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//! Smart pointer owning its pointee and generalization of [`std::borrow::Cow`]
//!
//! `deref_owned` provides a smart pointer [`Owned`], which points to a value
//! that can be borrowed from an owned inner value.
//! It's similar to [`Cow`], except that it's always owning a value.
//!
//! Moreover, a trait [`GenericCow`] is provided, which allows conversion from
//! certain pointers into an owned value though an
//! [`.into_owned()`](GenericCow::into_owned) method. The trait `GenericCow` is
//! implemented for:
//!
//! * `&'a B where B: ToOwned`
//! * `Cow<'a, B> where B: ToOwned`
//! * `Owned<'a, B, O> where O: Borrow<B>`
//!
//! Here
//!
//! * `B: ?Sized` is the borrowed type (and also the type of the
//!   [pointer target](Deref::Target)) and
//! * `O: Borrow<B>` is an owned type.

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

/// Smart pointer to value that can be borrowed from owned inner value
#[derive(Clone, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Owned<B, O>
where
    B: ?Sized,
{
    pub borrowed: PhantomData<*const B>,
    pub owned: O,
}

impl<B, O> Owned<B, O>
where
    B: ?Sized,
{
    pub const fn new(owned: O) -> Self {
        Owned {
            borrowed: PhantomData,
            owned,
        }
    }
}

impl<B, O> Deref for Owned<B, O>
where
    B: ?Sized,
    O: Borrow<B>,
{
    type Target = B;
    fn deref(&self) -> &Self::Target {
        self.owned.borrow()
    }
}

impl<B, O> Borrow<B> for Owned<B, O>
where
    B: ?Sized,
    O: Borrow<B>,
{
    fn borrow(&self) -> &B {
        self.owned.borrow()
    }
}

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

impl<B, O> fmt::Display for Owned<B, O>
where
    B: ?Sized,
    O: fmt::Display,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&self.owned, f)
    }
}

impl<B, O> fmt::Debug for Owned<B, O>
where
    B: ?Sized,
    O: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&self.owned, f)
    }
}

/// Pointer types that can be converted into an owned type
pub trait GenericCow: Deref + Sized {
    /// The type the pointer can be converted into
    type Owned: Borrow<<Self as Deref>::Target>;
    /// Convert into owned type
    fn into_owned(self) -> Self::Owned;
}

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

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

impl<B, O> GenericCow for Owned<B, O>
where
    B: ?Sized,
    O: Borrow<B>,
{
    type Owned = O;
    fn into_owned(self) -> Self::Owned {
        self.owned
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::borrow::Cow;
    #[test]
    fn test_owned() {
        let smart: Owned<str, _> = Owned::new("Alpha".to_string());
        let reference: &str = &*smart;
        assert_eq!(reference, "Alpha");
        let owned: String = smart.into_owned();
        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 smart: Owned<[i32], _> = Owned::new(vec![1, 2, 3]);
        assert_eq!(&*smart, &[1, 2, 3] as &[i32]);
        assert_eq!(GenericCow::into_owned(smart), vec![1, 2, 3]);
    }
    #[test]
    fn test_boxed_slice() {
        let smart: Owned<[i32], _> = Owned::new(vec![1, 2, 3].into_boxed_slice());
        assert_eq!(&*smart, &[1, 2, 3] as &[i32]);
        assert_eq!(
            GenericCow::into_owned(smart),
            vec![1, 2, 3].into_boxed_slice()
        );
    }
    #[test]
    fn test_generic_fn() {
        fn generic_fn(arg: impl Deref<Target = str> + GenericCow<Owned = String>) {
            let reference: &str = &*arg;
            assert_eq!(reference, "Echo");
            let owned: String = arg.into_owned();
            assert_eq!(owned, "Echo".to_string());
        }
        generic_fn(Owned::<str, _>::new("Echo".to_string()));
        generic_fn(Cow::Owned("Echo".to_string()));
        generic_fn(Cow::Borrowed("Echo"));
        generic_fn("Echo");
    }
}