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

    pub type Result<T> = result::Result<T, Error>;

    /// An error that occurred during the construction of an Aho-Corasick
    /// automaton.
    #[derive(Clone, Debug)]
    pub struct Error {
        kind: ErrorKind,
    }

    /// The kind of error that occurred.
    #[derive(Clone, Debug)]
    pub enum ErrorKind {
        /// An error that occurs when constructing an automaton would require the
        /// use of a state ID that overflows the chosen state ID representation.
        /// For example, if one is using `u8` for state IDs and builds a DFA with
        /// 257 states, then the last state's ID will be `256` which cannot be
        /// represented with `u8`.
        StateIDOverflow {
            /// The maximum possible state ID.
            max: usize,
        },
        /// An error that occurs when premultiplication of state IDs is requested
        /// when constructing an Aho-Corasick DFA, but doing so would overflow the
        /// chosen state ID representation.
        ///
        /// When `max == requested_max`, then the state ID would overflow `usize`.
        PremultiplyOverflow {
            /// The maximum possible state id.
            max: usize,
            /// The maximum ID required by premultiplication.
            requested_max: usize,
        }
    }

    impl Error {
        /// Return the kind of this error.
        pub fn kind(&self) -> &ErrorKind {
            &self.kind
        }

        pub(crate) fn state_id_overflow(max: usize) -> Error {
            Error { kind: ErrorKind::StateIDOverflow { max } }
        }

        pub(crate) fn premultiply_overflow(
            max: usize,
            requested_max: usize,
        ) -> Error {
            Error { kind: ErrorKind::PremultiplyOverflow { max, requested_max } }
        }
    }

    impl error::Error for Error {
        fn description(&self) -> &str {
            match self.kind {
                ErrorKind::StateIDOverflow { .. } => {
                    "state id representation too small"
                }
                ErrorKind::PremultiplyOverflow { .. } => {
                    "state id representation too small for premultiplication"
                }
            }
        }
    }

    impl fmt::Display for Error {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match self.kind {
                ErrorKind::StateIDOverflow { max } => {
                    write!(
                        f,
                        "building the automaton failed because it required \
                         building more states that can be identified, where the \
                         maximum ID for the chosen representation is {}",
                        max,
                    )
                }
                ErrorKind::PremultiplyOverflow { max, requested_max } => {
                    if max == requested_max {
                        write!(
                            f,
                            "premultiplication of states requires the ability to \
                             represent a state ID greater than what can fit on \
                             this platform's usize, which is {}",
                            ::std::usize::MAX,
                        )
                    } else {
                        write!(
                            f,
                            "premultiplication of states requires the ability to \
                             represent at least a state ID of {}, but the chosen \
                             representation only permits a maximum state ID of {}",
                            requested_max, max,
                        )
                    }
                }
            }
        }
    }

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