rssn 0.2.9

//! JSON-based FFI API for physics FVM functions.

use std::os::raw::c_char;

use serde::Deserialize;

use crate::ffi_apis::common::from_json_string;
use crate::ffi_apis::common::to_c_string;
use crate::ffi_apis::ffi_api::FfiResult;
use crate::physics::physics_fvm::Mesh;
use crate::physics::physics_fvm::SweState;
use crate::physics::physics_fvm::{
    self,
};

#[derive(Deserialize)]
struct AdvectionInput {
    num_cells: usize,
    domain_size: f64,
    velocity: f64,
    dt: f64,
    steps: usize,
    initial_values: Vec<f64>,
}

#[derive(Deserialize)]
struct SweInput {
    h: Vec<f64>,
    hu: Vec<f64>,
    dx: f64,
    dt: f64,
    steps: usize,
    g: f64,
}

/// Solves the 1D advection equation using Finite Volume Method (FVM) via JSON serialization.
///
/// The advection equation ∂u/∂t + v∂u/∂x = 0 models conservative transport
/// of a scalar quantity u with constant velocity v.
///
/// # Arguments
///
/// * `input` - A JSON string pointer containing:
///   - `num_cells`: Number of cells in the mesh
///   - `domain_size`: Total length of the 1D domain
///   - `velocity`: Advection velocity v
///   - `dt`: Time step size
///   - `steps`: Number of time steps to simulate
///   - `initial_values`: Initial field distribution
///
/// # Returns
///
/// A C string pointer containing JSON-encoded `FfiResult<Vec<f64>, String>` with
/// the final field distribution.
///
/// # Safety
///
/// This function is unsafe because it receives a raw C string pointer that must be
/// valid, null-terminated UTF-8. The caller must free the returned pointer.
///
/// # Safety
///
/// This function is unsafe because it dereferences raw pointers as part of the FFI boundary.
/// The caller must ensure:
/// 1. All pointer arguments are valid and point to initialized memory.
/// 2. The memory layout of passed structures matches the expected C-ABI layout.
/// 3. Any pointers returned by this function are managed according to the API's ownership rules.
///
/// # Panics
///
/// This function may panic if the FFI input is malformed, null where not expected,
/// or if internal state synchronization fails (e.g., poisoned locks).
#[unsafe(no_mangle)]
pub unsafe extern "C" fn rssn_physics_fvm_advection_json(input: *const c_char) -> *mut c_char {
    let input: AdvectionInput = match from_json_string(input) {
        | Some(i) => i,
        | None => {
            return to_c_string(
                serde_json::to_string(&FfiResult::<Vec<f64>, String>::err(
                    "Invalid JSON".to_string(),
                ))
                .unwrap(),
            );
        },
    };

    let mut mesh = Mesh::new(input.num_cells, input.domain_size, |_| 0.0);

    for (i, &val) in input.initial_values.iter().enumerate() {
        if i < mesh.cells.len() {
            mesh.cells[i].value = val;
        }
    }

    let result =
        physics_fvm::solve_advection_1d(&mut mesh, input.velocity, input.dt, input.steps, || {
            (0.0, 0.0)
        });

    to_c_string(serde_json::to_string(&FfiResult::<Vec<f64>, String>::ok(result)).unwrap())
}

/// Solves the 1D shallow water equations using Finite Volume Method (FVM) via JSON serialization.
///
/// The shallow water equations model conservation of mass and momentum in free-surface flows:
/// ∂h/∂t + ∂(hu)/∂x = 0 and ∂(hu)/∂t + ∂(hu² + gh²/2)/∂x = 0.
///
/// # Arguments
///
/// * `input` - A JSON string pointer containing:
///   - `h`: Initial water depth distribution
///   - `hu`: Initial momentum (h×velocity) distribution
///   - `dx`: Spatial step size
///   - `dt`: Time step size
///   - `steps`: Number of time steps to simulate
///   - `g`: Gravitational acceleration
///
/// # Returns
///
/// A C string pointer containing JSON-encoded `FfiResult<Vec<SweState>, String>` with
/// the time series of shallow water states (h, hu).
///
/// # Safety
///
/// This function is unsafe because it receives a raw C string pointer that must be
/// valid, null-terminated UTF-8. The caller must free the returned pointer.
///
/// # Safety
///
/// This function is unsafe because it dereferences raw pointers as part of the FFI boundary.
/// The caller must ensure:
/// 1. All pointer arguments are valid and point to initialized memory.
/// 2. The memory layout of passed structures matches the expected C-ABI layout.
/// 3. Any pointers returned by this function are managed according to the API's ownership rules.
///
/// # Panics
///
/// This function may panic if the FFI input is malformed, null where not expected,
/// or if internal state synchronization fails (e.g., poisoned locks).
#[unsafe(no_mangle)]
pub unsafe extern "C" fn rssn_physics_fvm_swe_json(input: *const c_char) -> *mut c_char {
    let input: SweInput = match from_json_string(input) {
        | Some(i) => i,
        | None => {
            return to_c_string(
                serde_json::to_string(&FfiResult::<Vec<SweState>, String>::err(
                    "Invalid JSON".to_string(),
                ))
                .unwrap(),
            );
        },
    };

    let result = physics_fvm::solve_shallow_water_1d(
        input.h,
        input.hu,
        input.dx,
        input.dt,
        input.steps,
        input.g,
    );

    to_c_string(serde_json::to_string(&FfiResult::<Vec<SweState>, String>::ok(result)).unwrap())
}