From e66cd52861b032fa38c86d54dbd08cbd47a3f209 Mon Sep 17 00:00:00 2001
From: Nilstrieb <48135649+Nilstrieb@users.noreply.github.com>
Date: Thu, 30 Dec 2021 17:59:48 +0100
Subject: [PATCH] start with gc

---
 src/value.rs | 82 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)

diff --git a/src/value.rs b/src/value.rs
index f716e9a..797e3f7 100644
--- a/src/value.rs
+++ b/src/value.rs
@@ -1,9 +1,91 @@
+use std::collections::LinkedList;
+use std::fmt::{Debug, Formatter};
+use std::ops::Deref;
+use std::ptr::NonNull;
+
 /// imagine interning or something here
 pub type Symbol = String;
 
+/// here is the actual interning or something
+pub type NewSym = Gc<str>;
+
 #[cfg(not(feature = "fxhash"))]
 #[allow(clippy::disallowed_type)]
 pub type HashMap<K, V> = std::collections::HashMap<K, V>;
 
 #[cfg(feature = "fxhash")]
 pub type HashMap<K, V> = rustc_hash::FxHashMap<K, V>;
+
+#[cfg(not(feature = "fxhash"))]
+#[allow(clippy::disallowed_type)]
+pub type HashSet<T> = std::collections::HashSet<T>;
+
+#[cfg(feature = "fxhash")]
+pub type HashSet<T> = rustc_hash::FxHashSet<T>;
+
+/// A pointer to a garbage collected value. This pointer *must* always be valid, and a value
+/// is only allowed to be freed once no Gc is pointing at it anymore. This is achieved through
+/// tracing through all objects from a few known roots and marking every reachable value. All other
+/// values will be swept.
+pub struct Gc<T: ?Sized> {
+    ptr: NonNull<T>,
+}
+
+impl<T: ?Sized> Deref for Gc<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: Gc will always point to a valid T, since T will only be freed once all Gc are gone
+        // This requires tracing through *all* roots without forgetting any
+        // I would guess that there will be some errors with the garbage collector, but once they are
+        // all fixed this will be sound. But who knows.
+        unsafe { &*self.ptr.as_ptr() }
+    }
+}
+
+impl<T: Debug + ?Sized> Debug for Gc<T> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        T::fmt(self, f)
+    }
+}
+
+impl<T: ?Sized> Clone for Gc<T> {
+    fn clone(&self) -> Self {
+        Self { ..*self }
+    }
+}
+
+impl<T: ?Sized> Copy for Gc<T> {}
+
+enum Object {
+    String(Gc<str>),
+}
+
+pub struct RtAlloc {
+    symbols: HashSet<NonNull<str>>,
+    objects: LinkedList<Object>,
+}
+
+impl RtAlloc {
+    pub fn alloc_str(&mut self, str: &str) -> Gc<str> {
+        let ptr = Box::into_raw(str.to_owned().into_boxed_str());
+        // SAFETY: Box cannot be null
+        let new_nonnull = unsafe { NonNull::new_unchecked(ptr) };
+        let gc = Gc { ptr: new_nonnull };
+        let object = Object::String(gc.clone());
+
+        self.objects.push_back(object);
+
+        gc
+    }
+
+    pub fn intern_string(&mut self, str: &str) -> NewSym {
+        let original_nonnull = NonNull::from(str);
+
+        if let Some(interned) = self.symbols.get(&original_nonnull) {
+            return Gc { ptr: *interned };
+        }
+
+        self.alloc_str(str)
+    }
+}