/*  Mycelium Scheme
 *  Copyright (C) 2025 Ava Affine
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

use core::ops::{Index, Deref, DerefMut};
use core::ptr::NonNull;
use core::cell::RefCell;

use alloc::rc::Rc;
use alloc::vec::Vec;
use alloc::boxed::Box;
use alloc::string::String;

use organelle::Number;

/* NOTE
 * decided not to implement a cache or a singleton heap manager
 * because I did not want to involve a datatype that would add
 * unneeded logic to where and how the Rcs get allocated or that
 * would require relocation if more Rcs were allocated. Any
 * ADT containing the source data referenced by Gc would add
 * overhead without value.
 *
 * Meanwhile, just using allocated-at-site Rcs provides accurate
 * reference counting garbage collection. We hack the Box::into_raw
 * function to pass around heap allocated Rcs.
 */

/* Gc
 * This is a heap allocated Rc passed around such that it fits into
 * a physical register. The pointer is to a Box<Rc<T>>, but custom
 * deref implementation will ensure that deref always points to the
 * encapsulated T
 */
#[repr(transparent)]
pub struct Gc<T>(NonNull<Rc<T>>);

impl<T> From<Rc<T>> for Gc<T> {
    fn from(src: Rc<T>) -> Self {
        Gc(NonNull::new(Box::into_raw(Box::new(src.clone())))
            .expect("GC obj from rc nonnull ptr check"))
    }
}

impl<T> From<T> for Gc<T> {
    fn from(value: T) -> Self {
        Gc(NonNull::new(Box::into_raw(Box::new(Rc::from(value))))
            .expect("GC obj from datum nonnull ptr check"))
    }
}

impl<T: PartialEq> PartialEq for Gc<T> {
    fn eq(&self, other: &Self) -> bool {
        self.deref().eq(other.deref())
    }

    fn ne(&self, other: &Self) -> bool {
        self.deref().ne(other.deref())
    }
}

impl<T> Deref for Gc<T> {
    type Target = T;
    fn deref(&self) -> &Self::Target {
        unsafe {
            Rc::<T>::as_ptr(self.0.as_ref())
                .as_ref()
                .expect("GC obj deref inconsistent rc ptr")
        }
    }
}

impl<T> DerefMut for Gc<T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        unsafe {
            (Rc::<T>::as_ptr(self.0.as_mut()) as *mut T)
                .as_mut()
                .expect("GC obj inconsistent rc ptr")
        }
    }
}

// takes a pointer to target Rc
macro_rules! shallow_copy_rc {
    ( $src:expr ) => {
        unsafe {
            NonNull::new(Box::into_raw(Box::new((*$src).clone())))
                .expect("GC obj shallow copy nonnull ptr check")
        }
    }
}

impl<T> Clone for Gc<T> {
    fn clone(&self) -> Self {
        Gc(shallow_copy_rc!(self.0.as_ptr()))
    }

    fn clone_from(&mut self, source: &Self) {
        self.0 = shallow_copy_rc!(source.0.as_ptr());
    }
}

impl<T> Drop for Gc<T> {
    fn drop(&mut self) {
        unsafe {
            let a = Box::<Rc<T>>::from_raw(self.0.as_ptr());
            drop(a)
        }
    }
}

impl<T: Clone> Gc<T> {
    #[inline]
    pub fn deep_copy(&self) -> Gc<T> {
        Gc(unsafe {
            NonNull::new(Box::into_raw(Box::new(Rc::from(
                            self.0.as_ref().clone()))))
                .expect("GC obj deep copy nonnull ptr check")
        })
    }
}

#[derive(PartialEq)]
pub enum Datum {
    Number(Number),
    Bool(bool),
    Cons(Cons),
    Symbol(String),
    Char(u8),
    String(Vec<u8>),
    Vector(RefCell<Vec<Gc<Datum>>>),
    ByteVector(RefCell<Vec<u8>>),
    None
}

// implemented by hand to force deep copy on Cons datum
impl Clone for Datum {
    fn clone(&self) -> Datum {
        match self {
            Datum::Number(n) => Datum::Number(n.clone()),
            Datum::Bool(n) => Datum::Bool(n.clone()),
            Datum::Cons(n) => Datum::Cons(n.deep_copy()),
            Datum::Symbol(n) => Datum::Symbol(n.clone()),
            Datum::Char(n) => Datum::Char(n.clone()),
            Datum::String(n) => Datum::String(n.clone()),
            Datum::Vector(n) =>
                Datum::Vector(n.clone()),
            Datum::ByteVector(n) =>
                Datum::ByteVector(n.clone()),
            Datum::None => Datum::None,
        }
    }

    fn clone_from(&mut self, source: &Self) {
        *self = source.clone();
    }
}

#[derive(Clone, PartialEq)]
pub struct Cons(pub Option<Gc<Datum>>, pub Option<Gc<Datum>>);

impl Cons {
    pub fn deep_copy(&self) -> Cons {
        macro_rules! car_cpy {
            () => {
                if let Some(ref car) = self.0 {
                    if let Datum::Cons(ref car) = **car {
                        Some(Datum::Cons(car.deep_copy()).into())
                    } else {
                        self.0.as_ref().map(|x| x.deep_copy())
                    }
                } else {
                    None
                }
            }
        }

        if let Some(ref next) = self.1 {
            if let Datum::Cons(ref next) = **next {
                Cons(car_cpy!(), Some(Datum::Cons(next.deep_copy()).into()))
            } else {
                Cons(car_cpy!(), self.1.as_ref().map(|x| x.deep_copy()))
            }
        } else {
            Cons(car_cpy!(), None)
        }
    }

    pub fn subsl(&self, start: isize, end: isize) -> Cons {
        if end - start == 1 {
            return Cons(Some(self[start as usize].clone()), None)
        }

        if end == 0 {
            return Cons(
                self.0.clone(),
                None
            )
        }

        let Some(ref next) = self.1 else {
            panic!("out of bounds subsl of cons list")
        };

        let Datum::Cons(ref next) = **next else {
            panic!("subsl of cons list not in standard form")
        };

        if start <= 0 {
            Cons(self.0.clone(),
                 Some(Datum::Cons(next.subsl(start - 1, end - 1))
                                    .into()))

        } else {
            next.subsl(start - 1, end - 1)
        }
    }

    pub fn len(&self) -> usize {
        let Some(_) = self.0 else {
            return 0
        };

        let Some(ref next) = self.1 else {
            return 1
        };

        let Datum::Cons(ref next) = **next else {
            // weird list but okay
            return 2
        };

        1 + next.len()
    }

    pub fn append(&mut self,  arg: Gc<Datum>) {
        let Some(_) = self.0 else {
            self.0 = Some(arg);
            return
        };

        if let Some(next) = &mut self.1 {
            if let Datum::Cons(next) = next.deref_mut() {
               next.append(arg);
               return;
            }
        }

        self.1 = Some(Datum::Cons(Cons(Some(arg), None)).into());
    }
}


impl Index<usize> for Cons {
    type Output = Gc<Datum>;
    fn index(&self, index: usize) -> &Self::Output {
        if index == 0 {
            if let Some(data) = &self.0 {
                data
            } else {
                panic!("out of bounds indexing into cons list")
            }
        } else {
            let Some(ref next) = self.1 else {
                panic!("out of bounds indexing into cons list")
            };

            let Datum::Cons(ref next) = **next else {
                panic!("cons list not in standard form")
            };

            &next[index - 1]
        }
    }
}


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

    #[derive(Debug)]
    struct GcTester<'a>(pub &'a mut bool);

    impl Clone for GcTester<'_> {
        fn clone(&self) -> Self {
            unsafe {
                GcTester(Box::into_raw(Box::new(self.0.clone()))
                    .as_mut()
                    .expect("Gc Test obj mut ref fail"))
            }
        }

        fn clone_from(&mut self, _: &Self) {
            unimplemented!("test impl")
        }
    }

    impl Drop for GcTester<'_> {
        fn drop(&mut self) {
            *self.0 = true;
        }
    }

    impl PartialEq for GcTester<'_> {
        fn eq(&self, other: &Self) -> bool {
            *(self.0) == *(other.0)
        }

        fn ne(&self, other: &Self) -> bool {
            *(self.0) != *(other.0)
        }
    }

    #[test]
    fn test_gc_basic_behavior() {
        let mut flag = false;
        let a = Into::<Gc<GcTester>>::into(GcTester(&mut flag));
        drop(a);
        assert!(flag)
    }

    #[test]
    fn test_gc_shallow_copy() {
        let mut flag = false;
        let a =
            Into::<Gc<GcTester>>::into(GcTester(&mut flag)).clone();
        drop(a);
        assert!(flag)
    }

    #[test]
    fn test_gc_deep_copy() {
        let mut flag = false;
        let reference_holder =
            Into::<Gc<GcTester>>::into(GcTester(&mut flag)).clone();
        let copied_data = reference_holder.deep_copy();

        assert_eq!(*reference_holder, *copied_data);
        drop(reference_holder);
        assert!(!*(*copied_data).0);
    }
}