AOC Day 7 Part 2 with Triangle Number Proof

use std::cmp::{max, min};
use std::io::{self, BufRead};
use std::fs::File;

fn main() {
    let file = File::open("./input.txt").unwrap();
    let input_lines = io::BufReader::new(file).lines();

    let mut str_lines: Vec<String> = Vec::new();

    for line in input_lines {
        if let Ok(input) = line {
            str_lines.push(input);
        }
    }

    let mut numbers: Vec<u64> = Vec::new();
    let mut max_num: u64 = 0;
    let mut min_num: u64 = u64::MAX;

    for line in str_lines {
        for token in line.split(",") {
            let num: u64 = token.parse().unwrap();
            numbers.push(num);
            max_num = max(max_num, num);
            min_num = min(min_num, num);
        }
    }

    let mut min_fuel: u64 = u64::MAX;
    for i in min_num..max_num {
        let mut fuel_spent: u64 = 0;
        for num in &numbers {
            let num_steps = if num > &i { num - i } else { i - num };
            fuel_spent += calc_fuel_from_steps(num_steps);
        }
        min_fuel = min(min_fuel, fuel_spent);
    }

    println!("Min fuel {}", min_fuel);
}

fn calc_fuel_from_steps(steps: u64) -> u64 {
    let fuel = (steps * (steps + 1)) / 2;
    fuel
}