use crate::en::Encoder;

/// Trait governing how a type is encoded as a packed value.
///
/// Packed encodings are ones where data follow one after another, with no
/// "metadata" indicating when one value starts and another stops.
///
/// This is typically used automatically through the `#[musli(packed)]`
/// attribute through the [`Decode` derive].
///
/// [`Decode` derive]: https://docs.rs/musli/latest/musli/_help/derives/
///
/// # Examples
///
/// ```
/// use musli::Encode;
///
/// #[derive(Encode)]
/// struct PackedType {
///     #[musli(packed)]
///     data: (u32, u32),
/// }
/// ```
///
/// Implementing manually:
///
/// ```
/// use musli::{Encode, Encoder};
/// use musli::en::SequenceEncoder;
///
/// struct PackedType {
///     data: (u32, u32),
/// }
///
/// impl<M> Encode<M> for PackedType {
///     type Encode = Self;
///
///     #[inline]
///     fn encode<E>(&self, encoder: E) -> Result<(), E::Error>
///     where
///         E: Encoder,
///     {
///         let mut pack = encoder.encode_pack()?;
///         pack.push(&self.data.0);
///         pack.push(&self.data.1);
///         pack.finish_sequence()
///     }
///
///     #[inline]
///     fn as_encode(&self) -> &Self::Encode {
///         self
///     }
/// }
/// ```
pub trait EncodePacked<M> {
    /// Encode the given output as bytes.
    fn encode_packed<E>(&self, encoder: E) -> Result<(), E::Error>
    where
        E: Encoder<Mode = M>;

    /// The number of fields in the type.
    #[inline]
    fn size_hint(&self) -> Option<usize> {
        None
    }
}

impl<T, M> EncodePacked<M> for &T
where
    T: ?Sized + EncodePacked<M>,
{
    #[inline]
    fn encode_packed<E>(&self, encoder: E) -> Result<(), E::Error>
    where
        E: Encoder<Mode = M>,
    {
        (**self).encode_packed(encoder)
    }
}

impl<T, M> EncodePacked<M> for &mut T
where
    T: ?Sized + EncodePacked<M>,
{
    #[inline]
    fn encode_packed<E>(&self, encoder: E) -> Result<(), E::Error>
    where
        E: Encoder<Mode = M>,
    {
        (**self).encode_packed(encoder)
    }
}
