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use nix;
use std::error;
use std::fmt;
use std::io;
use std::num;

/// This module implements a simple error type to match the errors that can be thrown from the C
/// functions as well as some extra errors resulting from internal validations.
///
/// As this crate exposes an opinionated API to the POSIX queues certain errors have been
/// ignored:
///
/// * ETIMEDOUT: The low-level timed functions are not exported and this error can not occur.
/// * EAGAIN: Non-blocking queue calls are not supported.
/// * EINVAL: Same reason as ETIMEDOUT
/// * EMSGSIZE: The message size is immutable after queue creation and this crate checks it.
/// * ENAMETOOLONG: This crate performs name validation
///
/// If an unexpected error is encountered it will be wrapped appropriately and should be reported
/// as a bug on https://b.tvl.fyi

#[derive(Debug)]
pub enum Error {
    // These errors are raised inside of the library
    InvalidQueueName(&'static str),
    ValueReadingError(io::Error),
    MessageSizeExceeded(),
    MaximumMessageSizeExceeded(),
    MaximumMessageCountExceeded(),

    // These errors match what is described in the man pages (from mq_overview(7) onwards).
    PermissionDenied(),
    InvalidQueueDescriptor(),
    QueueCallInterrupted(),
    QueueAlreadyExists(),
    QueueNotFound(),
    InsufficientMemory(),
    InsufficientSpace(),

    // These two are (hopefully) unlikely in modern systems
    ProcessFileDescriptorLimitReached(),
    SystemFileDescriptorLimitReached(),

    // If an unhandled / unknown / unexpected error occurs this error will be used.
    // In those cases bug reports would be welcome!
    UnknownForeignError(nix::errno::Errno),

    // Some other unexpected / unknown error occured. This is probably an error from
    // the nix crate. Bug reports also welcome for this!
    UnknownInternalError(),
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        use Error::*;
        f.write_str(match *self {
            // This error contains more sensible description strings already
            InvalidQueueName(e) => e,
            ValueReadingError(_) => "error reading system configuration for message queues",
            MessageSizeExceeded() => "message is larger than maximum size for specified queue",
            MaximumMessageSizeExceeded() => "specified queue message size exceeds system maximum",
            MaximumMessageCountExceeded() => "specified queue message count exceeds system maximum",
            PermissionDenied() => "permission to the specified queue was denied",
            InvalidQueueDescriptor() => "the internal queue descriptor was invalid",
            QueueCallInterrupted() => "queue method interrupted by signal",
            QueueAlreadyExists() => "the specified queue already exists",
            QueueNotFound() => "the specified queue could not be found",
            InsufficientMemory() => "insufficient memory to call queue method",
            InsufficientSpace() => "insufficient space to call queue method",
            ProcessFileDescriptorLimitReached() => {
                "maximum number of process file descriptors reached"
            }
            SystemFileDescriptorLimitReached() => {
                "maximum number of system file descriptors reached"
            }
            UnknownForeignError(_) => "unknown foreign error occured: please report a bug!",
            UnknownInternalError() => "unknown internal error occured: please report a bug!",
        })
    }
}

impl error::Error for Error {
    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
        match self {
            Error::ValueReadingError(e) => Some(e),
            Error::UnknownForeignError(e) => Some(e),
            _ => None,
        }
    }
}

/// This from implementation is used to translate errors from the lower-level
/// C-calls into sensible Rust errors.
impl From<nix::errno::Errno> for Error {
    fn from(err: nix::Error) -> Self {
        use nix::errno::Errno::*;
        match err {
            EACCES => Error::PermissionDenied(),
            EBADF => Error::InvalidQueueDescriptor(),
            EINTR => Error::QueueCallInterrupted(),
            EEXIST => Error::QueueAlreadyExists(),
            EMFILE => Error::ProcessFileDescriptorLimitReached(),
            ENFILE => Error::SystemFileDescriptorLimitReached(),
            ENOENT => Error::QueueNotFound(),
            ENOMEM => Error::InsufficientMemory(),
            ENOSPC => Error::InsufficientSpace(),
            _ => Error::UnknownForeignError(err),
        }
    }
}

// This implementation is used when reading system queue settings.
impl From<io::Error> for Error {
    fn from(e: io::Error) -> Self {
        Error::ValueReadingError(e)
    }
}

// This implementation is used when parsing system queue settings. The unknown error is returned
// here because the system is probably seriously broken if those files don't contain numbers.
impl From<num::ParseIntError> for Error {
    fn from(_: num::ParseIntError) -> Self {
        Error::UnknownInternalError()
    }
}