Advent of Code 2018 day 01 task 2.

advent-of-code-2018-day-01-task-2.rs

use std::io::{stdin, BufRead};
use std::collections::BTreeSet;

fn main() {
    let shifts: Vec<i32> = stdin().lock()
        .lines()
        .filter_map(|line| line.unwrap().parse().ok())
        .collect();

    let mut frequency = 0;
    let mut seen_frequencies = BTreeSet::new();

    for shift in shifts.iter().cycle() {
        frequency += shift;

        if !seen_frequencies.insert(frequency) {
            break;
        }
    }

    println!("Frequency is {}", frequency);
}

To run (not optimised but fast enough for this task):

cargo run < input.txt

where input.txt is a file containing the input data.