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use crate::{AsIs, AsIsMut, Is, IsMut};
use core::borrow::{Borrow, BorrowMut};
use core::cmp::Ordering;
use core::fmt;
use core::hash::{Hash, Hasher};
use core::ops::{Deref, DerefMut};
use core::str::{from_utf8, from_utf8_mut};

#[cfg(not(feature = "alloc"))]
use crate::ToOwned;

/// A stub for [`String`] used in a `no_std` environment.
///
/// [`String`]: https://doc.rust-lang.org/alloc/string/struct.String.html
#[derive(Default, Clone)]
pub struct StringStub([u8; 0]);

impl Deref for StringStub {
    type Target = str;

    fn deref(&self) -> &Self::Target {
        from_utf8(&self.0).ok().unwrap()
    }
}

impl DerefMut for StringStub {
    fn deref_mut(&mut self) -> &mut Self::Target {
        from_utf8_mut(&mut self.0).ok().unwrap()
    }
}

impl Borrow<str> for StringStub {
    fn borrow(&self) -> &str {
        self
    }
}

impl BorrowMut<str> for StringStub {
    fn borrow_mut(&mut self) -> &mut str {
        self
    }
}

impl PartialEq for StringStub {
    fn eq(&self, other: &Self) -> bool {
        (**self).eq(&**other)
    }
}

impl Eq for StringStub {}

impl PartialOrd for StringStub {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        (**self).partial_cmp(&**other)
    }
}

impl Ord for StringStub {
    fn cmp(&self, other: &Self) -> Ordering {
        (**self).cmp(&**other)
    }
}

impl Hash for StringStub {
    fn hash<H: Hasher>(&self, state: &mut H) {
        (**self).hash(state);
    }
}

impl fmt::Debug for StringStub {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[cfg(not(feature = "alloc"))]
impl ToOwned for str {
    type Owned = StringStub;
}

#[cfg(feature = "alloc")]
use alloc::string::String;

#[cfg(not(feature = "alloc"))]
use StringStub as String;

impl AsIs for String {
    type Is = String;

    fn as_is<'a>(self) -> Is<'a, Self::Is> {
        Is::Owned(self)
    }
}

impl AsIsMut for String {
    fn as_is_mut<'a>(self) -> IsMut<'a, Self::Is> {
        IsMut::Owned(self)
    }
}

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

    #[test]
    fn string_stub() {
        let s = StringStub::default();

        assert_eq!(s.clone(), s);
        assert_eq!(s < s, "" < "");
        assert_eq!(s.cmp(&s), "".cmp(&""));
    }
}