use std::fmt;

/// Errors originating from WAL operations.
///
/// Covers I/O failures, data corruption detected during recovery,
/// and lifecycle violations (e.g., appending after shutdown).
#[derive(Debug)]
pub enum WalError {
    /// Underlying filesystem I/O failure.
    Io(std::io::Error),
    /// Data corruption detected (BLAKE3 mismatch, invalid magic, etc.).
    Corruption { message: String },
    /// Current segment is full; internal signal to trigger rotation.
    SegmentFull,
    /// Attempted append after WAL has been shut down.
    Closed,
    /// Channel send to writer thread failed (writer thread panicked or exited).
    SendFailed,
    /// Writer thread join failed during shutdown.
    ShutdownFailed,
}

impl fmt::Display for WalError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Io(source) => write!(f, "WAL I/O error: {source}"),
            Self::Corruption { message } => write!(f, "WAL corruption: {message}"),
            Self::SegmentFull => f.write_str("WAL segment full"),
            Self::Closed => f.write_str("WAL closed"),
            Self::SendFailed => f.write_str("WAL channel send failed"),
            Self::ShutdownFailed => f.write_str("WAL shutdown failed"),
        }
    }
}

impl std::error::Error for WalError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            Self::Io(source) => Some(source),
            _ => None,
        }
    }
}

impl From<std::io::Error> for WalError {
    fn from(e: std::io::Error) -> Self {
        Self::Io(e)
    }
}

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

    #[test]
    fn display_io() {
        let e = WalError::Io(std::io::Error::new(
            std::io::ErrorKind::NotFound,
            "file not found",
        ));
        assert!(e.to_string().contains("I/O error"));
        assert!(e.to_string().contains("file not found"));
    }

    #[test]
    fn display_corruption() {
        let e = WalError::Corruption {
            message: "bad checksum".into(),
        };
        assert_eq!(e.to_string(), "WAL corruption: bad checksum");
    }

    #[test]
    fn display_segment_full() {
        assert_eq!(WalError::SegmentFull.to_string(), "WAL segment full");
    }

    #[test]
    fn display_closed() {
        assert_eq!(WalError::Closed.to_string(), "WAL closed");
    }

    #[test]
    fn display_send_failed() {
        assert_eq!(WalError::SendFailed.to_string(), "WAL channel send failed");
    }

    #[test]
    fn display_shutdown_failed() {
        assert_eq!(WalError::ShutdownFailed.to_string(), "WAL shutdown failed");
    }

    #[test]
    fn from_io_error() {
        let io_err = std::io::Error::new(std::io::ErrorKind::Other, "disk full");
        let wal_err: WalError = io_err.into();
        assert!(matches!(wal_err, WalError::Io(_)));
    }

    #[test]
    fn source_io() {
        use std::error::Error;
        let e = WalError::Io(std::io::Error::new(std::io::ErrorKind::Other, "test"));
        assert!(e.source().is_some());
    }

    #[test]
    fn source_corruption_is_none() {
        use std::error::Error;
        let e = WalError::Corruption {
            message: "test".into(),
        };
        assert!(e.source().is_none());
    }

    #[test]
    fn source_closed_is_none() {
        use std::error::Error;
        assert!(WalError::Closed.source().is_none());
    }
}