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

#[derive(Debug, Default, PartialEq)]
struct Args {
    verbose: bool,
    output: Option<String>,
    count: u32,
    inputs: Vec<String>,
}

#[derive(Debug, PartialEq)]
enum ParseError {
    MissingValue(&'static str),
    InvalidCount(String),
    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::InvalidCount(value) => write!(f, "invalid count: {value}"),
            Self::UnknownOption(option) => write!(f, "unknown option: {option}"),
        }
    }
}

fn parse_args<I, S>(args: I) -> Result<Args, ParseError>
where
    I: IntoIterator<Item = S>,
    S: Into<String>,
{
    let mut result = Args {
        count: 1,
        ..Args::default()
    };
    let mut args = args.into_iter().map(Into::into);

    while let Some(arg) = args.next() {
        match arg.as_str() {
            "-v" | "--verbose" => result.verbose = true,
            "-o" | "--output" => {
                result.output = Some(
                    args.next()
                        .ok_or(ParseError::MissingValue("--output"))?,
                );
            }
            "-n" | "--count" => {
                let value = args
                    .next()
                    .ok_or(ParseError::MissingValue("--count"))?;
                result.count = value
                    .parse()
                    .map_err(|_| ParseError::InvalidCount(value))?;
            }
            "--" => {
                result.inputs.extend(args);
                break;
            }
            _ if arg.starts_with('-') => return Err(ParseError::UnknownOption(arg)),
            _ => result.inputs.push(arg),
        }
    }

    Ok(result)
}

fn print_help(program: &str) {
    println!(
        "\
Usage: {program} [OPTIONS] [INPUTS...]

Options:
  -v, --verbose         Enable verbose output
  -o, --output FILE     Write output to FILE
  -n, --count NUMBER    Number of iterations (default: 1)
  -h, --help            Show this help"
    );
}

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

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

    match parse_args(args) {
        Ok(args) => println!("{args:#?}"),
        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([
            "--verbose",
            "--output",
            "result.txt",
            "-n",
            "3",
            "one",
            "two",
        ])
        .unwrap();

        assert_eq!(
            args,
            Args {
                verbose: true,
                output: Some("result.txt".into()),
                count: 3,
                inputs: vec!["one".into(), "two".into()],
            }
        );
    }

    #[test]
    fn double_dash_ends_option_parsing() {
        let args = parse_args(["--", "--verbose"]).unwrap();
        assert!(!args.verbose);
        assert_eq!(args.inputs, ["--verbose"]);
    }

    #[test]
    fn rejects_unknown_options() {
        assert_eq!(
            parse_args(["--unknown"]),
            Err(ParseError::UnknownOption("--unknown".into()))
        );
    }
}