Implementing a Basic Mark-and-Sweep Garbage Collector in Rust

Garbage collection (GC) is a crucial technique in memory management, automating the process of reclaiming memory that is no longer in use by a program. The Mark-and-Sweep algorithm is a foundational GC strategy. This challenge asks you to implement a simplified version of Mark-and-Sweep in Rust to understand its core principles.

Problem Description

Your task is to implement a basic Mark-and-Sweep garbage collector for a simplified heap. This heap will contain objects that can reference other objects. The GC should identify and reclaim memory occupied by objects that are unreachable from a set of "roots."

Key Requirements:

1. Heap Representation: You will need to define a structure to represent the heap, which will store the objects. Each object should have a unique identifier and a way to store references to other objects within the heap.
2. Object Referencing: Objects can hold references to other objects. These references are essentially pointers or IDs to other objects in the heap.
3. Roots: A set of "root" objects will be maintained. These are objects that are explicitly considered reachable by the program (e.g., global variables, stack frames).
4. Mark Phase: Traverse the heap starting from the roots. Any object reachable from the roots (directly or indirectly through other reachable objects) should be "marked" as alive.
5. Sweep Phase: Iterate through all objects in the heap. Any object that has not been marked during the mark phase is considered garbage and should be "swept" (i.e., reclaimed or removed from the heap).
6. Memory Reclamation: For simplicity, when an object is swept, you can simulate reclamation by removing it from the heap's collection of active objects.

Expected Behavior:

• The GC should correctly identify and remove unreferenced objects.
• Objects that are reachable from the roots, even indirectly, should be preserved.

Edge Cases to Consider:

• An empty heap.
• A heap with no roots.
• Circular references between objects.
• Objects that reference themselves.

Examples

Example 1:

Let's consider a simple scenario with objects and their references.

Heap State (Initial):

• Object 0: Value "A", References: [1]
• Object 1: Value "B", References: [2]
• Object 2: Value "C", References: []
• Object 3: Value "D", References: []

Roots: [0]

Mark Phase:

• Start with root Object 0. Mark Object 0.
• Object 0 references Object 1. Mark Object 1.
• Object 1 references Object 2. Mark Object 2.
• Object 2 has no references.

Sweep Phase:

• Check Object 0: Marked. Keep.
• Check Object 1: Marked. Keep.
• Check Object 2: Marked. Keep.
• Check Object 3: Not marked. Sweep (reclaim).

Output (Heap State After GC):

• Object 0: Value "A", References: [1]
• Object 1: Value "B", References: [2]
• Object 2: Value "C", References: []

Example 2:

Consider a scenario with an unreferenced branch.

Heap State (Initial):

• Object 0: Value "Root", References: [1]
• Object 1: Value "Branch1", References: [2]
• Object 2: Value "Branch2", References: []
• Object 3: Value "Orphan", References: [4]
• Object 4: Value "DeepOrphan", References: []

Roots: [0]

Mark Phase:

• Start with root Object 0. Mark Object 0.
• Object 0 references Object 1. Mark Object 1.
• Object 1 references Object 2. Mark Object 2.
• Object 2 has no references.

Sweep Phase:

• Check Object 0: Marked. Keep.
• Check Object 1: Marked. Keep.
• Check Object 2: Marked. Keep.
• Check Object 3: Not marked. Sweep.
• Check Object 4: Not marked. Sweep.

Output (Heap State After GC):

• Object 0: Value "Root", References: [1]
• Object 1: Value "Branch1", References: [2]
• Object 2: Value "Branch2", References: []

Example 3: Circular Reference

Heap State (Initial):

• Object 0: Value "X", References: [1]
• Object 1: Value "Y", References: [0]
• Object 2: Value "Z", References: []

Roots: [0]

Mark Phase:

• Start with root Object 0. Mark Object 0.
• Object 0 references Object 1. Mark Object 1.
• Object 1 references Object 0. Object 0 is already marked.

Sweep Phase:

• Check Object 0: Marked. Keep.
• Check Object 1: Marked. Keep.
• Check Object 2: Not marked. Sweep.

Output (Heap State After GC):

• Object 0: Value "X", References: [1]
• Object 1: Value "Y", References: [0]

Constraints

• The heap will not exceed 1000 objects.
• Object IDs will be non-negative integers.
• References will always point to valid (or potentially invalid, if simulating breakage) object IDs within the conceptual heap. You can assume for this exercise that references point to IDs that could exist.
• The primary goal is correctness of the Mark-and-Sweep logic, not extreme performance optimization.

Notes

• You'll likely want to use Rust's HashMap or Vec to represent your heap and store objects.
• Consider how to represent an object's "marked" state. A boolean flag within your object structure is a common approach.
• For simulating reclamation, removing the object from your heap's data structure is sufficient. You don't need to worry about deallocating raw memory.
• Think about how you will handle object IDs and ensure uniqueness.
• The traversal in the mark phase can be implemented using recursion or an explicit stack.