use std::io;
use zsh_history as zh;

fn print_usage(prog: &str) {
    eprintln!("Usage:");
    eprintln!("  {} merge <left> <right>            # two–way merge", prog);
    eprintln!("  {} merge <left> <right> <ancestor> # three–way merge", prog);
}

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let args: Vec<String> = std::env::args().collect();
    let prog = &args[0];

    if args.len() < 4 || args.len() > 5 || args[1] != "merge" {
        print_usage(prog);
        return Ok(());
    }

    // Open files and create readers in one go.
    let mut readers: Vec<_> = args[2..]
        .iter()
        .map(|name| {
            std::fs::File::open(name)
                .map_err(zh::Error::Io)
                .map(zh::Reader::new)
        })
        .collect::<Result<_, _>>()?;

    let mut out = io::stdout().lock();

    let result = match readers.len() {
        2 => {
            let ancestor = zh::Reader::new(io::empty());
            zh::merge(readers.remove(0), readers.remove(0), ancestor, &mut out)
        }
        3 => zh::merge(
            readers.remove(0),
            readers.remove(0),
            readers.remove(0),
            &mut out,
        ),
        _ => unreachable!(),
    };

    result?;
    Ok(())
}