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//! A container which provides typed scratch storage for external owners.

// Copyright 2023 Eric Michael Sumner
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use std::collections::{HashMap,HashSet};
use std::hash::{Hash,Hasher};
use std::borrow::Borrow;

use parking_lot::Mutex;

#[cfg(test)] mod tests;

type Id = u128;

/// A non-cloneable, guaranteed-unique `Id`
#[derive(Debug)]
struct UniqueId(Id);

impl Default for UniqueId {
    fn default()->Self {
        static NEXT: Mutex<Id> = Mutex::new(0);
        let mut guard = NEXT.lock();
        let result = *guard;
        *guard = result.checked_add(1).unwrap();
        UniqueId(result)
    }
}

impl std::ops::Deref for UniqueId {
    type Target = Id;
    fn deref(&self)->&Id { &self.0 }
}

/// A typed slot for `Guest`s to store private values.
/// Stores multiple values, one per guest.
#[derive(Default,Debug)]
pub struct GuestCell<T:?Sized> {
    map: Mutex<HashMap<Id, Box<T>>>,
    id: UniqueId
}

/// Accessor for a `GuestCell`.
/// Safety: `borrow` must always return the same `GuestCell`
pub unsafe trait IndirectGuestCell: Borrow<GuestCell<Self::Inner>> {
    type Inner: ?Sized;
}

unsafe impl<T:?Sized> IndirectGuestCell for &'_ GuestCell<T> { type Inner = T; }
unsafe impl<T:?Sized> IndirectGuestCell for std::sync::Arc<GuestCell<T>> { type Inner = T; }
unsafe impl<T:?Sized> IndirectGuestCell for std::rc::Rc<GuestCell<T>> { type Inner = T; }

trait ErasedGuestCell {
    fn guest_drop(&self, guest: &mut UniqueId);
    fn id(&self)->&UniqueId;
}

impl<T:?Sized> ErasedGuestCell for GuestCell<T> {
    fn guest_drop(&self, guest_id: &mut UniqueId) {
        self.map.lock().remove(&guest_id).unwrap();
    }

    fn id(&self)->&UniqueId { &self.id }
}

impl<Ptr:IndirectGuestCell> ErasedGuestCell for Ptr {
    fn guest_drop(&self, guest: &mut UniqueId) {
        self.borrow().guest_drop(guest)
    }

    fn id(&self)->&UniqueId {
        self.borrow().id()
    }
}

impl PartialEq for dyn ErasedGuestCell + '_ {
    fn eq(&self, rhs: &dyn ErasedGuestCell)->bool {
        **self.id() == **rhs.id()
    }
}

impl Eq for dyn ErasedGuestCell + '_ {}

impl Hash for dyn ErasedGuestCell + '_ {
    fn hash<H:Hasher>(&self, h:&mut H) {
        self.id().hash(h);
    }
}

/// An owner for private values stored in various `GuestCell`s.
#[derive(Default)]
pub struct Guest<'a> {
    id: UniqueId,
    cells: HashSet<Box<dyn ErasedGuestCell + 'a>>,
}

impl Drop for Guest<'_> {
    fn drop(&mut self) {
        for map in &self.cells {
            map.guest_drop(&mut self.id);
        }
    }
}

impl std::fmt::Debug for Guest<'_> {
    fn fmt(&self, f:&mut std::fmt::Formatter<'_>)->Result<(), std::fmt::Error> {
        f.debug_struct("Guest")
            .field("id", &self.id)
            .field("cells", &self.cells.iter().map(|x| x.id()).collect::<Vec<_>>())
            .finish()
    }
}

impl<'a> Guest<'a> {
    pub fn get<T:?Sized>(&self, cell: impl 'a + IndirectGuestCell<Inner=T>)->Option<&T> {
        let guard = cell.borrow().map.lock();
        let boxed:&Box<T> = guard.get(&self.id)?;

        // Safety: Access to this box is mediated through self.id 
        Some(unsafe { &*((&**boxed) as *const T) })
    }

    pub fn get_mut<T:?Sized>(&mut self, cell: impl 'a + IndirectGuestCell<Inner=T>)->Option<&mut T> {
        let mut guard = cell.borrow().map.lock();
        let boxed:&mut Box<T> = guard.get_mut(&self.id)?;

        // Safety: Access to this box is mediated through self.id 
        Some(unsafe { &mut *((&mut **boxed) as *mut T) })
    }

    pub fn set<T:?Sized>(&mut self, cell: impl 'a + IndirectGuestCell<Inner=T>, val:Box<T>)->Option<Box<T>> {
        let result = cell.borrow().map.lock().insert(*self.id, val);
        self.cells.insert(Box::new(cell));
        result
    }

    pub fn take<T:?Sized>(&mut self, cell: impl 'a + IndirectGuestCell<Inner=T>)->Option<Box<T>> {
        let result = cell.borrow().map.lock().remove(&self.id);
        self.cells.remove(&cell as &dyn ErasedGuestCell);
        result
    }
}