use std::{env, fmt, process};

#[derive(Debug, Default, PartialEq, Eq)]
struct Args {
    verbose: bool,
    output: Option<String>,
    inputs: Vec<String>,
}

#[derive(Debug, PartialEq, Eq)]
enum ParseError {
    MissingValue(&'static str),
    UnknownOption(String),
}

impl fmt::Display for ParseError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::MissingValue(option) => write!(f, "missing value for {option}"),
            Self::UnknownOption(option) => write!(f, "unknown option: {option}"),
        }
    }
}

// Parses arguments after the executable name.
fn parse_args<I, S>(arguments: I) -> Result<Args, ParseError>
where
    I: IntoIterator<Item = S>,
    S: Into<String>,
{
    let mut arguments = arguments.into_iter().map(Into::into);
    let mut parsed = Args::default();
    let mut options_enabled = true;

    while let Some(argument) = arguments.next() {
        if options_enabled {
            match argument.as_str() {
                "--" => {
                    options_enabled = false;
                    continue;
                }
                "-v" | "--verbose" => {
                    parsed.verbose = true;
                    continue;
                }
                "-o" | "--output" => {
                    parsed.output = Some(
                        arguments
                            .next()
                            .ok_or(ParseError::MissingValue("--output"))?,
                    );
                    continue;
                }
                _ if argument.starts_with("--output=") => {
                    parsed.output = Some(argument["--output=".len()..].to_owned());
                    continue;
                }
                _ if argument.starts_with('-') && argument != "-" => {
                    return Err(ParseError::UnknownOption(argument));
                }
                _ => {}
            }
        }

        parsed.inputs.push(argument);
    }

    Ok(parsed)
}

fn print_usage(program: &str) {
    println!("Usage: {program} [OPTIONS] [--] [INPUT ...]");
    println!();
    println!("Options:");
    println!("  -v, --verbose          Enable verbose output");
    println!("  -o, --output <PATH>    Set the output path");
    println!("  -h, --help             Show this help");
}

fn main() {
    let mut raw = env::args();
    let program = raw.next().unwrap_or_else(|| "app".to_owned());
    let arguments: Vec<String> = raw.collect();

    if arguments.iter().any(|arg| arg == "-h" || arg == "--help") {
        print_usage(&program);
        return;
    }

    match parse_args(arguments) {
        Ok(args) => {
            if args.verbose {
                eprintln!("parsed arguments: {args:?}");
            }

            println!("output: {}", args.output.as_deref().unwrap_or("<stdout>"));
            for input in args.inputs {
                println!("input: {input}");
            }
        }
        Err(error) => {
            eprintln!("{program}: {error}");
            eprintln!("Try '{program} --help' for more information.");
            process::exit(2);
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn parses_options_and_inputs() {
        let args = parse_args(["-v", "--output=result.txt", "one", "two"]).unwrap();

        assert_eq!(
            args,
            Args {
                verbose: true,
                output: Some("result.txt".to_owned()),
                inputs: vec!["one".to_owned(), "two".to_owned()],
            }
        );
    }

    #[test]
    fn double_dash_stops_option_parsing() {
        let args = parse_args(["--", "--verbose", "-o"]).unwrap();

        assert!(!args.verbose);
        assert_eq!(args.inputs, ["--verbose", "-o"]);
    }

    #[test]
    fn reports_missing_option_value() {
        assert_eq!(
            parse_args(["--output"]),
            Err(ParseError::MissingValue("--output"))
        );
    }
}