//! Reads a unit journal, with optional live output through the world watcher.
use sausage_init::name;
use std::{
    fs,
    io::{self, Write},
};
fn run() -> io::Result<()> {
    let mut args = std::env::args().skip(1);
    let (mut unit, mut lines, mut follow) = (String::new(), 10usize, false);
    while let Some(arg) = args.next() {
        match arg.as_str() {
            "-u" => {
                unit = name(
                    &args
                        .next()
                        .ok_or_else(|| io::Error::other("missing unit"))?,
                )
            }
            "-n" => {
                lines = args
                    .next()
                    .and_then(|n| n.parse().ok())
                    .ok_or_else(|| io::Error::other("missing count"))?
            }
            "-f" => follow = true,
            "--no-pager" => (),
            _ => return Err(io::Error::other("unknown argument")),
        }
    }
    let path = format!("/var/log/journal/{unit}.log");
    let watcher = sausage_spawn::Watcher::new(&[path.clone()]);
    let bytes = fs::read(&path).unwrap_or_default();
    let mut starts = vec![0];
    for (i, b) in bytes.iter().enumerate() {
        if *b == b'\n' && i + 1 < bytes.len() {
            starts.push(i + 1);
        }
    }
    let start = if lines == 0 {
        bytes.len()
    } else {
        starts[starts.len().saturating_sub(lines)]
    };
    io::stdout().write_all(&bytes[start..])?;
    io::stdout().flush()?;
    let mut offset = bytes.len();
    while follow {
        let pollable = watcher.subscribe();
        pollable.block();
        drop(pollable);
        watcher.changes();
        let bytes = fs::read(&path).unwrap_or_default();
        if bytes.len() < offset {
            offset = 0;
        }
        io::stdout().write_all(&bytes[offset..])?;
        io::stdout().flush()?;
        offset = bytes.len();
    }
    Ok(())
}
fn main() {
    if run().is_err() {
        std::process::exit(1);
    }
}