',
    'ox': 'oxen',
    'foot': 'feet',
    'tooth': 'teeth',
    'goose': 'geese',
    'mouse': 'mice',
    'louse': 'lice',
    'man': 'men',
    'woman': 'women',
    'goose': 'geese',
}
EXCEPTIONS_PLURAL = dict([p[::-1] for p in EXCEPTIONS_SINGULAR.iteritems()])
SAME = ('equipment', 'information', 'rice', 'money', 'species', 'series',
        'fish', 'sheep', 'deer', 'tuna', 'salmon', 'trout')


def apply_rules(word, rules):
    word = word.strip()

    if word in SAME:
        return word

    for pattern,replacement in rules:
        word, n =  re.subn(pattern + '$', replacement, word)

        if n:
            break

    return word


def singularize(word):
    if word in EXCEPTIONS_PLURAL:
        return EXCEPTIONS_PLURAL[word]

    return apply_rules(word, SINGULARS)


def pluralize(word):
    if word in EXCEPTIONS_SINGULAR:
        return EXCEPTIONS_SINGULAR[word]

    return apply_rules(word, PLURALS)