# Week 11 Update 11.1

During this week I spend most of my time trying to make the [Cryptoexperts](https://github.com/CryptoExperts/poseidon) Poseidon permutation work. Let us recall that the Cryptoexperts instantiation of Poseidon is written in C language. Hence, this required to make a C interface in Rust.

In this update I will present a guideline on how to compile the Cryptoexperts Poseidon permutation and generate a static library in order to use it from Rust. This writing will also serve as a future reference on how to use the C implementation in the Cryptoexperts repository.

1. Fork or clone the [repository](https://github.com/CryptoExperts/poseidon). In our [poseidon-benchmark](https://github.com/mdvillagra/poseidon-benchmarks) repository we added it as a submodule.
2. In the `sources` directory run the command `make only_c`. This action will create 5 files: `f251_int128.o`, `f251.o`, `poseidon_rc.o`, `poseidon.o` and `lib_pos.so`.
3. At this stage you can choose to use the `lib_pos.so` library but, in Linux, it will require you to set the environmental variable `LD_LIBRARY_PATH` to a directory containing `lib_pos.so` or to copy the file to a directory in `LD_LIBRARY_PATH`. It is easier, however, to generate an static library file that can be used from anywhere in the directory structure. To generate a static library file the following command suffices: `ar rcs lib_pos.a poseidon.o poseidon_rc.o f251.o f251_int128.o`. It is important to add the prefix `lib` to the `.a` file so that the linker recognizes it as a library file. After generating the static library file, we can start creating the Rust interface.
4. Create a build script `build.rs` in the root of the repository. In the script, we should tell `cargo` where to find the library file and its name. It should be something like this:

//build.rs
fn main() {
    println!("cargo:rustc-link-search=src");
    println!("cargo:rustc-link-lib=static=_pos");
}

5. The permutation functions by Cryptoexperts can be found in the [`poseidon.c`](https://github.com/CryptoExperts/poseidon/blob/main/sources/poseidon.c) file. There are four functions of the Poseidon permutation depending on the input size, namely, `void permutation_3(felt_t state[])`, `void permutation_4(felt_t state[])`, `void permutation_5(felt_t state[])` and `void permutation_9(felt_t state[])`, which correspond to arrays each of size 3, 4, 5 and 9, respectively. The type `felt_t` is a size 4 array of 64 bit unsigned integers, that is, four limbs each of type `u64`.
6. In order to make use of any of this functions we need to declare each function usign its signature as shown in the code below.

#[link(name = "_pos", kind = "static")]
extern "C" {
    fn permutation_3(state: *mut c_ulonglong);
}

With this setup, when we run `cargo` it will compile the Rust code and link it with the static library.

In order to fully use the Poseidon permutation I need to add a sponge function to it. I expect to finish that during Week 12. I also hope to finally be able to run the full benchmarks during this week.