//! Declare types for interning various objects.

use std::fmt::Debug;

use std::hash::Hash;

use std::sync::{Arc, Mutex, MutexGuard, OnceLock, Weak};

use derive_deftly::define_derive_deftly;

use derive_more::{Deref, Display, Into};

use educe::Educe;

/// Alias to force use of RandomState, regardless of features enabled in `weak_tables`.

///

/// See <https://github.com/tov/weak-table-rs/issues/23> for discussion.

type WeakHashSet<T> = weak_table::WeakHashSet<T, std::hash::RandomState>;

/// A wrapper around [`Arc`] representing owned [`InternCache`] entries.

///

/// The wrapper type serves the purpose of semantic meaning only, implying that

/// this value is cached in some way or another by this module.

///

/// We only conveniently allow obtaining the underlying [`Arc`] with a [`From`] but not the

/// other way around.  This means that interfacing code can make the type to

/// "forget" it originated from an [`InternCache`] but not the other way around,

/// i.e. cannot accidentally create fake entries that look like they came from an

/// [`InternCache`].  If one really has to circumvent this, then the

/// [`Intern::new_uncached_uninterned()`] method exists.

///

/// This ensures that interning is done everywhere that it's expected,

/// avoiding excess memory usage.

//

// Right now, this is the bare minimum of derives; it may need more in the

// future.  If so, just add them.

#[derive(Debug, Default, PartialEq, Eq, Hash, Display, Into, Deref, Educe)]

#[educe(Clone)]

pub struct Intern<T: ?Sized>(Arc<T>);

impl<T: ?Sized> Intern<T> {

    /// Creates an [`Intern`] from an arbitrary [`Arc`].

    ///

    /// The use of this is generally discouraged, as it effectively destroys

    /// the boundary of implying that certain cache entries come from the

    /// [`InternCache`].

}

// Some Arti code is pretty keen on using &Arc<T>.

impl<'a, T: ?Sized> From<&'a Intern<T>> for &'a Arc<T> {

}

/// Offers access to globally available cache for [`InternCache`].

///

/// Typically derived using [`crate::derive_deftly_template_GloballyInternable`].

pub trait GloballyInternable: Sized {

    /// Returns a reference to the global cache instance of this type.

    ///

    /// Implemented by implementors of this trait.

    /// Users of the trait should usually use [`GloballyInternable::into_intern()`].

    fn intern_cache() -> &'static InternCache<Self>;

    /// Places `self` into the global cache.

    ///

    /// Please use this instead of `T::intern_cache().intern(value)`.

    {

}

define_derive_deftly! {

    /// Implement the [`GloballyInternable`] trait for a specific type.

    ///

    /// The implementation in itself is trivial and straightforward with this

    /// macro primarily serving as a convenience method.

    export GloballyInternable for struct:

    impl $crate::intern::GloballyInternable for $ttype {

            static S: $crate::intern::InternCache::<$ttype> = $crate::intern::InternCache::new();

            &S

        }

    }

}

/// An InternCache is a lazily-constructed weak set of objects.

///

/// Let's break that down!  It's "lazily constructed" because it

/// doesn't actually allocate anything until you use it for the first

/// time.  That allows it to have a const [`new`](InternCache::new)

/// method, so you can make these static.

///

/// It's "weak" because it only holds weak references to its objects;

/// once every strong reference is gone, the object is unallocated.

/// Later, the hash entry is (lazily) removed.

pub struct InternCache<T: ?Sized> {

    /// Underlying hashset for interned objects

    //

    // TODO: If WeakHashSet::new is someday const, we can do away with OnceLock here.

    cache: OnceLock<Mutex<WeakHashSet<Weak<T>>>>,

}

impl<T: ?Sized> InternCache<T> {

    /// Create a new, empty, InternCache.

}

impl<T: ?Sized> Default for InternCache<T> {

}

impl<T: Eq + Hash + ?Sized> InternCache<T> {

    /// Helper: initialize the cache if needed, then lock it.

}

impl<T: Eq + Hash> InternCache<T> {

    /// Intern a given value into this cache.

    ///

    /// If `value` is already stored in this cache, we return a

    /// reference to the stored value.  Otherwise, we insert `value`

    /// into the cache, and return that.

        } else {

        }

}

impl<T: Hash + Eq + ?Sized> InternCache<T> {

    /// Intern an object by reference.

    ///

    /// Works with unsized types, but requires that the reference implements

    /// `Into<Arc<T>>`.

    {

        } else {

        }

}

#[cfg(test)]

mod test {

    // @@ begin test lint list maintained by maint/add_warning @@

    #![allow(clippy::bool_assert_comparison)]

    #![allow(clippy::clone_on_copy)]

    #![allow(clippy::dbg_macro)]

    #![allow(clippy::mixed_attributes_style)]

    #![allow(clippy::print_stderr)]

    #![allow(clippy::print_stdout)]

    #![allow(clippy::single_char_pattern)]

    #![allow(clippy::unwrap_used)]

    #![allow(clippy::unchecked_time_subtraction)]

    #![allow(clippy::useless_vec)]

    #![allow(clippy::needless_pass_by_value)]

    #![allow(clippy::string_slice)] // See arti#2571

    //! <!-- @@ end test lint list maintained by maint/add_warning @@ -->

    use super::*;

    #[test]

    fn interning_by_value() {

        // "intern" case.

        let c: InternCache<String> = InternCache::new();

        let s1: Arc<String> = c.intern("abc".to_string()).into();

        let s2 = c.intern("def".to_string()).into();

        let s3 = c.intern("abc".to_string()).into();

        assert!(Arc::ptr_eq(&s1, &s3));

        assert!(!Arc::ptr_eq(&s1, &s2));

        assert_eq!(s2.as_ref(), "def");

        assert_eq!(s3.as_ref(), "abc");

    }

    #[test]

    fn interning_by_ref() {

        // "intern" case.

        let c: InternCache<str> = InternCache::new();

        let s1: Arc<str> = c.intern_ref("abc").into();

        let s2 = c.intern_ref("def").into();

        let s3 = c.intern_ref("abc").into();

        assert!(Arc::ptr_eq(&s1, &s3));

        assert!(!Arc::ptr_eq(&s1, &s2));

        assert_eq!(&*s2, "def");

        assert_eq!(&*s3, "abc");

    }

}