use std::env;
use std::error::Error;
use std::fs::File;
use std::io::prelude::*;
use std::process::exit;
use std::slice::Iter;

use mailparse::dateparse;
use mailparse::MailHeaderMap;

fn newline(b: &u8) -> bool {
    *b == b'\n'
}

fn index_of(it: &mut Iter<u8>, needle: &[u8]) -> Option<usize> {
    let mut needle_ix = 0;
    it.position(move |&b| {
        if b == needle[needle_ix] {
            needle_ix += 1;
            if needle_ix == needle.len() {
                return true;
            }
        } else if b == needle[0] {
            needle_ix = 1;
        } else {
            needle_ix = 0;
        }
        false
    })
}

fn parse_mbox(mbox_bytes: &Vec<u8>) -> Result<(), Box<dyn Error>> {
    let mut it = mbox_bytes.iter();
    let mut ix = 0;

    loop {
        let mail_start = it.position(newline);
        if mail_start.is_none() {
            return Ok(());
        }
        ix += mail_start.unwrap() + 1;
        let start = ix;

        let delim = b"\nFrom ";
        let next_mail_start = index_of(&mut it, delim);

        let end = match next_mail_start {
            Some(x) => {
                ix += x + 1;
                ix - delim.len()
            }
            None => mbox_bytes.len(),
        };

        let mail_bytes = &mbox_bytes[start..end];

        let mail = mailparse::parse_mail(mail_bytes).unwrap();
        println!(
            "{:?} {:?} from {:?}",
            match mail.headers.get_first_value("Date")? {
                Some(date) => date,
                None => "NO DATE".to_string(),
            },
            match mail.headers.get_first_value("Subject")? {
                Some(subject) => subject,
                None => "NO SUBJECT".to_string(),
            },
            match mail.headers.get_first_value("From")? {
                Some(from) => from,
                None => "NO FROM".to_string(),
            },
        );
    }
}

fn main() {
    if env::args().count() <= 1 {
        println!("Provide mbox files as arguments");
        exit(1);
    }
    let mut args = env::args();
    args.next(); // drop executable name
    args.for_each(|mbox_path| {
        let mut mbox = File::open(mbox_path).unwrap();
        let mut mails = Vec::new();
        mbox.read_to_end(&mut mails).unwrap();
        parse_mbox(&mails);
    });
}