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

use raw::FstType;

/// An error that occurred while using a finite state transducer.
#[derive(Debug)]
pub enum Error {
    /// A version mismatch occurred while reading a finite state transducer.
    ///
    /// This occurs when the API version (of the crate) does not match the
    /// version encoded in the finite state transducer.
    ///
    /// When this error is encountered, there are only two ways to fix it:
    ///
    /// 1. Change the version of the library to one that is compatible with
    ///    the given finite state transducer.
    /// 2. Rebuild the finite state transducer.
    Version {
        /// The expected version, which is hard-coded into the current version
        /// of this crate.
        expected: u64,
        /// The version read from the finite state transducer.
        got: u64,
    },
    /// An unexpected error occurred while reading a finite state transducer.
    /// Usually this occurs because the data is corrupted or is not actually
    /// a finite state transducer serialized by this library.
    Format,
    /// A duplicate key was inserted into a finite state transducer, which is
    /// not allowed.
    DuplicateKey {
        /// The duplicate key.
        got: Vec<u8>,
    },
    /// A key was inserted out of order into a finite state transducer.
    ///
    /// Keys must always be inserted in lexicographic order.
    OutOfOrder {
        /// The last key successfully inserted.
        previous: Vec<u8>,
        /// The key that caused this error to occur.
        got: Vec<u8>,
    },
    /// A finite state transducer with an unexpected type was found.
    ///
    /// This is not currently used in this crate, but callers may wish to
    /// employ its use for alternative data structures implemented on top of
    /// finite state transducers.
    WrongType {
        /// The expected finite state transducer type.
        expected: FstType,
        /// The type read from a finite state transducer.
        got: FstType,
    },
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        use self::Error::*;
        match *self {
            Version { expected, got } => {
                write!(f, "\
Error opening FST: expected API version {}, got API version {}.
It looks like the FST you're trying to open is either not an FST file or it
was generated with a different version of the 'fst' crate. You'll either need
to change the version of the 'fst' crate you're using, or re-generate the
FST.", expected, got)
            }
            Format => write!(f, "\
Error opening FST: An unknown error occurred. This usually means you're trying
to read data that isn't actually an encoded FST."),
            DuplicateKey { ref got } => write!(f, "\
Error inserting duplicate key: {}.", format_bytes(&*got)),
            OutOfOrder { ref previous, ref got } => write!(f, "\
Error inserting out-of-order key: {}. (Previous key was {}.) Keys must be
inserted in lexicographic order.",
format_bytes(&*got), format_bytes(&*previous)),
            WrongType { expected, got } => write!(f, "\
Error opening FST: expected type {}, got type {}.", expected, got),
        }
    }
}

impl error::Error for Error {
    fn description(&self) -> &str {
        use self::Error::*;
        match *self {
            Version { .. } => "incompatible version found when opening FST",
            Format => "unknown invalid format found when opening FST",
            DuplicateKey { .. } => "duplicate key insertion",
            OutOfOrder { .. } => "out-of-order key insertion",
            WrongType { .. } => "incompatible type found when opening FST",
        }
    }

    fn cause(&self) -> Option<&error::Error> {
        None
    }
}

/// Attempt to convert an arbitrary byte string to a more convenient display
/// form.
///
/// Essentially, try to decode the bytes as UTF-8 and show that. Failing that,
/// just show the sequence of bytes.
fn format_bytes(bytes: &[u8]) -> String {
    match str::from_utf8(bytes) {
        Ok(s) => s.to_owned(),
        Err(_) => format!("{:?}", bytes),
    }
}