First excercise of the AoC in functional Rust

aoc_1a.rs

use itertools::Itertools;
use std::{
    fs::File,
    io::{BufRead, BufReader},
};

fn main() -> std::io::Result<()> {
    let f = File::open("test.txt")?;
    let reader = BufReader::new(f);

    let max = reader
        .lines()
        .map(|line| line.unwrap().parse::<i32>().unwrap_or(0))
        .group_by(|calories| *calories> 0)
        .into_iter()
        .map(|(_, backpack)| backpack.sum::<i32>())
        .max();

    if let Some(res) = max {
        println!("{}", res);
    }
    Ok(())
}