use alloc::string::{String, ToString};

use regex_automata::meta;

/// An error that occurred during parsing or compiling a regular expression.

#[non_exhaustive]

#[derive(Clone, PartialEq)]

pub enum Error {

    /// A syntax error.

    Syntax(String),

    /// The compiled program exceeded the set size

    /// limit. The argument is the size limit imposed by

    /// [`RegexBuilder::size_limit`](crate::RegexBuilder::size_limit). Even

    /// when not configured explicitly, it defaults to a reasonable limit.

    ///

    /// If you're getting this error, it occurred because your regex has been

    /// compiled to an intermediate state that is too big. It is important to

    /// note that exceeding this limit does _not_ mean the regex is too big to

    /// _work_, but rather, the regex is big enough that it may wind up being

    /// surprisingly slow when used in a search. In other words, this error is

    /// meant to be a practical heuristic for avoiding a performance footgun,

    /// and especially so for the case where the regex pattern is coming from

    /// an untrusted source.

    ///

    /// There are generally two ways to move forward if you hit this error.

    /// The first is to find some way to use a smaller regex. The second is to

    /// increase the size limit via `RegexBuilder::size_limit`. However, if

    /// your regex pattern is not from a trusted source, then neither of these

    /// approaches may be appropriate. Instead, you'll have to determine just

    /// how big of a regex you want to allow.

    CompiledTooBig(usize),

}

impl Error {

    pub(crate) fn from_meta_build_error(err: meta::BuildError) -> Error {

        if let Some(size_limit) = err.size_limit() {

            Error::CompiledTooBig(size_limit)

        } else if let Some(ref err) = err.syntax_error() {

            Error::Syntax(err.to_string())

        } else {

            // This is a little suspect. Technically there are more ways for

            // a meta regex to fail to build other than "exceeded size limit"

            // and "syntax error." For example, if there are too many states

            // or even too many patterns. But in practice this is probably

            // good enough. The worst thing that happens is that Error::Syntax

            // represents an error that isn't technically a syntax error, but

            // the actual message will still be shown. So... it's not too bad.

            //

            // We really should have made the Error type in the regex crate

            // completely opaque. Rookie mistake.

            Error::Syntax(err.to_string())

        }

    }

}

#[cfg(feature = "std")]

impl std::error::Error for Error {

    // TODO: Remove this method entirely on the next breaking semver release.

    #[allow(deprecated)]

    fn description(&self) -> &str {

        match *self {

            Error::Syntax(ref err) => err,

            Error::CompiledTooBig(_) => "compiled program too big",

        }

    }

}

impl core::fmt::Display for Error {

    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {

        match *self {

            Error::Syntax(ref err) => err.fmt(f),

            Error::CompiledTooBig(limit) => write!(

                f,

                "Compiled regex exceeds size limit of {limit} bytes.",

            ),

        }

    }

}

// We implement our own Debug implementation so that we show nicer syntax

// errors when people use `Regex::new(...).unwrap()`. It's a little weird,

// but the `Syntax` variant is already storing a `String` anyway, so we might

// as well format it nicely.

impl core::fmt::Debug for Error {

    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {

        match *self {

            Error::Syntax(ref err) => {

                let hr: String = core::iter::repeat('~').take(79).collect();

                writeln!(f, "Syntax(")?;

                writeln!(f, "{hr}")?;

                writeln!(f, "{err}")?;

                writeln!(f, "{hr}")?;

                write!(f, ")")?;

                Ok(())

            }

            Error::CompiledTooBig(limit) => {

                f.debug_tuple("CompiledTooBig").field(&limit).finish()

            }

        }

    }

}