use std::fs;

use itertools::Itertools;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let input = fs::read_to_string("input")?;
    let input = parse_number_list(input)?;
    let numbers = find_2020_numbers(input, 3)?;
    println!("{}", numbers.iter().product::<u32>());
    Ok(())
}

fn parse_number_list(input: String) -> Result<Vec<u32>, std::num::ParseIntError> {
    input.lines().map(|line| line.parse::<u32>()).collect()
}

fn find_2020_numbers(input: Vec<u32>, sum_number: usize) -> Result<Vec<u32>, String> {
    input
        .into_iter()
        .combinations(sum_number)
        .find(|i| i.iter().sum::<u32>() == 2020)
        .ok_or("Could not find 2020.".to_string())
}

#[cfg(test)]
mod tests {
    use super::*;
    use indoc::indoc;

    #[test]
    fn test_first_example() -> Result<(), Box<dyn std::error::Error>> {
        let example = indoc!(
            "
            1721
            979
            366
            299
            675
            1456
        "
        );

        let example = parse_number_list(example.to_string())?;
        let numbers = find_2020_numbers(example, 2)?;

        assert_eq!(numbers.iter().product::<u32>(), 514579);

        Ok(())
    }

    #[test]
    fn test_second_example() -> Result<(), Box<dyn std::error::Error>> {
        let example = indoc!(
            "
            1721
            979
            366
            299
            675
            1456
        "
        );

        let example = parse_number_list(example.to_string())?;
        let numbers = find_2020_numbers(example, 3)?;

        assert_eq!(numbers.iter().to_owned().product::<u32>(), 241861950);

        Ok(())
    }
}