mail_header_fp.rs

use std::collections::HashMap;
use std::fs::File;
use std::io;
use std::io::prelude::*;
use regex::Regex;
use itertools::Itertools;

type MailHeader = HashMap<String, Vec<String>>;

fn read_file(path: &str) -> io::Result<String> {
    let mut file = File::open(path)?;
    let mut contents = String::new();
    file.read_to_string(&mut contents)?;
    Ok(contents)
}

fn unfold_header(header: String) -> String {
    let re = Regex::new(r"\r\n[ \t]+").unwrap();
    re.replace_all(&header, " ").to_string()
}

fn split_field(line: &str) -> Option<(String, String)> {
    let re = Regex::new(r"(\S+): *(.+)?").unwrap();
    re.captures(line).map(|caps| {
        let fname = caps.get(1).unwrap().as_str().to_string();
        let fvalue = caps.get(2).map(|m| m.as_str());
        (fname, fvalue.unwrap_or("").to_string())
    })
}

fn parse_header(header: String) -> Vec<(String, String)> {
    header.lines().filter_map(split_field).collect()
}

fn split_header_body(text: String) -> Option<(String, String)> {
    match text.splitn(2, "\r\n\r\n").collect::<Vec<&str>>().as_slice() {
        [header, body] => Some((header.to_string(), body.to_string())),
        _ => None,
    }
}

fn tuple_to_hash(header: Vec<(String, String)>) -> MailHeader {
    let mut mheader = header.clone();
    mheader.sort_by(|a, b| a.0.cmp(&b.0));

    mheader.iter()
        .group_by(|(fst, _)| fst)
        .into_iter()
        .map(|(fname, group)| (fname.to_lowercase(), group.map(|(_, snd)| snd.to_string()).collect()))
        .collect()
}

fn main() {
    let text = read_file("data.txt").expect("IO error");
    let (header, _) = split_header_body(text).expect("Invalid mail-header");
    let unfolded_header = unfold_header(header);
    let tupled_header = parse_header(unfolded_header);
    let mail_header = tuple_to_hash(tupled_header);

    //println!("{:?}", mail_header);
    //mail_header.iter().for_each(|(fname, fvalue)| println!("{}: {:?}", fname, fvalue));
    println!("{}", mail_header.iter().map(|(fname, fvalue)| format!("{}: {:?}", fname, fvalue)).join("\n"));
}