rssn 0.2.9

//! JSON-based FFI API for physics EM 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_em::{
    self,
};
use crate::physics::physics_rkm::DampedOscillatorSystem;
use crate::physics::physics_rkm::LorenzSystem;

#[derive(Deserialize)]
struct EulerInput {
    system_type: String, // "lorenz", "oscillator", "orbital"
    params: serde_json::Value,
    y0: Vec<f64>,
    t_span: (f64, f64),
    dt: f64,
    method: String, // "forward", "midpoint", "heun"
}

/// Solves ODE systems using Euler methods (forward, midpoint, or Heun) via JSON serialization.
///
/// Supports various dynamical systems including Lorenz attractor, damped oscillators,
/// and orbital mechanics, using explicit Euler integration methods.
///
/// # Arguments
///
/// * `input` - A JSON string pointer containing:
///   - `system_type`: System identifier ("lorenz", "oscillator", "orbital")
///   - `params`: System parameters as JSON object
///   - `y0`: Initial state vector
///   - `t_span`: Time interval [`t_start`, `t_end`]
///   - `dt`: Time step size
///   - `method`: Integration method ("forward", "midpoint", "heun")
///
/// # Returns
///
/// A C string pointer containing JSON-encoded `FfiResult<Vec<(f64, Vec<f64>)>, String>` with
/// the trajectory as (time, state) pairs.
///
/// # 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_em_solve_json(input: *const c_char) -> *mut c_char {
    let input: EulerInput = match from_json_string(input) {
        | Some(i) => i,
        | None => {
            return to_c_string(
                serde_json::to_string(&FfiResult::<Vec<(f64, Vec<f64>)>, String>::err(
                    "Invalid JSON".to_string(),
                ))
                .unwrap(),
            );
        },
    };

    let res = match input.system_type.as_str() {
        | "lorenz" => {
            let sys: LorenzSystem = match serde_json::from_value(input.params) {
                | Ok(s) => s,
                | Err(e) => {
                    return to_c_string(
                        serde_json::to_string(&FfiResult::<Vec<(f64, Vec<f64>)>, String>::err(
                            e.to_string(),
                        ))
                        .unwrap(),
                    );
                },
            };

            solve_with_method(&sys, &input.y0, input.t_span, input.dt, &input.method)
        },
        | "oscillator" => {
            let sys: DampedOscillatorSystem = match serde_json::from_value(input.params) {
                | Ok(s) => s,
                | Err(e) => {
                    return to_c_string(
                        serde_json::to_string(&FfiResult::<Vec<(f64, Vec<f64>)>, String>::err(
                            e.to_string(),
                        ))
                        .unwrap(),
                    );
                },
            };

            solve_with_method(&sys, &input.y0, input.t_span, input.dt, &input.method)
        },
        | _ => {
            return to_c_string(
                serde_json::to_string(&FfiResult::<Vec<(f64, Vec<f64>)>, String>::err(
                    "Unknown system \
                     type"
                        .to_string(),
                ))
                .unwrap(),
            );
        },
    };

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

pub(crate) fn solve_with_method<S: crate::physics::physics_rkm::OdeSystem>(
    sys: &S,
    y0: &[f64],
    t_span: (f64, f64),
    dt: f64,
    method: &str,
) -> Vec<(f64, Vec<f64>)> {
    match method {
        | "midpoint" => physics_em::solve_midpoint_euler(sys, y0, t_span, dt),
        | "heun" => physics_em::solve_heun_euler(sys, y0, t_span, dt),
        | _ => physics_em::solve_forward_euler(sys, y0, t_span, dt),
    }
}