Project Euler #12: Highly divisible triangular number

This article is part of a series where I'll be diving head first into the Project Euler puzzles. I want to document the challenge of solving such a puzzle and how I got to the answer. I want to prefix this by stating that I can't cheat for any of these challenges; with that I mean I can't look up any other implementations online. After the implementation, I will validate the answer by using this document or a similar sheet.

In this article I'll be solving: Project Euler #12.

This article features only an answer, because I’ve started writing from problem 14.

fn num_factors(n: i64) -> i64 {
    if n == 1 {
        return n;
    }

    let t = (n as f64).sqrt() as i64;
    let mut d = 2; // You always have 2 divisors, 1 and yourself

    for i in 2..=t {
        if n % i == 0 {
            d += 2
        }
    }

    d
}

fn problem_12(max_divs: i64) -> i64 {
    let mut i = 1;
    let mut j = 1;

    loop {
        i += 1;
        j += i;

        let o = num_factors(j);

        if o > max_divs {
            break j
        }
    }
}

#[test]
fn test_triangle_five_hundred() {
    assert_eq!(problem_12(5), 28);
    assert_eq!(problem_12(500), 76576500);
}

#[test]
fn test_num_factors() {
    assert_eq!(num_factors(60), 12);
    assert_eq!(num_factors(1), 1);
    assert_eq!(num_factors(3), 2);
    assert_eq!(num_factors(6), 4);
    assert_eq!(num_factors(10), 4);
}

The full solution is available on GitHub.

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