Implementing a Stack Allocator in Go

Many programming languages provide automatic memory management, but understanding how memory is allocated can be crucial for performance optimization. In Go, while garbage collection is the primary mechanism, you can achieve stack-like allocation for temporary, short-lived data structures. This challenge asks you to build a simple stack allocator that allows for efficient allocation and deallocation of memory in a LIFO (Last-In, First-Out) manner.

Problem Description

Your task is to implement a StackAllocator struct in Go. This allocator will manage a contiguous block of memory and provide methods to Allocate and Deallocate memory segments. The allocator should behave like a traditional stack: the last allocated block is the first one to be deallocated.

Key Requirements:

• NewStackAllocator(size int): A constructor function that creates a new StackAllocator with a pre-allocated underlying memory buffer of the specified size in bytes.
• Allocate(size int): A method that attempts to allocate a contiguous block of memory of the requested size. It should return a pointer to the allocated memory (a []byte slice) and an error if the allocation fails (e.g., not enough space). The returned slice should be a view into the allocator's underlying buffer.
• Deallocate(ptr []byte): A method that marks the given memory segment ptr as free. This should only deallocate the most recently allocated block that matches the provided pointer and size. Deallocating in an arbitrary order is not supported by this simple stack allocator.
• No Garbage Collection Interference: The allocated memory should be managed by your allocator and not directly by Go's garbage collector in a way that would interfere with your LIFO deallocation.

Expected Behavior:

The allocator should maintain a pointer to the next available free space within its buffer. Allocation increments this pointer, and deallocation effectively rewinds it.

Edge Cases:

• Attempting to allocate more memory than available in the entire buffer.
• Attempting to deallocate a block that was not the most recently allocated.
• Attempting to deallocate an empty or invalid pointer.
• Allocating and deallocating repeatedly to test boundary conditions.

Examples

Example 1:

Input:
allocator := NewStackAllocator(100) // Buffer of 100 bytes
block1, err1 := allocator.Allocate(20)
block2, err2 := allocator.Allocate(30)

Output:
block1: A []byte slice of length 20, pointing to the start of the buffer.
err1: nil
block2: A []byte slice of length 30, starting immediately after block1 in the buffer.
err2: nil

Explanation:
Two blocks are successfully allocated sequentially. block1 starts at offset 0, and block2 starts at offset 20.

Example 2:

Input:
allocator := NewStackAllocator(50) // Buffer of 50 bytes
block1, _ := allocator.Allocate(40)
block2, _ := allocator.Allocate(20) // This allocation should fail

Output:
block2: nil
err2: "not enough space to allocate 20 bytes"

Explanation:
The first allocation of 40 bytes succeeds. The buffer has only 10 bytes remaining, so the second allocation of 20 bytes fails.

Example 3:

Input:
allocator := NewStackAllocator(100)
block1, _ := allocator.Allocate(20)
block2, _ := allocator.Allocate(30)
allocator.Deallocate(block1) // Attempting to deallocate an older block first

Output:
Error from Deallocate(block1): "invalid deallocation, block not most recently allocated"

Explanation:
block2 was the most recently allocated. Attempting to deallocate block1 before block2 violates the LIFO principle of this stack allocator.

Example 4:

Input:
allocator := NewStackAllocator(100)
block1, _ := allocator.Allocate(20)
block2, _ := allocator.Allocate(30)
allocator.Deallocate(block2) // Deallocating the most recent block
block3, _ := allocator.Allocate(25) // Should succeed and reuse space

Output:
block3: A []byte slice of length 25, starting at the same offset as block2 did.

Explanation:
block2 is deallocated, freeing up its space. block3 is then allocated and can fit into the newly available space.

Constraints

• The size parameter for NewStackAllocator will be between 1 and 1024 bytes (inclusive).
• The size parameter for Allocate will be between 1 and 1024 bytes (inclusive).
• The total number of allocations and deallocations in any test case will not exceed 1000.
• The Deallocate method will only be called with slices that were previously returned by Allocate from the same allocator instance.

Notes

• Consider how you will track the current position of the free space within the buffer.
• You'll need a way to remember the size of each allocated block to correctly manage deallocation. A simple approach might involve storing metadata alongside the allocated blocks or within a separate structure.
• For simplicity, this challenge does not require thread-safety.
• Think about how to represent the allocated blocks and their positions. You might need an auxiliary data structure to keep track of active allocations.
• The []byte slices returned by Allocate should be "views" into the underlying buffer, meaning they don't copy the data. Modifying the data in the returned slice should modify the allocator's internal buffer.