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//! Generalization of [`std::borrow::Cow`]
//!
//! `deref_owned` provides a wrapper [`Owned`] which holds an inner value and
//! points to it by implementing [`Deref`]. `Owned` is similar to [`Cow`],
//! except that it's always owning a value. This can be used to avoid runtime
//! overhead in certain scenarios.
//!
//! Moreover, a trait [`GenericCow<B: ?Sized + ToOwned>: Borrow<B>`](GenericCow)
//! is provided, which allows conversion from certain types into an owned type
//! [`<B as ToOwned>::Owned`](ToOwned::Owned) through an
//! [`.into_owned()`](GenericCow::into_owned) method. The trait can be seen as a
//! generalization of `Cow<'_, B>` and is implemented for:
//!
//! * `&'a B`
//! * `Cow<'a, B>`
//! * `Owned<<B as ToOwned>::Owned>`

use std::borrow::{Borrow, Cow};
use std::fmt;
use std::ops::Deref;

/// Wrapper holding an owned value of type `T` and implementing
/// [`Deref<Target = T>`](Deref)
#[derive(Clone, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Owned<T>(pub T);

impl<T> Deref for Owned<T> {
    type Target = T;
    fn deref(&self) -> &T {
        &self.0
    }
}

impl<B> Borrow<B> for Owned<<B as ToOwned>::Owned>
where
    B: ?Sized + ToOwned,
{
    fn borrow(&self) -> &B {
        self.0.borrow()
    }
}

impl<T, U> AsRef<U> for Owned<T>
where
    T: AsRef<U>,
    U: ?Sized,
{
    fn as_ref(&self) -> &U {
        self.0.as_ref()
    }
}

impl<T: fmt::Display> fmt::Display for Owned<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&self.0, f)
    }
}

impl<T: fmt::Debug> fmt::Debug for Owned<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&self.0, f)
    }
}

/// Types which implement [`Borrow<B>`](Borrow) and can be converted into
/// [`<B as ToOwned>::Owned`](ToOwned::Owned)
pub trait GenericCow<B>: Sized + Borrow<B>
where
    B: ?Sized + ToOwned,
{
    /// Convert into owned type
    fn into_owned(self) -> <B as ToOwned>::Owned;
}

impl<'a, B> GenericCow<B> for &'a B
where
    B: ?Sized + ToOwned,
{
    fn into_owned(self) -> <B as ToOwned>::Owned {
        self.to_owned()
    }
}

impl<'a, B> GenericCow<B> for Cow<'a, B>
where
    B: ?Sized + ToOwned,
{
    fn into_owned(self) -> <B as ToOwned>::Owned {
        Cow::into_owned(self)
    }
}

impl<B> GenericCow<B> for Owned<<B as ToOwned>::Owned>
where
    B: ?Sized + ToOwned,
{
    fn into_owned(self) -> <B as ToOwned>::Owned {
        self.0
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::borrow::Cow;
    #[test]
    fn test_owned() {
        let wrapped: Owned<String> = Owned("Alpha".to_string());
        let reference: &str = &*wrapped;
        assert_eq!(reference, "Alpha");
        let owned: String = GenericCow::<str>::into_owned(wrapped);
        assert_eq!(owned, "Alpha".to_string());
    }
    #[test]
    fn test_cow() {
        let cow_owned: Cow<'static, str> = Cow::Owned("Bravo".to_string());
        assert_eq!(GenericCow::into_owned(cow_owned), "Bravo".to_string());
        let cow_borrowed: Cow<'static, str> = Cow::Borrowed("Charlie");
        assert_eq!(GenericCow::into_owned(cow_borrowed), "Charlie".to_string());
    }
    #[test]
    fn test_ref() {
        let reference: &str = "Delta";
        assert_eq!(GenericCow::into_owned(reference), "Delta".to_string());
    }
    #[test]
    fn test_vec() {
        let wrapped: Owned<Vec<i32>> = Owned(vec![1, 2, 3]);
        assert_eq!(&*wrapped, &[1, 2, 3] as &[i32]);
        assert_eq!(GenericCow::<[i32]>::into_owned(wrapped), vec![1, 2, 3]);
    }
    #[test]
    fn test_generic_fn() {
        fn generic_fn(arg: impl GenericCow<str>) {
            let reference: &str = arg.borrow();
            assert_eq!(reference, "Echo");
            let owned: String = arg.into_owned();
            assert_eq!(owned, "Echo".to_string());
        }
        generic_fn(Owned("Echo".to_string()));
        generic_fn(Cow::Owned("Echo".to_string()));
        generic_fn(Cow::Borrowed("Echo"));
        generic_fn("Echo");
    }
}