Git Clone Implementation

The `git clone` command is used to create a local copy of a remote Git repository. This process involves several complex steps to fetch all the repository's history, branches, and tags, and then set up the local working directory. A full implementation of `git clone` from scratch is a significant undertaking, as it requires understanding and implementing Git's internal object model, packfile format, and various network protocols.

Here's a high-level breakdown of what a `git clone` operation typically entails:

1. Repository URL Parsing: The client first parses the provided URL to determine the protocol (HTTP/S, SSH, Git protocol) and the remote repository's location.

2. Protocol Negotiation: The client initiates a connection to the remote server using the appropriate protocol. For HTTP/S, this usually involves requesting a service URL (e.g., `'/info/refs?service=git-upload-pack'`) to discover available references and capabilities. For SSH or the native Git protocol, a direct connection is established.

3. Fetch Ref-advertisement: The server responds with a list of its available references (branches, tags) and their corresponding SHA-1 or SHA-256 hashes. This 'ref-advertisement' allows the client to see what's available and decide which objects it needs.

4. Object Request & Packfile Transfer: Based on the ref-advertisement, the client determines which objects (commits, trees, blobs) it doesn't have locally (for a clone, it initially has none) and requests them from the server. The server typically responds by sending these objects in a highly compressed 'packfile' format. A packfile bundles multiple Git objects into a single file to optimize transfer speed and storage.

5. Object Deserialization and Storage: The client receives the packfile, unpacks it, and stores the individual Git objects (blobs, trees, commits, annotated tags) into its local object database, usually located in the `.git/objects` directory.

6. Reference Creation: After receiving all necessary objects, the client creates local references (e.g., `HEAD`, `refs/heads/main`, `refs/remotes/origin/main`) in its `.git` directory, mirroring the remote's default branch and setting up remote-tracking branches.

7. Working Tree Checkout: Finally, the client checks out the files for the initial commit (usually the tip of the default branch) into the working directory, making the repository ready for use.

Implementing all these steps involves:
* Network programming: Handling HTTP/S, SSH, or Git protocol requests and responses.
* Git object model understanding: Parsing and reconstructing Git's fundamental objects.
* Packfile format parsing: Decompressing and extracting objects from packfiles.
* SHA-1/SHA-256 hashing: Verifying object integrity.
* Filesystem interaction: Creating directories, writing files, and setting up the `.git` structure.

The provided Rust example will demonstrate a very small, initial step of the `git clone` process: fetching the `info/refs` advertisement over HTTP. This gives a glimpse into the network interaction involved but does not implement the full object parsing or packfile handling, which are far more complex.

```rust
use reqwest::Error;
use std::io::Write;

// This example demonstrates a highly simplified part of the git clone process:
// fetching the 'info/refs' advertisement from a remote Git repository over HTTP/S.
// This is an initial step to discover available branches and tags before fetching objects.
// A full git clone implementation would involve parsing the response, negotiating capabilities,
// requesting packfiles, decompressing objects, and setting up the local repository structure.

#[tokio::main]
async fn main() -> Result<(), Error> {
    let repo_url = "https://github.com/rust-lang/rust.git";
    // The 'info/refs' endpoint with the 'git-upload-pack' service is used by Git's smart HTTP protocol
    // to advertise references (branches, tags) and their SHAs.
    let info_refs_url = format!("{}/info/refs?service=git-upload-pack", repo_url);

    println!("Attempting to fetch ref advertisement from: {}", info_refs_url);

    // 1. Make an HTTP GET request to the 'info/refs' endpoint.
    let client = reqwest::Client::builder()
        .user_agent("git/2.x (rust-simulated-clone)") // Mimic a Git client user agent
        .build()?;

    let response = client.get(&info_refs_url).send().await?;

    // Check if the request was successful
    if response.status().is_success() {
        println!("Successfully fetched ref advertisement.");
        let body = response.text().await?;

        // 2. Print the raw response body. This typically contains Git's pkt-line formatted data
        //    listing references and their SHAs.
        println!("\n--- Raw Ref Advertisement Body ---");
        println!("{}", body);
        println!("--- End Raw Ref Advertisement Body ---\n");

        // In a real clone, this body would be parsed to understand the remote's branches and tags.
        // For demonstration, let's also save it to a file.
        let mut file = std::fs::File::create("git_info_refs.txt")
            .expect("Failed to create file");
        file.write_all(body.as_bytes())
            .expect("Failed to write to file");
        println!("Ref advertisement saved to 'git_info_refs.txt'");

    } else {
        eprintln!("Failed to fetch ref advertisement. Status: {}", response.status());
        eprintln!("Response body: {:?}", response.text().await?);
    }

    Ok(())
}
```