From d09d945323f0c36ab112bf88e0866c2be3528ef1 Mon Sep 17 00:00:00 2001
From: Jesse Brault <jbrault@mac.com>
Date: Tue, 14 Oct 2025 21:18:06 -0500
Subject: [PATCH] Experimenting with garbage collection algorithm.

---
 src/vm/mem/mod.rs | 205 ++++++++++++++++++++++++++++++++++++++++++++++
 src/vm/mod.rs     |   1 +
 2 files changed, 206 insertions(+)
 create mode 100644 src/vm/mem/mod.rs

diff --git a/src/vm/mem/mod.rs b/src/vm/mem/mod.rs
new file mode 100644
index 0000000..61fbf20
--- /dev/null
+++ b/src/vm/mem/mod.rs
@@ -0,0 +1,205 @@
+use std::cell::RefCell;
+use std::marker::PhantomData;
+use std::ops::Mul;
+
+#[derive(Debug)]
+pub struct Gc<T> {
+    inner: *mut GcInner,
+    data: *const T,
+    _phantom: PhantomData<T>,
+}
+
+#[derive(Debug)]
+pub struct GcInner {
+    color: GcColor,
+    size: usize,
+}
+
+impl GcInner {
+    pub fn new(size: usize) -> Self {
+        Self {
+            color: GcColor::White,
+            size,
+        }
+    }
+}
+
+#[derive(PartialEq, Eq, Copy, Clone, Debug)]
+pub enum GcColor {
+    White,
+    Black,
+}
+
+impl<T> Gc<T> {
+    pub fn new(data: T) -> Self {
+        let inner = Box::into_raw(Box::new(GcInner::new(size_of::<T>())));
+        let boxed_data = Box::into_raw(Box::new(data)) as *const T;
+        Self {
+            inner,
+            data: boxed_data,
+            _phantom: PhantomData,
+        }
+    }
+
+    pub fn set_color(&mut self, color: GcColor) {
+        unsafe {
+            (*self.inner).color = color;
+        }
+    }
+
+    pub fn color(&self) -> GcColor {
+        unsafe { (*self.inner).color }
+    }
+
+    pub fn size(&self) -> usize {
+        unsafe { (*self.inner).size }
+    }
+
+    pub fn data(&self) -> &T {
+        unsafe { &*self.data }
+    }
+
+    pub fn dealloc(&mut self) {
+        unsafe {
+            drop(Box::from_raw(self.inner));
+            drop(Box::from_raw(self.data as *mut T));
+        }
+    }
+}
+
+impl<T> Clone for Gc<T> {
+    fn clone(&self) -> Self {
+        Self {
+            inner: self.inner,
+            data: self.data,
+            _phantom: PhantomData,
+        }
+    }
+}
+
+#[derive(Debug)]
+enum DvmValue {
+    Primitive(usize),
+    Object(Gc<RefCell<DvmObject>>),
+    Nil,
+}
+
+#[derive(Debug)]
+struct DvmObject {
+    fields: Vec<DvmValue>,
+}
+
+impl DvmObject {
+    pub fn new() -> Self {
+        Self { fields: vec![] }
+    }
+
+    pub fn fields(&self) -> &[DvmValue] {
+        &self.fields
+    }
+
+    pub fn fields_mut(&mut self) -> &mut Vec<DvmValue> {
+        &mut self.fields
+    }
+}
+
+fn collect_garbage(
+    next_gc: &mut usize,
+    current_size: &mut usize,
+    stack: &Vec<DvmValue>,
+    gcs: &mut Vec<Gc<RefCell<DvmObject>>>,
+) {
+    let mut gray_stack: Vec<Gc<RefCell<DvmObject>>> = vec![];
+
+    for stack_value in stack {
+        match stack_value {
+            DvmValue::Object(gc) => {
+                if gc.color() == GcColor::White {
+                    gray_stack.push(gc.clone());
+                }
+            }
+            _ => {}
+        }
+    }
+
+    while let Some(mut current) = gray_stack.pop() {
+        for field in current.data().borrow().fields() {
+            match field {
+                DvmValue::Object(object) => {
+                    gray_stack.push(object.clone());
+                }
+                _ => {}
+            }
+        }
+        current.set_color(GcColor::Black);
+    }
+
+    let mut collected_size: usize = 0;
+
+    for i in (0..gcs.len()).rev() {
+        if gcs[i].color() == GcColor::White {
+            let mut gc = gcs.remove(i);
+            let size = gc.size();
+            *current_size -= size;
+            collected_size += size;
+            gc.dealloc();
+        } else {
+            gcs[i].set_color(GcColor::White);
+        }
+    }
+
+    *next_gc = std::cmp::max(1024 * 1024, current_size.mul(2));
+    println!(
+        "collected_size: {}, current_size: {}, next_gc: {}",
+        collected_size, current_size, next_gc
+    );
+}
+
+fn maybe_collect_garbage(
+    next_gc: &mut usize,
+    current_size: &mut usize,
+    stack: &Vec<DvmValue>,
+    gcs: &mut Vec<Gc<RefCell<DvmObject>>>,
+) {
+    if current_size > next_gc {
+        collect_garbage(next_gc, current_size, stack, gcs);
+    }
+}
+
+fn alloc_dvm_object(
+    next_gc: &mut usize,
+    current_size: &mut usize,
+    stack: &Vec<DvmValue>,
+    gcs: &mut Vec<Gc<RefCell<DvmObject>>>,
+) -> Gc<RefCell<DvmObject>> {
+    maybe_collect_garbage(next_gc, current_size, stack, gcs);
+    let mut object = DvmObject::new();
+    object.fields_mut().push(DvmValue::Nil);
+    let gc = Gc::new(RefCell::new(object));
+    *current_size += gc.size();
+    gcs.push(gc.clone());
+    gc
+}
+
+fn set_field(obj: &mut Gc<RefCell<DvmObject>>, field_index: usize, value: DvmValue) {
+    let mut binding = obj.data().borrow_mut();
+    binding.fields_mut()[field_index] = value;
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::vm::mem::{alloc_dvm_object, collect_garbage, DvmObject, DvmValue, Gc};
+    use std::cell::RefCell;
+
+    #[test]
+    fn test() {
+        let mut stack: Vec<DvmValue> = vec![];
+        let mut heap: Vec<Gc<RefCell<DvmObject>>> = vec![];
+        let mut next_gc: usize = 1024 * 1024;
+        let mut current_size: usize = 0;
+        for i in 0..100_000_000 {
+            let mut o = alloc_dvm_object(&mut next_gc, &mut current_size, &stack, &mut heap);
+        }
+        collect_garbage(&mut next_gc, &mut current_size, &stack, &mut heap);
+    }
+}
diff --git a/src/vm/mod.rs b/src/vm/mod.rs
index 3fa6b74..050abdb 100644
--- a/src/vm/mod.rs
+++ b/src/vm/mod.rs
@@ -6,6 +6,7 @@ pub mod implementation;
 pub mod instruction;
 pub mod interface;
 pub mod lib;
+mod mem;
 pub mod method;
 pub mod object;
 pub mod op_codes;