use std::{fmt, error};

pub fn prog_name() -> &'static str
{
    lazy_static! {
	static ref NAME: String = std::env::args().next().unwrap();
    }

    &NAME[..]
}

/// Print usage then exit with error code `0`.
pub fn usage() -> !
{
    println!("Unit conversion v{}", env!("CARGO_PKG_VERSION"));
    println!(" by {} with <3 (license GPL3+)", env!("CARGO_PKG_AUTHORS"));
    println!();
    println!("This program takes a value with a unit and outputs the smallest value corresponding to that type to `stdout`. It then outputs the multiplyer used to convert this unit to `stderr`, if there is one needed.");
    println!();
    println!("usage: {} <unit> <value>", prog_name());
    println!("usage: {} --help", prog_name());
    println!();
    //TODO: generalise units
    println!(r#"Units:
 BYTES		Convert byte unit (k, m, g, p) into single bytes"#);
    std::process::exit(0)
}

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub enum Operation
{
    Help,
    Convert(String, String),
}

#[inline] pub fn parse_args() -> Result<Operation, ArgParseError>
{
    parse(std::env::args().skip(1))
}

fn parse<T: Into<String>>(from: impl IntoIterator<Item = T>) -> Result<Operation, ArgParseError>
{
    let mut from = from.into_iter().map(Into::into);

    let first = from.next().ok_or(ArgParseError)?;
    match first.to_lowercase().trim() {
	"--help" => Ok(Operation::Help),
	_ => {
	    Ok(Operation::Convert(first, from.next().ok_or(ArgParseError)?))
	},
    }
}

#[derive(Debug)]
pub struct ArgParseError;

impl error::Error for ArgParseError{}
impl fmt::Display for ArgParseError
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
    {
	write!(f, "Failed to parse args. Try passing `--help`.")
    }
}