//! Tri-color mark-and-sweep garbage collector.
//!
//! Manages heap-allocated JS objects (plain objects, arrays, functions) using
//! the tri-color invariant: white (unreached), gray (reached, children not yet
//! scanned), black (fully scanned). The collector is stop-the-world and
//! triggered when live object count exceeds a configurable threshold.

/// Mark color for tri-color marking.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Color {
    /// Not yet visited — candidate for collection.
    White,
    /// Visited but children not yet scanned.
    Gray,
    /// Fully scanned — reachable and all children traced.
    Black,
}

/// A reference (handle) to a GC-managed object.
///
/// This is a lightweight index into the GC heap. It is `Copy` so it can be
/// freely duplicated — the GC is responsible for tracking liveness.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct GcRef(u32);

/// Trait for types that can be traced by the garbage collector.
///
/// Implementors must report all `GcRef` values they hold so the GC can
/// traverse the object graph.
pub trait Traceable {
    /// Call `visitor` once for each `GcRef` this object holds.
    fn trace(&self, visitor: &mut dyn FnMut(GcRef));
}

/// A heap slot containing GC metadata and the object data.
struct HeapSlot<T> {
    color: Color,
    data: T,
}

/// Statistics about the GC heap.
#[derive(Debug, Clone)]
pub struct GcStats {
    /// Number of live (allocated) objects.
    pub live_count: usize,
    /// Total heap slots (including free slots).
    pub total_slots: usize,
    /// Number of collections performed so far.
    pub collections: usize,
}

/// The garbage collector.
///
/// Manages a heap of `T` objects. Objects are allocated via [`alloc`] and
/// accessed via [`get`]/[`get_mut`]. Collection is triggered manually or
/// when [`should_collect`] returns true.
pub struct Gc<T: Traceable> {
    /// The heap: a vector of optional slots (None = free).
    heap: Vec<Option<HeapSlot<T>>>,
    /// Free slot indices for O(1) allocation.
    free_list: Vec<u32>,
    /// Number of currently live objects.
    live_count: usize,
    /// Collection is suggested when live_count exceeds this.
    threshold: usize,
    /// Total collections performed.
    collections: usize,
}

/// Initial collection threshold.
const INITIAL_THRESHOLD: usize = 256;

impl<T: Traceable> Gc<T> {
    /// Create a new, empty GC heap.
    pub fn new() -> Self {
        Self {
            heap: Vec::new(),
            free_list: Vec::new(),
            live_count: 0,
            threshold: INITIAL_THRESHOLD,
            collections: 0,
        }
    }

    /// Allocate a new object on the heap, returning a `GcRef` handle.
    pub fn alloc(&mut self, data: T) -> GcRef {
        let slot = HeapSlot {
            color: Color::White,
            data,
        };
        let idx = if let Some(free_idx) = self.free_list.pop() {
            let i = free_idx as usize;
            self.heap[i] = Some(slot);
            free_idx
        } else {
            let i = self.heap.len();
            self.heap.push(Some(slot));
            i as u32
        };
        self.live_count += 1;
        GcRef(idx)
    }

    /// Get an immutable reference to the object at `r`.
    ///
    /// Returns `None` if the slot has been freed (stale reference).
    pub fn get(&self, r: GcRef) -> Option<&T> {
        let idx = r.0 as usize;
        self.heap
            .get(idx)
            .and_then(|slot| slot.as_ref().map(|s| &s.data))
    }

    /// Get a mutable reference to the object at `r`.
    ///
    /// Returns `None` if the slot has been freed (stale reference).
    pub fn get_mut(&mut self, r: GcRef) -> Option<&mut T> {
        let idx = r.0 as usize;
        self.heap
            .get_mut(idx)
            .and_then(|slot| slot.as_mut().map(|s| &mut s.data))
    }

    /// Returns `true` if a collection should be triggered.
    pub fn should_collect(&self) -> bool {
        self.live_count >= self.threshold
    }

    /// Perform a garbage collection pass.
    ///
    /// `roots` must contain every `GcRef` reachable from the mutator (VM
    /// registers, globals, call stack). Any object not transitively reachable
    /// from a root is freed.
    pub fn collect(&mut self, roots: &[GcRef]) {
        self.mark(roots);
        self.sweep();
        self.collections += 1;

        // Grow threshold so we don't collect too often.
        if self.live_count >= self.threshold {
            self.threshold = self.live_count * 2;
        }
    }

    /// Return heap statistics.
    pub fn stats(&self) -> GcStats {
        GcStats {
            live_count: self.live_count,
            total_slots: self.heap.len(),
            collections: self.collections,
        }
    }

    /// Mark phase: starting from roots, color all reachable objects black.
    fn mark(&mut self, roots: &[GcRef]) {
        let mut gray_stack: Vec<GcRef> = Vec::new();

        // Color roots gray.
        for &root in roots {
            let idx = root.0 as usize;
            if idx < self.heap.len() {
                if let Some(entry) = &mut self.heap[idx] {
                    if entry.color == Color::White {
                        entry.color = Color::Gray;
                        gray_stack.push(root);
                    }
                }
            }
        }

        // Process the gray stack.
        while let Some(gc_ref) = gray_stack.pop() {
            let idx = gc_ref.0 as usize;

            // Collect child references (scoped borrow).
            let children = {
                let mut c = Vec::new();
                if let Some(entry) = &self.heap[idx] {
                    entry.data.trace(&mut |child| c.push(child));
                }
                c
            };

            // Mark current object black.
            if let Some(entry) = &mut self.heap[idx] {
                entry.color = Color::Black;
            }

            // Mark white children gray.
            for child in children {
                let cidx = child.0 as usize;
                if cidx < self.heap.len() {
                    if let Some(entry) = &mut self.heap[cidx] {
                        if entry.color == Color::White {
                            entry.color = Color::Gray;
                            gray_stack.push(child);
                        }
                    }
                }
            }
        }
    }

    /// Sweep phase: free all white objects, reset black objects to white.
    fn sweep(&mut self) {
        for i in 0..self.heap.len() {
            let should_free = match &self.heap[i] {
                Some(entry) => entry.color == Color::White,
                None => false,
            };
            if should_free {
                self.heap[i] = None;
                self.free_list.push(i as u32);
                self.live_count -= 1;
            } else if let Some(entry) = &mut self.heap[i] {
                // Reset black → white for next cycle.
                entry.color = Color::White;
            }
        }
    }
}

impl<T: Traceable> Default for Gc<T> {
    fn default() -> Self {
        Self::new()
    }
}

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

    /// A simple test object that holds GcRef children.
    struct TestObj {
        children: Vec<GcRef>,
    }

    impl Traceable for TestObj {
        fn trace(&self, visitor: &mut dyn FnMut(GcRef)) {
            for &child in &self.children {
                visitor(child);
            }
        }
    }

    #[test]
    fn test_alloc_and_get() {
        let mut gc = Gc::new();
        let r = gc.alloc(TestObj {
            children: Vec::new(),
        });
        assert!(gc.get(r).is_some());
        assert_eq!(gc.stats().live_count, 1);
    }

    #[test]
    fn test_collect_unreachable() {
        let mut gc = Gc::new();
        let r1 = gc.alloc(TestObj {
            children: Vec::new(),
        });
        let r2 = gc.alloc(TestObj {
            children: Vec::new(),
        });
        assert_eq!(gc.stats().live_count, 2);

        // Collect with no roots — everything is freed.
        gc.collect(&[]);
        assert_eq!(gc.stats().live_count, 0);
        assert!(gc.get(r1).is_none());
        assert!(gc.get(r2).is_none());
    }

    #[test]
    fn test_collect_reachable() {
        let mut gc = Gc::new();
        let r1 = gc.alloc(TestObj {
            children: Vec::new(),
        });
        let r2 = gc.alloc(TestObj {
            children: Vec::new(),
        });
        assert_eq!(gc.stats().live_count, 2);

        // Only r1 is a root.
        gc.collect(&[r1]);
        assert_eq!(gc.stats().live_count, 1);
        assert!(gc.get(r1).is_some());
        assert!(gc.get(r2).is_none());
    }

    #[test]
    fn test_collect_transitive() {
        let mut gc = Gc::new();
        let r1 = gc.alloc(TestObj {
            children: Vec::new(),
        });
        let r2 = gc.alloc(TestObj {
            children: Vec::new(),
        });
        let r3 = gc.alloc(TestObj {
            children: Vec::new(),
        });

        // r1 -> r2 -> r3
        gc.get_mut(r1).unwrap().children.push(r2);
        gc.get_mut(r2).unwrap().children.push(r3);

        gc.collect(&[r1]);
        assert_eq!(gc.stats().live_count, 3);
        assert!(gc.get(r1).is_some());
        assert!(gc.get(r2).is_some());
        assert!(gc.get(r3).is_some());
    }

    #[test]
    fn test_cycle_collection() {
        let mut gc = Gc::new();
        let r1 = gc.alloc(TestObj {
            children: Vec::new(),
        });
        let r2 = gc.alloc(TestObj {
            children: Vec::new(),
        });

        // Create a cycle: r1 -> r2 -> r1.
        gc.get_mut(r1).unwrap().children.push(r2);
        gc.get_mut(r2).unwrap().children.push(r1);

        // With r1 as root, both survive.
        gc.collect(&[r1]);
        assert_eq!(gc.stats().live_count, 2);

        // With no roots, both are freed (cycle is collected).
        gc.collect(&[]);
        assert_eq!(gc.stats().live_count, 0);
    }

    #[test]
    fn test_free_list_reuse() {
        let mut gc = Gc::new();
        let _r1 = gc.alloc(TestObj {
            children: Vec::new(),
        });
        let _r2 = gc.alloc(TestObj {
            children: Vec::new(),
        });

        // Free both.
        gc.collect(&[]);
        assert_eq!(gc.stats().live_count, 0);
        assert_eq!(gc.stats().total_slots, 2);

        // Allocate again — should reuse freed slots.
        let _r3 = gc.alloc(TestObj {
            children: Vec::new(),
        });
        let _r4 = gc.alloc(TestObj {
            children: Vec::new(),
        });
        assert_eq!(gc.stats().live_count, 2);
        assert_eq!(gc.stats().total_slots, 2); // No growth.
    }

    #[test]
    fn test_should_collect_threshold() {
        let mut gc: Gc<TestObj> = Gc::new();
        assert!(!gc.should_collect());

        // Allocate up to threshold.
        for _ in 0..INITIAL_THRESHOLD {
            gc.alloc(TestObj {
                children: Vec::new(),
            });
        }
        assert!(gc.should_collect());
    }

    #[test]
    fn test_stress() {
        let mut gc = Gc::new();
        let mut live_refs = Vec::new();

        for i in 0..1000 {
            let r = gc.alloc(TestObj {
                children: Vec::new(),
            });
            if i % 3 == 0 {
                live_refs.push(r);
            }
        }

        gc.collect(&live_refs);
        // Only refs kept in live_refs should survive.
        assert_eq!(gc.stats().live_count, live_refs.len());

        for r in &live_refs {
            assert!(gc.get(*r).is_some());
        }
    }
}