//! A pathologically simple command line tokenizer.
//!
//! Most argument parsers are declarative: you tell them what to parse,
//! and they do it.
//!
//! This one provides you with a stream of options and values and lets you
//! figure out the rest.
//!
//! ## Example
//! ```no_run
//! struct Args {
//!     thing: String,
//!     number: u32,
//!     shout: bool,
//! }
//!
//! fn parse_args() -> Result<Args, lexopt::Error> {
//!     use lexopt::prelude::*;
//!
//!     let mut thing = None;
//!     let mut number = 1;
//!     let mut shout = false;
//!     let mut parser = lexopt::Parser::from_env();
//!     while let Some(arg) = parser.next()? {
//!         match arg {
//!             Short('n') | Long("number") => {
//!                 number = parser.value()?.parse()?;
//!             }
//!             Long("shout") => {
//!                 shout = true;
//!             }
//!             Value(val) if thing.is_none() => {
//!                 thing = Some(val.into_string()?);
//!             }
//!             Long("help") => {
//!                 println!("Usage: hello [-n|--number=NUM] [--shout] THING");
//!                 std::process::exit(0);
//!             }
//!             _ => return Err(arg.unexpected()),
//!         }
//!     }
//!
//!     Ok(Args {
//!         thing: thing.ok_or("missing argument THING")?,
//!         number,
//!         shout,
//!     })
//! }
//!
//! fn main() -> Result<(), lexopt::Error> {
//!     let args = parse_args()?;
//!     let mut message = format!("Hello {}", args.thing);
//!     if args.shout {
//!         message = message.to_uppercase();
//!     }
//!     for _ in 0..args.number {
//!         println!("{}", message);
//!     }
//!     Ok(())
//! }
//! ```

#![forbid(unsafe_code)]
#![warn(missing_docs)]
#![allow(clippy::should_implement_trait)]
#![allow(clippy::map_clone)] // Because of the MSRV (setting MSRV in clippy.toml doesn't help)

use std::{ffi::OsString, fmt::Display, str::FromStr};

/// A parser for command line arguments.
pub struct Parser {
    source: Box<dyn Iterator<Item = OsString> + 'static>,
    // The current string of short options being processed
    shorts: Option<(Vec<u8>, usize)>,
    #[cfg(windows)]
    // The same thing, but encoded as UTF-16
    shorts_utf16: Option<(Vec<u16>, usize)>,
    // Temporary storage for a long option so it can be borrowed
    long: Option<String>,
    // The pending value for the last long option
    long_value: Option<OsString>,
    // The last option we emitted
    last_option: LastOption,
    // Whether we encountered "--" and know no more options are coming
    finished_opts: bool,
    // The name of the command (argv[0])
    bin_name: Option<OsString>,
}

// source may not implement Debug
impl std::fmt::Debug for Parser {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        let mut f = f.debug_struct("Parser");
        f.field("source", &"<iterator>")
            .field("shorts", &self.shorts);
        #[cfg(windows)]
        f.field("shorts_utf16", &self.shorts_utf16);
        f.field("long", &self.long)
            .field("long_value", &self.long_value)
            .field("last_option", &self.last_option)
            .field("finished_opts", &self.finished_opts)
            .field("bin_name", &self.bin_name)
            .finish()
    }
}

/// We use this to keep track of the last emitted option, for error messages when
/// an expected value is not found.
///
/// A long option can be recovered from the `long` field, so that variant doesn't
/// need to contain data.
///
/// Our short option storage is cleared more aggressively, so we do need to
/// duplicate that.
#[derive(Debug, PartialEq, Clone, Copy)]
enum LastOption {
    None,
    Short(char),
    Long,
}

/// A command line argument found by [`Parser`], either an option or a positional argument.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Arg<'a> {
    /// A short option, e.g. `-q`.
    Short(char),
    /// A long option, e.g. `--verbose`. (The dashes are not included.)
    Long(&'a str),
    /// A positional argument, e.g. `/dev/null`.
    Value(OsString),
}

impl Parser {
    /// Get the next option or positional argument.
    ///
    /// A return value of `Ok(None)` means the command line has been exhausted.
    ///
    /// Options that are not valid unicode are transformed with replacement
    /// characters as by [`String::from_utf8_lossy`].
    ///
    /// # Errors
    ///
    /// [`Error::UnexpectedValue`] is returned if the last option had a
    /// value that hasn't been consumed, as in `--option=value`.
    ///
    /// It's possible to continue parsing after an error (but this is rarely useful).
    pub fn next(&mut self) -> Result<Option<Arg<'_>>, Error> {
        self.check_state();

        if let Some(value) = self.long_value.take() {
            // Last time we got `--long=value`, and `value` hasn't been used.
            return Err(Error::UnexpectedValue {
                option: self.long.clone(),
                value,
            });
        }

        if let Some((ref arg, ref mut pos)) = self.shorts {
            // We're somewhere inside a -abc chain. Because we're in .next(),
            // not .value(), we can assume that the next character is another option.
            match first_codepoint(&arg[*pos..]) {
                Ok(None) => {
                    self.shorts = None;
                }
                // If we find '=' here we assume it's part of an option.
                // Another possibility would be to see it as a value separator.
                // `-=` as an option exists in the wild!
                // See https://linux.die.net/man/1/a2ps
                Ok(Some(ch)) => {
                    *pos += ch.len_utf8();
                    self.last_option = LastOption::Short(ch);
                    return Ok(Some(Arg::Short(ch)));
                }
                Err(_) => {
                    // Advancing may allow recovery.
                    // This is a little iffy, there might be more bad unicode next.
                    *pos += 1;
                    self.last_option = LastOption::Short('�');
                    return Ok(Some(Arg::Short('�')));
                }
            }
        }

        #[cfg(windows)]
        {
            if let Some((ref arg, ref mut pos)) = self.shorts_utf16 {
                match first_utf16_codepoint(&arg[*pos..]) {
                    Ok(None) => {
                        self.shorts_utf16 = None;
                    }
                    Ok(Some(ch)) => {
                        *pos += ch.len_utf16();
                        self.last_option = LastOption::Short(ch);
                        return Ok(Some(Arg::Short(ch)));
                    }
                    Err(_) => {
                        *pos += 1;
                        self.last_option = LastOption::Short('�');
                        return Ok(Some(Arg::Short('�')));
                    }
                }
            }
        }

        let arg = match self.source.next() {
            Some(arg) => arg,
            None => return Ok(None),
        };

        if self.finished_opts {
            return Ok(Some(Arg::Value(arg)));
        }
        if arg == "--" {
            self.finished_opts = true;
            return self.next();
        }

        #[cfg(any(unix, target_os = "wasi"))]
        {
            // Fast solution for platforms where OsStrings are just UTF-8-ish bytes
            #[cfg(unix)]
            use std::os::unix::ffi::{OsStrExt, OsStringExt};
            #[cfg(target_os = "wasi")]
            use std::os::wasi::ffi::{OsStrExt, OsStringExt};

            let bytes = arg.as_bytes();
            if bytes.starts_with(b"--") {
                // Long options have two forms: --option and --option=value.
                let option = if let Some(ind) = bytes.iter().position(|&b| b == b'=') {
                    // The value can be an OsString...
                    self.long_value = Some(OsString::from_vec(bytes[ind + 1..].into()));
                    // ...but the option has to be a string.
                    String::from_utf8_lossy(&bytes[..ind]).into()
                } else {
                    // arg.to_string_lossy().into_owned() would work, but its
                    // return type is Cow: if the original was valid a borrowed
                    // version is returned, and then into_owned() does an
                    // unnecessary copy.
                    // By trying .into_string() first we avoid that copy if arg
                    // is already UTF-8.
                    // reqwest does a similar maneuver more efficiently with unsafe:
                    // https://github.com/seanmonstar/reqwest/blob/e6a1a09f0904e06de4ff1317278798c4ed28af66/src/async_impl/response.rs#L194

                    match arg.into_string() {
                        Ok(text) => text,
                        Err(arg) => arg.to_string_lossy().into_owned(),
                    }
                };
                Ok(Some(self.set_long(option)))
            } else if bytes.len() > 1 && bytes[0] == b'-' {
                self.shorts = Some((arg.into_vec(), 1));
                self.next()
            } else {
                Ok(Some(Arg::Value(arg)))
            }
        }

        #[cfg(not(any(unix, target_os = "wasi")))]
        {
            // Platforms where looking inside an OsString is harder

            #[cfg(windows)]
            {
                // Fast path for Windows
                use std::os::windows::ffi::OsStrExt;
                let mut bytes = arg.encode_wide();
                const DASH: u16 = b'-' as u16;
                match (bytes.next(), bytes.next()) {
                    (Some(DASH), Some(_)) => {
                        // This is an option, we'll have to do more work.
                        // (We already checked for "--" earlier.)
                    }
                    _ => {
                        // Just a value, return early.
                        return Ok(Some(Arg::Value(arg)));
                    }
                }
            }

            let arg = match arg.into_string() {
                Ok(arg) => arg,
                Err(arg) => {
                    // The argument is not valid unicode.
                    // If it's an option we'll have to do something nasty,
                    // otherwise we can return it as-is.

                    #[cfg(windows)]
                    {
                        // On Windows we can only get here if this is an option, otherwise
                        // we return earlier.
                        // Unlike on Unix, we can't efficiently process invalid unicode.
                        // Semantically it's UTF-16, but internally it's WTF-8 (a superset of UTF-8).
                        // So we only process the raw version here, when we know we really have to.
                        use std::os::windows::ffi::{OsStrExt, OsStringExt};
                        let arg: Vec<u16> = arg.encode_wide().collect();
                        const DASH: u16 = b'-' as u16;
                        const EQ: u16 = b'=' as u16;
                        if arg.starts_with(&[DASH, DASH]) {
                            if let Some(ind) = arg.iter().position(|&u| u == EQ) {
                                self.long_value = Some(OsString::from_wide(&arg[ind + 1..]));
                                let long = self.set_long(String::from_utf16_lossy(&arg[..ind]));
                                return Ok(Some(long));
                            } else {
                                let long = self.set_long(String::from_utf16_lossy(&arg));
                                return Ok(Some(long));
                            }
                        } else {
                            assert!(arg.starts_with(&[DASH]));
                            assert!(arg.len() > 1);
                            self.shorts_utf16 = Some((arg, 1));
                            return self.next();
                        }
                    };

                    #[cfg(not(windows))]
                    {
                        // This code may be reachable on Hermit and SGX, but probably
                        // not on wasm32-unknown-unknown, which is unfortunate as that's
                        // the only one we can easily test.

                        // This allocates unconditionally, sadly.
                        let text = arg.to_string_lossy();
                        if text.starts_with('-') {
                            // Use the lossily patched version and hope for the best.
                            // This may be incorrect behavior. Our only other option
                            // is an error but I don't want to write complicated code
                            // I can't actually test.
                            // Please open an issue if this behavior affects you!
                            text.into_owned()
                        } else {
                            // It didn't look like an option, so return it as a value.
                            return Ok(Some(Arg::Value(arg)));
                        }
                    }
                }
            };

            // The argument is valid unicode. This is the ideal version of the
            // code, the previous mess was purely to deal with invalid unicode.
            if arg.starts_with("--") {
                let mut parts = arg.splitn(2, '=');
                if let (Some(option), Some(value)) = (parts.next(), parts.next()) {
                    self.long_value = Some(value.into());
                    Ok(Some(self.set_long(option.into())))
                } else {
                    Ok(Some(self.set_long(arg)))
                }
            } else if arg.starts_with('-') && arg != "-" {
                self.shorts = Some((arg.into(), 1));
                self.next()
            } else {
                Ok(Some(Arg::Value(arg.into())))
            }
        }
    }

    /// Get a value for an option.
    ///
    /// This function should normally be called right after seeing an option
    /// that expects a value, with positional arguments being collected
    /// using [`next()`][Parser::next].
    ///
    /// A value is collected even if it looks like an option
    /// (i.e., starts with `-`).
    ///
    /// This method can alternatively be used to collect the remaining
    /// arguments if you want to stop processing options. For more information,
    /// see [`examples/posixly_correct.rs`](https://github.com/blyxxyz/lexopt/blob/master/examples/posixly_correct.rs).
    ///
    /// # Errors
    ///
    /// An [`Error::MissingValue`] is returned if the end of the command
    /// line is reached.
    pub fn value(&mut self) -> Result<OsString, Error> {
        self.check_state();

        if let Some(value) = self.optional_value() {
            return Ok(value);
        }

        if let Some(value) = self.source.next() {
            return Ok(value);
        }

        let option = match self.last_option {
            LastOption::None => None,
            LastOption::Short(ch) => Some(format!("-{}", ch)),
            LastOption::Long => self.long.clone(),
        };
        Err(Error::MissingValue { option })
    }

    /// The name of the command, as in the zeroth argument of the process.
    ///
    /// This may be useful to include in help or error messages.
    ///
    /// Returns `None` if:
    /// - The parser was not constructed by [`Parser::from_env`]
    /// - The name is not valid unicode
    /// - The name was not supplied by the system
    ///
    /// Using [`Option::unwrap_or`] for a fallback value is recommended.
    pub fn bin_name(&self) -> Option<&str> {
        self.bin_name.as_ref().and_then(|s| s.to_str())
    }

    /// Get a value only if it's concatenated to an option, as in `-fvalue` or
    /// `--option=value`.
    ///
    /// I'm unsure about making this public. It'd contribute to parity with
    /// GNU getopt but it'd also detract from the cleanness of the interface.
    fn optional_value(&mut self) -> Option<OsString> {
        if let Some(value) = self.long_value.take() {
            return Some(value);
        }

        if let Some((arg, pos)) = self.shorts.take() {
            if pos < arg.len() {
                #[cfg(any(unix, target_os = "wasi"))]
                {
                    #[cfg(unix)]
                    use std::os::unix::ffi::OsStringExt;
                    #[cfg(target_os = "wasi")]
                    use std::os::wasi::ffi::OsStringExt;
                    return Some(OsString::from_vec(arg[pos..].into()));
                }
                #[cfg(not(any(unix, target_os = "wasi")))]
                {
                    let arg = String::from_utf8(arg[pos..].into())
                        .expect("short option args on exotic platforms must be unicode");
                    return Some(arg.into());
                }
            }
        }

        #[cfg(windows)]
        {
            if let Some((arg, pos)) = self.shorts_utf16.take() {
                if pos < arg.len() {
                    use std::os::windows::ffi::OsStringExt;
                    return Some(OsString::from_wide(&arg[pos..]));
                }
            }
        }

        None
    }

    /// Create a parser from the environment using [`std::env::args_os`].
    pub fn from_env() -> Parser {
        let mut source = std::env::args_os();
        let bin_name = source.next();
        Parser {
            source: Box::new(source),
            shorts: None,
            #[cfg(windows)]
            shorts_utf16: None,
            long: None,
            long_value: None,
            last_option: LastOption::None,
            finished_opts: false,
            bin_name,
        }
    }

    /// Create a parser from an iterator. This may be useful for testing.
    ///
    /// The executable name must not be included.
    pub fn from_args<I>(args: I) -> Parser
    where
        I: IntoIterator + 'static,
        I::Item: Into<OsString>,
    {
        Parser {
            source: Box::new(args.into_iter().map(Into::into)),
            shorts: None,
            #[cfg(windows)]
            shorts_utf16: None,
            long: None,
            long_value: None,
            last_option: LastOption::None,
            finished_opts: false,
            bin_name: None,
        }
    }

    /// Store a long option so the caller can borrow it.
    /// We go through this trouble because matching an owned string is a pain.
    fn set_long(&mut self, value: String) -> Arg {
        self.last_option = LastOption::Long;
        // Option::insert would work but it didn't exist in 1.31 (our MSRV)
        self.long = None;
        Arg::Long(&self.long.get_or_insert(value)[2..])
    }

    /// Some basic sanity checks for the internal state.
    ///
    /// Particularly nice for fuzzing.
    fn check_state(&self) {
        if let Some((ref arg, pos)) = self.shorts {
            assert!(pos <= arg.len());
            assert!(arg.len() > 1);
            if pos > 1 {
                assert!(self.last_option != LastOption::None);
                assert!(self.last_option != LastOption::Long);
            }
            assert!(self.long_value.is_none());
        }

        #[cfg(windows)]
        {
            if let Some((ref arg, pos)) = self.shorts_utf16 {
                assert!(pos <= arg.len());
                assert!(arg.len() > 1);
                if pos > 1 {
                    assert!(self.last_option != LastOption::None);
                    assert!(self.last_option != LastOption::Long);
                }
                assert!(self.shorts.is_none());
                assert!(self.long_value.is_none());
            }
        }

        match self.last_option {
            LastOption::None => {
                assert!(self.long.is_none());
                assert!(self.long_value.is_none());
            }
            LastOption::Short(_) => {
                assert!(self.long_value.is_none());
            }
            LastOption::Long => {
                assert!(self.long.is_some());
            }
        }

        if self.long_value.is_some() {
            assert!(self.long.is_some());
            assert!(self.last_option == LastOption::Long);
        }
    }
}

impl<'a> Arg<'a> {
    /// Convert an unexpected argument into an error.
    pub fn unexpected(self) -> Error {
        match self {
            Arg::Short(short) => Error::UnexpectedOption(format!("-{}", short)),
            Arg::Long(long) => Error::UnexpectedOption(format!("--{}", long)),
            Arg::Value(value) => Error::UnexpectedArgument(value),
        }
    }
}

/// An error during argument parsing.
///
/// This implements `From<String>` and `From<&str>`, for easy ad-hoc error
/// messages.
///
/// It also implements `From<OsString>`, as that's used as an error type
/// by [`OsString::into_string`], so that method may be used with the try (`?`)
/// operator.
//
// This is not #[non_exhaustive] because of the MSRV. I'm hoping no more
// variants will turn out to be needed: this seems reasonable, if the scope
// of the library doesn't change. Worst case scenario it can be stuffed inside
// Error::Custom.
pub enum Error {
    /// An option argument was expected but was not found.
    MissingValue {
        /// The most recently emitted option.
        option: Option<String>,
    },

    /// An unexpected option was found.
    UnexpectedOption(String),

    /// A positional argument was found when none was expected.
    UnexpectedArgument(OsString),

    /// An option had a value when none was expected.
    UnexpectedValue {
        /// The option. This is always a long option.
        option: Option<String>,
        /// The value.
        value: OsString,
    },

    /// Parsing a value failed. Returned by methods on [`ValueExt`].
    ParsingFailed {
        /// The string that failed to parse.
        value: String,
        /// The error returned while parsing.
        error: Box<dyn std::error::Error + Send + Sync + 'static>,
    },

    /// A value was found that was not valid unicode.
    ///
    /// This can be returned by some methods on [`ValueExt`].
    NonUnicodeValue(OsString),

    /// For custom error messages in application code.
    Custom(Box<dyn std::error::Error + Send + Sync + 'static>),
}

impl Display for Error {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        use crate::Error::*;
        match self {
            MissingValue { option: None } => write!(f, "missing argument at end of command"),
            MissingValue {
                option: Some(option),
            } => {
                write!(f, "missing argument for option '{}'", option)
            }
            UnexpectedOption(option) => write!(f, "invalid option '{}'", option),
            UnexpectedArgument(value) => write!(f, "unexpected argument {:?}", value),
            UnexpectedValue {
                option: Some(option),
                value,
            } => {
                write!(
                    f,
                    "unexpected argument for option '{}': {:?}",
                    option, value
                )
            }
            UnexpectedValue {
                option: None,
                value,
            } => {
                write!(f, "unexpected argument for option: {:?}", value)
            }
            NonUnicodeValue(value) => write!(f, "argument is invalid unicode: {:?}", value),
            ParsingFailed { value, error } => {
                write!(f, "cannot parse argument {:?}: {}", value, error)
            }
            Custom(err) => write!(f, "{}", err),
        }
    }
}

// This is printed when returning an error from main(), so defer to Display
impl std::fmt::Debug for Error {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        Display::fmt(self, f)
    }
}

impl std::error::Error for Error {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            Error::ParsingFailed { error, .. } | Error::Custom(error) => Some(error.as_ref()),
            _ => None,
        }
    }
}

impl From<String> for Error {
    fn from(msg: String) -> Self {
        Error::Custom(msg.into())
    }
}

impl<'a> From<&'a str> for Error {
    fn from(msg: &'a str) -> Self {
        Error::Custom(msg.into())
    }
}

/// For [`OsString::into_string`].
impl From<OsString> for Error {
    fn from(arg: OsString) -> Self {
        Error::NonUnicodeValue(arg)
    }
}

mod private {
    pub trait Sealed {}
    impl Sealed for std::ffi::OsString {}
}

/// An optional extension trait with methods for parsing [`OsString`]s.
///
/// They may fail in two cases:
/// - The value cannot be decoded because it's invalid unicode
///   ([`Error::NonUnicodeValue`])
/// - The value can be decoded, but parsing fails ([`Error::ParsingFailed`])
///
/// If parsing fails the error will be wrapped in lexopt's own [`Error`] type.
pub trait ValueExt: private::Sealed {
    /// Decode the value and parse it using [`FromStr`].
    ///
    /// This will fail if the value is not valid unicode or if the subsequent
    /// parsing fails.
    fn parse<T: FromStr>(&self) -> Result<T, Error>
    where
        T::Err: Into<Box<dyn std::error::Error + Send + Sync + 'static>>;

    /// Decode the value and parse it using a custom function.
    fn parse_with<F, T, E>(&self, func: F) -> Result<T, Error>
    where
        F: FnOnce(&str) -> Result<T, E>,
        E: Into<Box<dyn std::error::Error + Send + Sync + 'static>>;

    // There is no parse_os_with() because I can't think of any useful
    // fallible operations on an OsString. Typically you'd either decode it,
    // use it as is, or do an infallible conversion to a PathBuf or such.
    //
    // If you have a use for parse_os_with() please open an issue with an
    // example.
}

impl ValueExt for OsString {
    fn parse<T: FromStr>(&self) -> Result<T, Error>
    where
        T::Err: Into<Box<dyn std::error::Error + Send + Sync + 'static>>,
    {
        self.parse_with(FromStr::from_str)
    }

    fn parse_with<F, T, E>(&self, func: F) -> Result<T, Error>
    where
        F: FnOnce(&str) -> Result<T, E>,
        E: Into<Box<dyn std::error::Error + Send + Sync + 'static>>,
    {
        match self.to_str() {
            Some(text) => match func(text) {
                Ok(value) => Ok(value),
                Err(err) => Err(Error::ParsingFailed {
                    value: text.to_owned(),
                    error: err.into(),
                }),
            },
            None => Err(Error::NonUnicodeValue(self.into())),
        }
    }
}

/// A small prelude for processing arguments.
///
/// It allows you to write `Short`/`Long`/`Value` without an [`Arg`] prefix
/// and adds convenience methods to [`OsString`].
///
/// If this is used it's best to import it inside a function, not in module
/// scope:
/// ```ignore
/// fn parse_args() -> Result<Args, lexopt::Error> {
///     use lexopt::prelude::*;
///     ...
/// }
/// ```
pub mod prelude {
    pub use super::Arg::*;
    pub use super::ValueExt;
}

/// Take the first codepoint of a bytestring. On error, return the first
/// (and therefore in some way invalid) byte/code unit.
///
/// The rest of the bytestring does not have to be valid unicode.
fn first_codepoint(bytes: &[u8]) -> Result<Option<char>, u8> {
    // We only need the first 4 bytes
    let bytes = bytes.get(..4).unwrap_or(bytes);
    let text = match std::str::from_utf8(bytes) {
        Ok(text) => text,
        Err(err) if err.valid_up_to() > 0 => {
            std::str::from_utf8(&bytes[..err.valid_up_to()]).unwrap()
        }
        Err(_) => return Err(bytes[0]),
    };
    Ok(text.chars().next())
}

#[cfg(windows)]
/// As before, but for UTF-16.
fn first_utf16_codepoint(units: &[u16]) -> Result<Option<char>, u16> {
    match std::char::decode_utf16(units.iter().map(|ch| *ch)).next() {
        Some(Ok(ch)) => Ok(Some(ch)),
        Some(Err(_)) => Err(units[0]),
        None => Ok(None),
    }
}

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

    fn parse(args: &'static str) -> Parser {
        Parser::from_args(args.split_whitespace().map(bad_string))
    }

    /// Specialized backport of matches!()
    macro_rules! assert_matches {
        ($expression: expr, $pattern: pat) => {
            match $expression {
                $pattern => true,
                _ => panic!(
                    "{:?} does not match {:?}",
                    stringify!($expression),
                    stringify!($pattern)
                ),
            }
        };
    }

    #[test]
    fn test_basic() -> Result<(), Error> {
        let mut p = parse("-n 10 foo - -- baz -qux");
        assert_eq!(p.next()?.unwrap(), Short('n'));
        assert_eq!(p.value()?.parse::<i32>()?, 10);
        assert_eq!(p.next()?.unwrap(), Value("foo".into()));
        assert_eq!(p.next()?.unwrap(), Value("-".into()));
        assert_eq!(p.next()?.unwrap(), Value("baz".into()));
        assert_eq!(p.next()?.unwrap(), Value("-qux".into()));
        assert_eq!(p.next()?, None);
        assert_eq!(p.next()?, None);
        assert_eq!(p.next()?, None);
        Ok(())
    }

    #[test]
    fn test_combined() -> Result<(), Error> {
        let mut p = parse("-abc -fvalue -xfvalue");
        assert_eq!(p.next()?.unwrap(), Short('a'));
        assert_eq!(p.next()?.unwrap(), Short('b'));
        assert_eq!(p.next()?.unwrap(), Short('c'));
        assert_eq!(p.next()?.unwrap(), Short('f'));
        assert_eq!(p.value()?, "value");
        assert_eq!(p.next()?.unwrap(), Short('x'));
        assert_eq!(p.next()?.unwrap(), Short('f'));
        assert_eq!(p.value()?, "value");
        assert_eq!(p.next()?, None);
        Ok(())
    }

    #[test]
    fn test_long() -> Result<(), Error> {
        let mut p = parse("--foo --bar=qux --foobar=qux=baz");
        assert_eq!(p.next()?.unwrap(), Long("foo"));
        assert_eq!(p.next()?.unwrap(), Long("bar"));
        assert_eq!(p.value()?, "qux");
        assert_eq!(p.next()?.unwrap(), Long("foobar"));
        match p.next().unwrap_err() {
            Error::UnexpectedValue {
                option: Some(option),
                value,
            } => {
                assert_eq!(option, "--foobar");
                assert_eq!(value, "qux=baz");
            }
            _ => panic!(),
        }
        assert_eq!(p.next()?, None);
        Ok(())
    }

    #[test]
    fn test_dash_args() -> Result<(), Error> {
        // "--" should indicate the end of the options
        let mut p = parse("-x -- -y");
        assert_eq!(p.next()?.unwrap(), Short('x'));
        assert_eq!(p.next()?.unwrap(), Value("-y".into()));
        assert_eq!(p.next()?, None);

        // ...unless it's an argument of an option
        let mut p = parse("-x -- -y");
        assert_eq!(p.next()?.unwrap(), Short('x'));
        assert_eq!(p.value()?, "--");
        assert_eq!(p.next()?.unwrap(), Short('y'));
        assert_eq!(p.next()?, None);

        // "-" is a valid value that should not be treated as an option
        let mut p = parse("-x - -y");
        assert_eq!(p.next()?.unwrap(), Short('x'));
        assert_eq!(p.next()?.unwrap(), Value("-".into()));
        assert_eq!(p.next()?.unwrap(), Short('y'));
        assert_eq!(p.next()?, None);

        // '-' is a silly and hard to use short option, but other parsers treat
        // it like an option in this position
        let mut p = parse("-x-y");
        assert_eq!(p.next()?.unwrap(), Short('x'));
        assert_eq!(p.next()?.unwrap(), Short('-'));
        assert_eq!(p.next()?.unwrap(), Short('y'));
        assert_eq!(p.next()?, None);

        Ok(())
    }

    #[test]
    fn test_missing_value() -> Result<(), Error> {
        let mut p = parse("-o");
        assert_eq!(p.next()?.unwrap(), Short('o'));
        match p.value() {
            Err(Error::MissingValue {
                option: Some(option),
            }) => assert_eq!(option, "-o"),
            _ => panic!(),
        }

        let mut q = parse("--out");
        assert_eq!(q.next()?.unwrap(), Long("out"));
        match q.value() {
            Err(Error::MissingValue {
                option: Some(option),
            }) => assert_eq!(option, "--out"),
            _ => panic!(),
        }

        let mut r = parse("");
        assert_matches!(r.value(), Err(Error::MissingValue { option: None }));

        Ok(())
    }

    #[test]
    fn test_weird_args() -> Result<(), Error> {
        let mut p = Parser::from_args(&[
            "", "--=", "--=3", "-", "-x", "--", "-", "-x", "--", "", "-", "-x",
        ]);
        assert_eq!(p.next()?.unwrap(), Value(OsString::from("")));

        // These are weird and questionable, but this seems to be the standard
        // interpretation
        // GNU getopt_long and argparse complain that it could be an abbreviation
        // of every single long option
        // clap complains that "--" is not expected, which matches its treatment
        // of unknown long options
        assert_eq!(p.next()?.unwrap(), Long(""));
        assert_eq!(p.value()?, OsString::from(""));
        assert_eq!(p.next()?.unwrap(), Long(""));
        assert_eq!(p.value()?, OsString::from("3"));

        assert_eq!(p.next()?.unwrap(), Value(OsString::from("-")));
        assert_eq!(p.next()?.unwrap(), Short('x'));
        assert_eq!(p.value()?, OsString::from("--"));
        assert_eq!(p.next()?.unwrap(), Value(OsString::from("-")));
        assert_eq!(p.next()?.unwrap(), Short('x'));
        assert_eq!(p.next()?.unwrap(), Value(OsString::from("")));
        assert_eq!(p.next()?.unwrap(), Value(OsString::from("-")));
        assert_eq!(p.next()?.unwrap(), Value(OsString::from("-x")));
        assert_eq!(p.next()?, None);

        #[cfg(any(unix, target_os = "wasi", windows))]
        {
            let mut q = parse("--=@");
            assert_eq!(q.next()?.unwrap(), Long(""));
            assert_eq!(q.value()?, bad_string("@"));
            assert_eq!(q.next()?, None);
        }

        let mut r = parse("");
        assert_eq!(r.next()?, None);

        Ok(())
    }

    #[test]
    fn test_unicode() -> Result<(), Error> {
        let mut p = parse("-aµ --µ=10 µ --foo=µ");
        assert_eq!(p.next()?.unwrap(), Short('a'));
        assert_eq!(p.next()?.unwrap(), Short('µ'));
        assert_eq!(p.next()?.unwrap(), Long("µ"));
        assert_eq!(p.value()?, "10");
        assert_eq!(p.next()?.unwrap(), Value("µ".into()));
        assert_eq!(p.next()?.unwrap(), Long("foo"));
        assert_eq!(p.value()?, "µ");
        Ok(())
    }

    #[cfg(any(unix, target_os = "wasi", windows))]
    #[test]
    fn test_mixed_invalid() -> Result<(), Error> {
        let mut p = parse("--foo=@@@");
        assert_eq!(p.next()?.unwrap(), Long("foo"));
        assert_eq!(p.value()?, bad_string("@@@"));

        let mut q = parse("-💣@@@");
        assert_eq!(q.next()?.unwrap(), Short('💣'));
        assert_eq!(q.value()?, bad_string("@@@"));

        let mut r = parse("-f@@@");
        assert_eq!(r.next()?.unwrap(), Short('f'));
        assert_eq!(r.next()?.unwrap(), Short('�'));
        assert_eq!(r.next()?.unwrap(), Short('�'));
        assert_eq!(r.next()?.unwrap(), Short('�'));
        assert_eq!(r.next()?, None);

        let mut s = parse("--foo=bar=@@@");
        assert_eq!(s.next()?.unwrap(), Long("foo"));
        assert_eq!(s.value()?, bad_string("bar=@@@"));

        Ok(())
    }

    #[cfg(any(unix, target_os = "wasi", windows))]
    #[test]
    fn test_separate_invalid() -> Result<(), Error> {
        let mut p = parse("--foo @@@");
        assert_eq!(p.next()?.unwrap(), Long("foo"));
        assert_eq!(p.value()?, bad_string("@@@"));
        Ok(())
    }

    #[cfg(any(unix, target_os = "wasi", windows))]
    #[test]
    fn test_invalid_long_option() -> Result<(), Error> {
        let mut p = parse("--@=10");
        assert_eq!(p.next()?.unwrap(), Long("�"));
        assert_eq!(p.value().unwrap(), OsString::from("10"));
        assert_eq!(p.next()?, None);

        let mut q = parse("--@");
        assert_eq!(q.next()?.unwrap(), Long("�"));
        assert_eq!(q.next()?, None);

        Ok(())
    }

    #[test]
    fn test_value_ext() -> Result<(), Error> {
        let s = OsString::from("-10");
        assert_eq!(s.parse::<i32>()?, -10);
        assert_eq!(
            s.parse_with(|s| match s {
                "-10" => Ok(0),
                _ => Err("bad"),
            })?,
            0,
        );
        match s.parse::<u32>() {
            Err(Error::ParsingFailed { value, .. }) => assert_eq!(value, "-10"),
            _ => panic!(),
        }
        match s.parse_with(|s| match s {
            "11" => Ok(0_i32),
            _ => Err("bad"),
        }) {
            Err(Error::ParsingFailed { value, .. }) => assert_eq!(value, "-10"),
            _ => panic!(),
        }
        assert_eq!(s.into_string()?, "-10");
        Ok(())
    }

    #[cfg(any(unix, target_os = "wasi", windows))]
    #[test]
    fn test_value_ext_invalid() -> Result<(), Error> {
        let s = bad_string("foo@");
        assert_matches!(s.parse::<i32>(), Err(Error::NonUnicodeValue(_)));
        assert_matches!(
            s.parse_with(<f32 as FromStr>::from_str),
            Err(Error::NonUnicodeValue(_))
        );
        assert_matches!(
            s.into_string().map_err(Error::from),
            Err(Error::NonUnicodeValue(_))
        );
        Ok(())
    }

    #[test]
    fn test_first_codepoint() {
        assert_eq!(first_codepoint(b"foo").unwrap(), Some('f'));
        assert_eq!(first_codepoint(b"").unwrap(), None);
        assert_eq!(first_codepoint(b"f\xFF\xFF").unwrap(), Some('f'));
        assert_eq!(first_codepoint(b"\xC2\xB5bar").unwrap(), Some('µ'));
        first_codepoint(b"\xFF").unwrap_err();
        assert_eq!(first_codepoint(b"foo\xC2\xB5").unwrap(), Some('f'));
    }

    /// Transform @ characters into invalid unicode.
    fn bad_string(text: &str) -> OsString {
        #[cfg(any(unix, target_os = "wasi"))]
        {
            #[cfg(unix)]
            use std::os::unix::ffi::OsStringExt;
            #[cfg(target_os = "wasi")]
            use std::os::wasi::ffi::OsStringExt;
            let mut text = text.as_bytes().to_vec();
            for ch in &mut text {
                if *ch == b'@' {
                    *ch = b'\xFF';
                }
            }
            OsString::from_vec(text)
        }
        #[cfg(windows)]
        {
            use std::os::windows::ffi::OsStringExt;
            let mut out = Vec::new();
            for ch in text.chars() {
                if ch == '@' {
                    out.push(0xD800);
                } else {
                    let mut buf = [0; 2];
                    out.extend(&*ch.encode_utf16(&mut buf));
                }
            }
            OsString::from_wide(&out)
        }
        #[cfg(not(any(unix, target_os = "wasi", windows)))]
        {
            if text.contains('@') {
                unimplemented!("Don't know how to create invalid OsStrings on this platform");
            }
            text.into()
        }
    }

    /// Very basic exhaustive testing of short combinations of "interesting"
    /// arguments. Should not panic or hang, anything goes otherwise.
    ///
    /// Partially copied from fuzz/fuzz_targets/fuzz_target_1.
    ///
    /// The advantage compared to full fuzzing is that it runs on all platforms
    /// and together with the other tests. cargo-fuzz doesn't work on Windows
    /// and requires a special incantation.
    ///
    /// A disadvantage is that it's still limited by arguments I could think of
    /// and only does very short sequences. Another is that it's bad at
    /// reporting failure, though the println!() helps.
    #[test]
    fn basic_fuzz() {
        #[cfg(any(windows, unix, target_os = "wasi"))]
        const VOCABULARY: &[&str] = &[
            "", "-", "--", "a", "-a", "-aa", "@", "-@", "-a@", "-@a", "--a", "--@", "--a=a",
            "--a=", "--a=@", "--@=a", "--=", "--=@", "--=a", "-@@",
        ];
        #[cfg(not(any(windows, unix, target_os = "wasi")))]
        const VOCABULARY: &[&str] = &[
            "", "-", "--", "a", "-a", "-aa", "--a", "--a=a", "--a=", "--=", "--=a",
        ];
        let vocabulary: Vec<OsString> = VOCABULARY.iter().map(|&s| bad_string(s)).collect();
        let mut permutations = vec![vec![]];
        for _ in 0..3 {
            let mut new = Vec::new();
            for old in permutations {
                for word in &vocabulary {
                    let mut extended = old.clone();
                    extended.push(word);
                    new.push(extended);
                }
            }
            permutations = new;
            for permutation in &permutations {
                println!("{:?}", permutation);
                let permutation: Vec<OsString> = permutation.iter().map(|&s| s.clone()).collect();
                let p = Parser::from_args(permutation.into_iter());
                exhaust(p, 0);
            }
        }
    }

    /// Run every sequence of .next()/.value() on a Parser.
    fn exhaust(parser: Parser, depth: u16) {
        if depth > 100 {
            panic!("Stuck in loop");
        }
        let (mut a, mut b) = dup_parser(parser);
        match a.next() {
            Ok(None) => (),
            _ => exhaust(a, depth + 1),
        }
        match b.value() {
            Err(_) => (),
            _ => exhaust(b, depth + 1),
        }
    }

    /// Clone a parser.
    ///
    /// This is not a Clone impl because the original has to be modified and
    /// I don't want to add interior mutability. It also runs forever if the
    /// iterator is infinite, which seems impolite.
    fn dup_parser(mut parser: Parser) -> (Parser, Parser) {
        let args: Vec<OsString> = parser.source.collect();
        parser.source = Box::new(args.clone().into_iter());
        let new = Parser {
            source: Box::new(args.into_iter()),
            shorts: parser.shorts.clone(),
            #[cfg(windows)]
            shorts_utf16: parser.shorts_utf16.clone(),
            long: parser.long.clone(),
            long_value: parser.long_value.clone(),
            last_option: parser.last_option,
            finished_opts: parser.finished_opts,
            bin_name: parser.bin_name.clone(),
        };
        (parser, new)
    }
}