roman.py

#!/usr/bin/env python3


ROMAN_LEGEND = {
  'I': 1,
  'V': 5,
  'X': 10,
  'L': 50,
  'C': 100,
  'D': 500,
  'M': 1000,
}
LEGEND_ROMAN = {v: k for k, v in ROMAN_LEGEND.items()}


def _subtractive(roman, i):
  return any(ROMAN_LEGEND[roman[j]] > ROMAN_LEGEND[roman[i]] for j in range(i + 1, len(roman)))


def roman_to_decimal(roman):
  roman_values = [ROMAN_LEGEND[c] for c in roman]
  ##return sum(roman_values)  # naive, misses e.g. IX, XC, etc.
  #new_romans = []
  #for i, x in enumerate(roman_values):
  #  new_romans.append(x)
  #  try:
  #    if roman_values[i + 1] > x:
  #      new_romans[i] = -x
  #  except IndexError:
  #    break
  #return sum(new_romans)  # almost, misses alternative forms e.g. XIIX, IIXX, etc.
  return sum(-x if _subtractive(roman, i) else x for i, x in enumerate(roman_values))



def decimal_to_roman(decimal):
  roman = ''
  for key in sorted(LEGEND_ROMAN.keys(), reverse=True):
    while decimal >= key:
      roman += LEGEND_ROMAN[key]
      decimal -= key
  return roman  # naive, does not use the subtractive form


ROMAN_DECIMALS = {
  'I': 1,
  'V': 5,
  'VI': 6,
  'IV': 4,
  'X': 10,
  'IX': 9,
  'XI': 11,
  'XV': 15,
  'L': 50,
  'C': 100,
  'XC': 90,
  'XCIX': 99,
  'CM': 900,
  'MDCCLXXVI': 1776,
  'MCMIII': 1903,
  'MCMX': 1910,
  'MCMLIV': 1954,
  'MCMXC': 1990,
  'MMXIV': 2014,
}

ROMAN_ALTERNATIVES = {
  'IIII': 4,
  'VIIII': 9,
  'XIIX': 18,
  'IIXX': 18,
  'IC': 99,
  'MDCDIII': 1903,
  'MDCCCCX': 1910,
}



def test_roman_to_decimal():
  #assert all(roman_to_decimal(roman) == decimal for roman, decimal in ROMAN_DECIMALS.items())
  for roman, decimal in ROMAN_DECIMALS.items():
    assert roman_to_decimal(roman) == decimal


def test_roman_to_decimal_alt():
  #assert all(roman_to_decimal(roman) == decimal for roman, decimal in ROMAN_ALTERNATIVES.items())
  for roman, decimal in ROMAN_ALTERNATIVES.items():
    assert roman_to_decimal(roman) == decimal


def test_decimal_to_roman():
  #assert all(decimal_to_roman(decimal) == roman for roman, decimal in ROMAN_DECIMALS.items())
  for roman, decimal in ROMAN_DECIMALS.items():
    assert decimal_to_roman(decimal) == roman


# It doesn't make sense to test alternative forms against test_decimal_to_roman, which should return the canonical form.