romans.py

import re

mapping = {
    'i': (1, 1, 1),
    'v': (5, 5, 1),
    'x': (10, 1, 2),
    'l': (50, 5, 2),
    'c': (100, 1, 3),
    'd': (500, 5, 3),
    'm': (1000, 1, 4)
}


def get_same_degree_val(typ, degree):
    for x in mapping:
        if mapping[x][2] == degree and typ != mapping[x][1]:
            return x


def read_chunk(rom_str, prev_degree=None):
    first = rom_str[0]
    s = 0
    val, typ, degree = mapping[first]
    if prev_degree and degree >= prev_degree:
        raise Exception('malformed input: prev {} vs current {} in degrees for {}'.format(prev_degree, degree, rom_str))
    if typ == 5:
        same_degree = get_same_degree_val(typ, degree)
        s += val
        re_m = '^'+first+same_degree+'{0,3}'
        match = re.match(re_m, rom_str, re.I).group()
        s += (len(match)-1)*mapping[same_degree][0]
        out = rom_str[len(match):]
    else:
        if len(rom_str) == 1:
            return val, degree, ''
        if rom_str[1] != first and mapping[rom_str[1]][2] > degree:
            if mapping[rom_str[1]][2] == degree+1:
                return mapping[rom_str[1]][0] - val, degree, rom_str[2:]
            else:
                raise Exception('malformed input')
        re_m = '^'+first+'{,3}'
        match = re.match(re_m, rom_str).group()
        s = len(match) * val
        out = rom_str[len(match):]

    return s, degree, out


def convert_roman(rom_str):
    degree = None
    val = 0
    while True:
        cur_val, degree, rom_str = read_chunk(rom_str, degree)
        val += cur_val
        if not rom_str:
            break
        if degree == 1 and rom_str:
            raise Exception('malformed input')
    return val

print(convert_roman('cccviii'.lower()))