Python/project_euler/problem_002/sol5.py

"""
Project Euler Problem 2: https://projecteuler.net/problem=2

Even Fibonacci Numbers

Each new term in the Fibonacci sequence is generated by adding the previous
two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

By considering the terms in the Fibonacci sequence whose values do not exceed
four million, find the sum of the even-valued terms.

References:
    - https://en.wikipedia.org/wiki/Fibonacci_number
"""


def solution(n: int = 4000000) -> int:
    """
    Returns the sum of all even fibonacci sequence elements that are lower
    or equal to n.

    >>> solution(10)
    10
    >>> solution(15)
    10
    >>> solution(2)
    2
    >>> solution(1)
    0
    >>> solution(34)
    44
    """

    fib = [0, 1]
    i = 0
    while fib[i] <= n:
        fib.append(fib[i] + fib[i + 1])
        if fib[i + 2] > n:
            break
        i += 1
    total = 0
    for j in range(len(fib) - 1):
        if fib[j] % 2 == 0:
            total += fib[j]

    return total


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