use std::error::Error;
use std::num::ParseIntError;
use csv::{Reader, Writer};

fn main() -> Result<(), Box<dyn Error>> {
    let mut reader = Reader::from_path("data.csv")?;
    let mut writer = Writer::from_path("output.csv")?;

    // headers() returns an immutable reference, so clone() before appending
    let mut headers = reader.headers()?.clone();
    headers.push_field("SUM");
    writer.write_record(headers.iter())?;

    for row in reader.records() {
        let mut row = row?;

        // `sum` needs the type annotation so that `parse::<i64>` knows what error type to return
        let sum: Result<_, ParseIntError> = row.iter().try_fold(0, |accum, s| {
            Ok(accum + s.parse::<i64>()?)
        });

        row.push_field(&sum?.to_string());
        writer.write_record(row.iter())?;
    }

    writer.flush()?;
    Ok(())
}