The Results of the Expressive C++17 Coding Challenge in Rust

main.rs

use std::io::{Read, Write};
use std::fs::File;

fn main() {
    let args = ::std::env::args().collect::<Vec<_>>();
    // skip the program name
    if args.len() != 5 {
        println!("Usage: {} <filename> <columnname> <replacement> <output_filename>", args[0]);
    }
    
    let filename = &args[1];
    let column_name = &args[2];
    let replacement = &args[3];
    let output_filename = &args[4];

    load_csv(filename)
        .and_then(|csv_data|replace_column(&csv_data, column_name, replacement))
        .and_then(|modified_data|write_csv(&modified_data, output_filename))
        .unwrap();
}

fn load_csv(filename: &str) -> Result<String, &'static str> {
    let mut buffer = String::new();
    File::open(filename)
        .ok()
        .into_iter()
        .flat_map(|mut f|f.read_to_string(&mut buffer).ok())
        .filter(|&n| n > 0)
        .next().map(|_|buffer).ok_or("input file missing")
}

fn replace_column(data: &str, column: &str, replacement: &str) -> Result<String, &'static str> {
    let mut lines = data.lines();
    lines.next()
        .ok_or("no first line!")
        .and_then(|c|c.split(',').position(|e|e == column).map(|pos|(c, pos)).ok_or("column name doesn't exist in the input file"))
        .map(|(c, pos)|lines.fold(c.to_owned() + "\n", |mut res, line| {
            res.extend(line.split(',')
                .enumerate()
                .map(|(i, val)|if pos == i { replacement } else { val }));
            res.push_str("\n");
            res
        }))
}

fn write_csv(data: &str, filename: &str) -> Result<(), &'static str> {
    File::create(filename)
        .ok()
        .and_then(|mut f| f.write_all(data.as_bytes()).ok())
        .ok_or("Could not write to file!")
}