Python/project_euler/problem_003/sol1.py

"""
Project Euler Problem 3: https://projecteuler.net/problem=3

Largest prime factor

The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143?

References:
    - https://en.wikipedia.org/wiki/Prime_number#Unique_factorization
"""

import math


def is_prime(number: int) -> bool:
    """Checks to see if a number is a prime in O(sqrt(n)).
    A number is prime if it has exactly two factors: 1 and itself.
    Returns boolean representing primality of given number (i.e., if the
    result is true, then the number is indeed prime else it is not).

    >>> is_prime(2)
    True
    >>> is_prime(3)
    True
    >>> is_prime(27)
    False
    >>> is_prime(2999)
    True
    >>> is_prime(0)
    False
    >>> is_prime(1)
    False
    """

    if 1 < number < 4:
        # 2 and 3 are primes
        return True
    elif number < 2 or number % 2 == 0 or number % 3 == 0:
        # Negatives, 0, 1, all even numbers, all multiples of 3 are not primes
        return False

    # All primes number are in format of 6k +/- 1
    for i in range(5, int(math.sqrt(number) + 1), 6):
        if number % i == 0 or number % (i + 2) == 0:
            return False
    return True


def solution(n: int = 600851475143) -> int:
    """
    Returns the largest prime factor of a given number n.

    >>> solution(13195)
    29
    >>> solution(10)
    5
    >>> solution(17)
    17
    >>> solution(3.4)
    3
    >>> solution(0)
    Traceback (most recent call last):
        ...
    ValueError: Parameter n must be greater than or equal to one.
    >>> solution(-17)
    Traceback (most recent call last):
        ...
    ValueError: Parameter n must be greater than or equal to one.
    >>> solution([])
    Traceback (most recent call last):
        ...
    TypeError: Parameter n must be int or castable to int.
    >>> solution("asd")
    Traceback (most recent call last):
        ...
    TypeError: Parameter n must be int or castable to int.
    """

    try:
        n = int(n)
    except (TypeError, ValueError):
        raise TypeError("Parameter n must be int or castable to int.")
    if n <= 0:
        raise ValueError("Parameter n must be greater than or equal to one.")
    max_number = 0
    if is_prime(n):
        return n
    while n % 2 == 0:
        n //= 2
    if is_prime(n):
        return n
    for i in range(3, int(math.sqrt(n)) + 1, 2):
        if n % i == 0:
            if is_prime(n // i):
                max_number = n // i
                break
            elif is_prime(i):
                max_number = i
    return max_number


if __name__ == "__main__":
    print(f"{solution() = }")