pcast/

lib.rs

//! Same-size tagged data structure conversions.
//!
//! This crate provides a few boilerplate macros to enable conversions between
//! types that are unions with a built-in discriminatory field. An example is a
//! network protocol that consists of multiple packet-types with their
//! respective packet-type indicated by a field on the struct:
//!
//! ```
//! #[repr(C)]
//! pub struct Packet {
//!     packet_type: u8,
//!     // an unknown (depends on packet type) payload
//!     data: [u8; 7],
//! }
//!
//! #[repr(C)]
//! pub struct StatusPacket {
//!     /// must be 0x02 for a status packet
//!     packet_type: u8,
//!     status_0: u8,
//!     status_1: u8,
//!     status_2: u8,
//!     ts: [u8; 4],
//! }
//!
//! #[macro_use]
//! extern crate pcast;
//!
//! pub enum ConversionError {
//!     WrongPacketType
//! }
//!
//! subtype_of!(Packet => StatusPacket | ConversionError {
//!     Ok(())
//! });
//!
//! fn main() {}
//! ```
//!
//! The `StatusPacket` has three fields for various flags and a four byte
//! timestamp here; its presence is indicated by a value of 0x02 in
//! `packet_type`.
//!
//! The `subtype_of` macro can now be used to declare express this. As a
//! result, a `Packet` can be `try_into`'d into a `StatusPacket` and references
//! can be passed because `&StatusPacket` will `Deref` to `&Packet`.
//!
//! A conversion from `&mut StatusPacket` to `&mut Packet` is not included,
//! as altering the `Packet`-structure might violate invariants required
//! by `StatusPacket`.
//!
//! ## TryFrom and TryInto
//!
//! The `TryFrom` and `TryInto` traits are not stabilized yet. `pcast`
//! reexports these for the time being from the `try_from` crate; as soon as
//! they are stabilized in `std::convert`, the export will be updated.

extern crate try_from;

pub use try_from::TryInto;
pub use try_from::TryFrom;

/// Conversion trait used internally by the `subtype_of` macro.
pub trait SubtypeCheck<F, T, E> {
    fn check_is_valid_subtype(&self) -> Result<(), E>;
}

/// Generation conversion traits for subtype of base.
///
/// Syntax:
///
/// ```rust,no_run
/// subtype_of!(Base => Sub | Error { ... })
/// ```
///
/// The macro generates four conversion traits for `Sub`:
///
/// * `Deref<Target=Base>` for `Sub`
/// * `TryFrom<Base, Err=Error>` for `Sub`
/// * `TryFrom<&Base, Err=Error>` for `&Sub`
/// * `TryFrom<&mut Base, Err=Error>` for `&mut Sub`
///
/// All traits use the `{ ... }` block which must expand to a `Result<(),
/// Error>` to check whether or not a conversion is possible.
#[macro_export]
macro_rules! subtype_of {
    ($base:ty => $sub:ty | $cerr:ty $check_fn:block) => (
        impl $crate::SubtypeCheck<$base, $sub, $cerr> for $base {
            fn check_is_valid_subtype(&self) -> Result<(), $cerr> $check_fn
        }

        impl ::std::ops::Deref for $sub {
            type Target = $base;

            #[inline(always)]
            fn deref(&self) -> &$base {
                unsafe { ::std::mem::transmute::<&$sub, &$base>(self) }
            }
        }

        impl $crate::TryFrom<$base> for $sub {
            type Err = $cerr;

            #[inline(always)]
            fn try_from(base: $base) -> Result<Self, Self::Err> {
                try!($crate::SubtypeCheck::<$base, $sub, $cerr>::check_is_valid_subtype(&base));
                Ok(unsafe { ::std::mem::transmute::<$base, $sub>(base) })
            }
        }

        impl<'a> $crate::TryFrom<&'a $base> for &'a $sub {
            type Err = $cerr;

            #[inline(always)]
            fn try_from(base_ref: &$base) -> Result<Self, Self::Err> {
                try!($crate::SubtypeCheck::<$base, $sub, $cerr>::check_is_valid_subtype(base_ref));
                Ok(unsafe { ::std::mem::transmute::<&$base, &$sub>(base_ref) })
            }
        }

        impl<'a> $crate::TryFrom<&'a mut $base> for &'a mut $sub {
            type Err = $cerr;

            #[inline(always)]
            fn try_from(base_ref: &mut $base) -> Result<Self, Self::Err> {
                try!($crate::SubtypeCheck::<$base, $sub, $cerr>::check_is_valid_subtype(base_ref));
                Ok(unsafe { ::std::mem::transmute::<&mut $base, &mut $sub>(base_ref) })
            }
        }

    )
}

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

    #[repr(C)]
    pub struct Packet {
        // packet type: 0x02 is "status"
        packet_type: u8,

        // 7 byte payload.
        // status: 4 byte u32 in big endian byteorder for node id, 3x1 byte status
        data: [u8; 7],
    }

    #[repr(C)]
    pub struct StatusPacket {
        packet_type: u8,
        ts: [u8; 4],
        status_0: u8,
        status_1: u8,
        status_2: u8,
    }

    #[repr(C, packed)]
    pub struct PingPacket {
        packet_type: u8,
        dummy: u32,
        unused: [u8; 3],
    }

    #[repr(C, packed)]
    pub struct PongPacket {
        packet_type: u8,
        dummy: u32,
        unused: [u8; 3],
    }

    pub struct PongConvError {

    }

    subtype_of!(Packet => PingPacket | ConversionError {
        Ok(())
    });
    subtype_of!(Packet => PongPacket | PongConvError {
        Err(PongConvError {})
    });
    subtype_of!(Packet => StatusPacket | () {
        Ok(())
    });

    #[derive(Debug)]
    pub enum ConversionError {}

    impl Packet {
        pub fn get_raw_payload(&self) -> &[u8] {
            &self.data
        }

        pub fn set_raw_payload(&mut self, data: [u8; 7]) {
            self.data = data
        }

        pub fn new(packet_type: u8, data: [u8; 7]) -> Packet {
            Packet {
                packet_type: packet_type,
                data: data,
            }
        }
    }

    impl StatusPacket {
        pub fn get_status_2(&self) -> u8 {
            self.status_2
        }

        pub fn set_status_2(&mut self, v: u8) {
            self.status_2 = v
        }
    }

    /// send takes a raw packet to send
    fn send(packet: &Packet) {
        let _ = packet.get_raw_payload();
        // ...
    }

    fn swallow_status_packet(_: StatusPacket) {
        // goodbye, s!
    }

    #[test]
    fn test_send() {
        let mut owned = Packet::new(2, b"0123456".to_owned());
        send(&owned);

        {
            let status_view: &StatusPacket = (&owned).try_into().unwrap();
            send(status_view);
        }

        let mut status_mut_ref: &mut StatusPacket = (&mut owned).try_into().unwrap();
        status_mut_ref.set_status_2(0x12);
        assert_eq!(status_mut_ref.get_status_2(), 0x12);
        send(&status_mut_ref);
    }

    #[test]
    fn send_from_ref() {
        let mut owned = Packet::new(2, b"0123456".to_owned());

        let pref: &mut Packet = &mut owned;

        send(pref);

        {
            // FIXME: DerefMut not a good idea beacause we don't want
            //        to allow manipulations on a reference -- it might
            //        invalidate StatusPacket
            //        &(*pref) seems kind of silly though to drop the mut
            let status_view: &StatusPacket = (&(*pref)).try_into().unwrap();
            send(&status_view);
        }

        let mut status_mut_view: &mut StatusPacket = pref.try_into().unwrap();
        status_mut_view.set_status_2(0x12);
        assert_eq!(status_mut_view.get_status_2(), 0x12);
        send(&status_mut_view);
    }

    #[test]
    fn call_base_and_sub_methods() {
        let mut owned = Packet::new(2, b"0123456".to_owned());
        owned.set_raw_payload(b"xxxxxxx".to_owned());

        {
            let status_view: &StatusPacket = (&owned).try_into().unwrap();
            status_view.get_status_2();
            status_view.get_raw_payload();
        }

        let mut status_mut_view: &mut StatusPacket = (&mut owned).try_into().unwrap();
        status_mut_view.get_status_2();
        status_mut_view.get_raw_payload();
        status_mut_view.set_status_2(0x34);

        // does not work (and shouldn't):
        // status_mut_view.set_raw_payload(b"xxxxxxx".to_owned());
    }

    #[test]
    fn create_from_immutable_ref() {
        let v = vec![Packet::new(2, b"0123456".to_owned())];

        for p in v.iter() {
            let status_view: &StatusPacket = (&(*p)).try_into().unwrap();
            status_view.get_status_2();
        }
    }

    #[test]
    fn use_owned_status() {
        let p = Packet::new(2, b"0123456".to_owned());

        let s: StatusPacket = p.try_into().unwrap();

        swallow_status_packet(s);
    }
}