Implement Pointer Compression in Rust

Pointer compression is a technique used to reduce the memory footprint of data structures by representing pointers to large memory regions with smaller, relative offsets. This is particularly useful in scenarios where you have many pointers to objects within a fixed-size memory pool. Your challenge is to implement a system in Rust that can compress and decompress these pointers.

Problem Description

You need to create a Rust struct, let's call it PointerCompressor, that manages a contiguous memory buffer and provides methods to:

1. new(capacity: usize): Initialize the compressor with a given memory capacity in bytes. This will allocate the underlying buffer.
2. add_object<T: 'static>(&mut self, object: T) -> Result<CompressedPointer, CompressionError>: Add an object of any 'static type to the memory buffer. This method should return a CompressedPointer representing the object's location. The object should be stored in a way that maintains its original type for later retrieval.
3. get_object<T: 'static>(&self, ptr: &CompressedPointer) -> Result<&T, CompressionError>: Retrieve a reference to an object given its CompressedPointer. The type T must match the type of the object stored at that pointer.
4. get_object_mut<T: 'static>(&mut self, ptr: &CompressedPointer) -> Result<&mut T, CompressionError>: Retrieve a mutable reference to an object given its CompressedPointer.

Key Requirements:

• Memory Management: The PointerCompressor should own and manage a Vec<u8> as its memory buffer.
• Object Storage: Objects need to be stored in a way that preserves their type information. This implies using some form of serialization or storing type information alongside the object data. You can assume objects will not be moved or deallocated after being added.
• Pointer Representation: A CompressedPointer should be a simple struct representing the compressed pointer. For this challenge, a CompressedPointer can be represented by a u32 value, which will store the offset within the PointerCompressor's buffer.
• Error Handling: Implement appropriate error types for situations like memory overflow, invalid pointer access, or type mismatches.
• 'static Bound: The 'static lifetime bound on T simplifies memory management, meaning objects will live for the entire duration of the program.

Expected Behavior:

When add_object is called, the object should be serialized (or otherwise converted into bytes) and appended to the PointerCompressor's buffer. A CompressedPointer containing the starting offset of the object in the buffer should be returned.

When get_object or get_object_mut is called, the CompressedPointer's offset should be used to locate the object's byte representation in the buffer. This byte representation must then be deserialized (or otherwise converted back) into the expected type T.

Important Edge Cases:

• Buffer Full: What happens if add_object is called when there isn't enough space in the buffer?
• Invalid Pointer: What happens if get_object is called with a CompressedPointer that is out of bounds or points to corrupted data?
• Type Mismatch: What happens if get_object is called with a CompressedPointer but the requested type T does not match the type of the object stored there?
• Zero-Sized Types (ZSTs): How should ZSTs be handled? (For simplicity, you might choose to disallow or handle them specifically).

Examples

Example 1: Adding and Retrieving a Simple Type

Input:
let mut compressor = PointerCompressor::new(1024);
let data1 = vec![1, 2, 3];
let ptr1 = compressor.add_object(data1).unwrap();

let data2 = "hello";
let ptr2 = compressor.add_object(data2).unwrap();

// Assume ptr1 and ptr2 are valid CompressedPointer instances

Output:
let retrieved_data1: &Vec<i32> = compressor.get_object(&ptr1).unwrap();
assert_eq!(*retrieved_data1, vec![1, 2, 3]);

let retrieved_data2: &&str = compressor.get_object(&ptr2).unwrap();
assert_eq!(*retrieved_data2, "hello");

Explanation:

data1 and data2 are added to the PointerCompressor's buffer. add_object returns CompressedPointers that point to their respective locations. Later, get_object uses these pointers to retrieve references to the original data, verifying their content and type.

Example 2: Attempting to Retrieve with a Wrong Type

Input:
let mut compressor = PointerCompressor::new(1024);
let data = 123i32;
let ptr = compressor.add_object(data).unwrap();

// Assume ptr is a valid CompressedPointer instance

Output:
// Attempt to retrieve as a string, which will fail
let result: Result<&str, CompressionError> = compressor.get_object(&ptr);
assert!(result.is_err());
// The error should indicate a type mismatch.

Explanation:

An i32 is stored. Attempting to retrieve it as a &str will result in a type mismatch error.

Example 3: Buffer Overflow Scenario

Input:
let mut compressor = PointerCompressor::new(10); // Very small capacity
let large_data = vec![0u8; 20]; // Data larger than capacity

Output:
let result = compressor.add_object(large_data);
assert!(result.is_err());
// The error should indicate a buffer overflow.

Explanation:

Attempting to add data that exceeds the allocated buffer capacity will result in an error.

Constraints

• The memory capacity provided during initialization will be a usize and will not exceed 2^32 - 1 bytes (to fit within a u32 offset).
• The CompressedPointer will be represented by a u32 offset.
• Objects added to the compressor must implement serde::Serialize and serde::Deserialize for easy serialization and deserialization. You will need to add the serde crate to your Cargo.toml.
• Performance is important. Serialization and deserialization should be reasonably efficient. The primary goal is memory reduction, so the overhead of compression/decompression should be acceptable.
• You can assume that the total size of all objects added will not exceed the initial capacity.

Notes

• Consider how you will store the type information of objects. A common approach is to serialize the type identifier along with the object data.
• You'll need to decide on a serialization format (e.g., Bincode, JSON, etc.). Bincode is a good choice for binary serialization that is efficient and compact.
• For get_object, you'll need to ensure that the deserialized object can be safely cast to the requested type T. This is where the type information stored during add_object becomes crucial.
• The 'static bound on T means you don't have to worry about complex lifetime management for the stored objects.
• Think about how to handle the possibility of an object's size changing if you were to use more dynamic storage strategies (though for this challenge, assume fixed sizes after addition).
• You might want to create an enum for CompressionError to cover different failure cases.