roman.py

#! /usr/bin/env python
"""
Convert roman numerals. To test: `nosetests roman.py`.
"""

import nose.tools

KNOWN_VALUES = [
    (1, 'I'),
    (5, 'V'),
    (10,'X'),
    (50,'L'),
    (100,'C'),
    (500,'D'),
    (1000,'M'),
    
    (2, "II"),
    (3, "III"),
    (4, "IV"),
    
    (200,"CC"),
    
    (9, "IX"),
    (19, "XIX"),
    (49, "XLIX"),
    (99, "XCIX"),
    
    (207, "CCVII"),
    (1904, "MCMIV"),
    (1954, "MCMLIV"),
    (2013, "MMXIII"),
    (400, "CD"),
    (900, "CM"),
    (10000, "MMMMMMMMMM"),
    ]



SYMBOLS = [
    (1, 'I'),
    (5, 'V'),
    (10,'X'),
    (50,'L'),
    (100,'C'),
    (500,'D'),
    (1000,'M'),
    ]


def convert(number):
    big_string = number*'I'
    return big_string_to_small(big_string)

def big_string_to_small(big_string):
    """
    >>> big_string_to_small('I'*1000)
    'M'
    >>> big_string_to_small('I'*500)
    'D'
    >>> big_string_to_small('I'*100)
    'C'
    >>> big_string_to_small('I'*105)
    'CV'
    """
    result = big_string
    for (number, text) in reversed(SYMBOLS):
        if number*'I' in result:
            # Replace all runs of "I" with the appropriate symbol
            result = result.replace(number*'I',text)
    
    return replace_repeats(result)

def replace_repeats(roman):
    """
    >>> replace_repeats('IIII')
    'IV'
    >>> replace_repeats('XXXX')
    'XL'
    >>> replace_repeats(convert(404))
    'CDIV'
    """
    result = roman
    
    # iterate over all of the symbols, looking two ahead each iteration
    symbols = zip(SYMBOLS, SYMBOLS[1:], SYMBOLS[2:])
    for (_, smaller), (_, bigger), (_, BIGGER) in symbols:
    
        # Substitute four consecutive symbols (IIII) for a more simple
        # representation (IV).
        if 4*smaller in result:
            target = "{}{}".format(smaller, bigger)
            result = result.replace(4*smaller, target)
        
        # If a pattern like "VIV" exists, there is a more compact
        # representation: IX.
        aba = "{0}{1}{0}".format(bigger, smaller)
        if aba in result:
            target = "{}{}".format(smaller, BIGGER)
            result = result.replace(aba, target)
            result = replace_repeats(result)
        
    return result
    
def test_known_value():
    for input_, output in KNOWN_VALUES:
        yield _check_known_value, input_, output

def _check_known_value(input_, output):
    nose.tools.eq_(convert(input_), output)

Awesome!