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use std::error;
use std::fmt;

use regex_syntax;

/// An error that occurred while compiling a regular expression.
#[derive(Debug)]
pub enum Error {
    /// A problem with the syntax of a regular expression.
    Syntax(regex_syntax::Error),
    /// Too many instructions resulting from the regular expression.
    ///
    /// The number given is the limit that was exceeded.
    CompiledTooBig(usize),
    /// Too many automata states resulting from the regular expression.
    ///
    /// This is distinct from `CompiledTooBig` because `TooManyStates` refers
    /// to the DFA construction where as `CompiledTooBig` refers to the NFA
    /// construction.
    ///
    /// The number given is the limit that was exceeded.
    TooManyStates(usize),
    /// Lazy quantifiers are not allowed (because they have no useful
    /// interpretation when used purely for automata intersection, as is the
    /// case in this crate).
    NoLazy,
    /// Word boundaries are currently not allowed.
    ///
    /// This restriction may be lifted in the future.
    NoWordBoundary,
    /// Empty or "zero width assertions" such as `^` or `$` are currently
    /// not allowed.
    ///
    /// This restriction may be lifted in the future.
    NoEmpty,
}

impl From<regex_syntax::Error> for Error {
    fn from(err: regex_syntax::Error) -> Error {
        Error::Syntax(err)
    }
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        use self::Error::*;
        match *self {
            Syntax(ref err) => err.fmt(f),
            CompiledTooBig(size_limit) => {
                write!(f, "Compiled regex exceeds size limit of {} bytes",
                       size_limit)
            }
            TooManyStates(size_limit) => {
                write!(f, "Compiled regex exceeds size limit of {} states",
                       size_limit)
            }
            NoLazy => write!(f, "Lazy reptition operators such as '+?' are \
                                 not allowed."),
            NoWordBoundary => write!(f, "Word boundary operators are not \
                                           allowed."),
            NoEmpty => write!(f, "Empty match operators are not allowed \
                                  (hopefully temporary)."),
        }
    }
}

impl error::Error for Error {
    fn description(&self) -> &str {
        use self::Error::*;
        match *self {
            Syntax(ref err) => err.description(),
            CompiledTooBig(_) => "compiled regex is too big",
            TooManyStates(_) => "compiled regex has too many states",
            NoLazy => "lazy repetition operators are not allowed",
            NoWordBoundary => "word boundary operators are not allowed",
            NoEmpty => "empty match operators are not allowed",
        }
    }

    fn cause(&self) -> Option<&error::Error> {
        use self::Error::*;
        match *self {
            Syntax(ref err) => Some(err),
            _ => None,
        }
    }
}