use miette::Result;

#[tracing::instrument]
pub fn process(input: &str) -> Result<usize> {
    let sum = {
        let (firsts, lasts): (Vec<_>, Vec<_>) = input
            .lines()
            .filter(|line| !line.is_empty())
            .map(|line| {
                let nums = line
                    .split_whitespace()
                    .map(|num| num.parse::<usize>().unwrap())
                    .collect::<Vec<_>>();
                (*nums.first().unwrap(), *nums.last().unwrap())
            })
            .unzip();

        firsts
            .iter()
            .map(|x| {
                let count = lasts.iter().filter(|&y| y == x).count();
                x * count
            })
            .sum()
    };
    Ok(sum)
}

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

    #[test]
    fn test_process() -> Result<()> {
        let input = "
3   4
4   3
2   5
1   3
3   9
3   3
";
        let result = 31;
        assert_eq!(process(input)?, result);
        Ok(())
    }
}