Python/project_euler/problem_017/sol1.py

"""
Number letter counts
Problem 17: https://projecteuler.net/problem=17

If the numbers 1 to 5 are written out in words: one, two, three, four, five,
then there are 3 + 3 + 5 + 4 + 4 = 19 letters used in total.

If all the numbers from 1 to 1000 (one thousand) inclusive were written out in
words, how many letters would be used?


NOTE: Do not count spaces or hyphens. For example, 342 (three hundred and
forty-two) contains 23 letters and 115 (one hundred and fifteen) contains 20
letters. The use of "and" when writing out numbers is in compliance withBritish
usage.
"""


def solution(n: int = 1000) -> int:
    """Returns the number of letters used to write all numbers from 1 to n.
    where n is lower or equals to 1000.
    >>> solution(1000)
    21124
    >>> solution(5)
    19
    """
    # number of letters in zero, one, two, ..., nineteen (0 for zero since it's
    # never said aloud)
    ones_counts = [0, 3, 3, 5, 4, 4, 3, 5, 5, 4, 3, 6, 6, 8, 8, 7, 7, 9, 8, 8]
    # number of letters in twenty, thirty, ..., ninety (0 for numbers less than
    # 20 due to inconsistency in teens)
    tens_counts = [0, 0, 6, 6, 5, 5, 5, 7, 6, 6]

    count = 0

    for i in range(1, n + 1):
        if i < 1000:
            if i >= 100:
                # add number of letters for "n hundred"
                count += ones_counts[i // 100] + 7

                if i % 100 != 0:
                    # add number of letters for "and" if number is not multiple
                    # of 100
                    count += 3

            if 0 < i % 100 < 20:
                # add number of letters for one, two, three, ..., nineteen
                # (could be combined with below if not for inconsistency in
                # teens)
                count += ones_counts[i % 100]
            else:
                # add number of letters for twenty, twenty one, ..., ninety
                # nine
                count += ones_counts[i % 10]
                count += tens_counts[(i % 100 - i % 10) // 10]
        else:
            count += ones_counts[i // 1000] + 8
    return count


if __name__ == "__main__":
    print(solution(int(input().strip())))