Numbah Convertah

number_to_spoken_number.py

#!/usr/bin/env python

"""
Number-to-spoken-numbers converter.
"""

max_value = 10**66 - 1
big_words = {
    3: "thousand",
    6: "million",
    9: "billion",
    12: "trillion",
    15: "quadrillion",
    18: "quintillion",
    21: "sextillion",
    24: "septillion",
    27: "octillion",
    30: "nonillion",
    33: "decillion",
    36: "undecillion",
    39: "duodecillion",
    42: "tredecillion",
    45: "quattuordecillion",
    48: "quindecillion",
    51: "sexdecillion (sedecillion)",
    54: "septendecillion",
    57: "octodecillion",
    60: "novemdecillion (novendecillion)",
    63: "vigintillion",
}
exponents = big_words.keys()

ones_words = ['one', 'two', 'three', 'four', 'five', 'six', 
                'seven', 'eight', 'nine']
teens_words = {
    10: 'ten', 
    11: 'eleven', 
    12: 'twelve', 
    13: 'thirteen', 
    14: 'fourteen',
    15:'fifteen', 
    16: 'sixteen',
    17: 'seventeen',
    18: 'eighteen',
    19: 'nineteen'
}

tens_words = ['twenty', 'thirty', 'fourty', 'fifty', 'sixty', 'seventy',
             'eighty', 'ninety']

def num_to_words(n):
    """return the english spoken-language representation of a non-negative
    integer. max value: 10^66 - 1.

    >>> num_to_words(3)
    'three'
    >>> num_to_words(12)
    'twelve'
    >>> num_to_words(355)
    'three hundred fifty five'
    >>> num_to_words(1051213)
    'one million fifty one thousand two hundred thirteen'
    >>> num_to_words(-1)
    Traceback (most recent call last):
        ...
    ValueError: n must be a non-negative integer
    >>> num_to_words("biscuits")
    Traceback (most recent call last):
        ...
    ValueError: n must be a non-negative integer
    >>> num_to_words(10**66)
    Traceback (most recent call last):
        ...
    OverflowError: that's a huuuuuuuge number, too big for me!
    """

    # check for max value
    if isinstance(n, long) and n > max_value:
        raise OverflowError("that's a huuuuuuuge number, too big for me!")

    # check for positive int.
    if not isinstance(n, int) or n < 0:
        raise ValueError("n must be a non-negative integer")

    if n == 0:
        return 'zero'

    # iteratively build the string.
    words = [] 
    for exponent in reversed(exponents):
        divisor = 10**exponent
        int_part = n/divisor
        if int_part > 0:
            words.extend(small_num_to_words(int_part) + 
                         [big_words[exponent]])
            n -= int_part * divisor

    words.extend(small_num_to_words(n))

    return " ".join(words)


# helper function for numbers > 0, < 1000.
def small_num_to_words(small_n):
    words = []

    hundreds = small_n/100
    if hundreds > 0:
        words.extend([ones_words[hundreds-1]] + ['hundred'])
        small_n %= 100

    if small_n >= 10 and small_n < 20:
        words.append(teens_words[small_n])
    else:
        tens = small_n/10
        if tens > 1:
            words.append(tens_words[tens-2])
        ones = small_n % 10
        words.append(ones_words[ones-1])
    return words


if __name__ == "__main__":
    import doctest
    doctest.testmod()