//! RNS carrier (Smol Mail 1.2, RNS.md sec 7): Reticulum Links to the
//! `smolmail.server` IN/SINGLE destination, dispatched through the same
//! `Session` as TCP.
//!
//! microReticulum's request handler is a bare function pointer with no
//! userdata, so this module's state lives in a `static OnceLock` and the
//! shim reaches it through the `smolmail_rns_on_*` C callbacks below. The
//! link identifier is the session key: bind values are derived from
//! destination || link_id (RNS.md sec 2), so AUTH on one link cannot replay
//! on another.

use std::collections::{HashMap, HashSet};
use std::ffi::CString;
use std::sync::{Mutex, OnceLock};

use crate::bind::TransportBindValues;
use crate::proto::{AUTH_FULL_TOKENS, INTERNAL_ERROR, MALFORMED, RATE_LIMITED, TOO_LARGE};
use crate::ratelimit::{ByteRateLimiter, RateLimiter};
use crate::session::{ServerConfig, Session};
use crate::store::Store;

mod ffi {
    use std::os::raw::{c_char, c_int};

    extern "C" {
        // shim/smolmail_rns.cpp
        pub fn smolmail_rns_start(
            identity: *const u8,
            storage_dir: *const c_char,
            udp_listen_host: *const c_char,
            udp_listen_port: u16,
            udp_forward_host: *const c_char,
            udp_forward_port: u16,
            destination_hash_out: *mut u8,
        ) -> c_int;

        pub fn smolmail_rns_destination_hash(
            identity: *const u8,
            destination_hash_out: *mut u8,
        ) -> c_int;
    }
}

/// The identity file is 64 raw bytes: x25519 private (32) || ed25519 private
/// (32), the layout microReticulum's `Identity::to_file` writes and
/// `load_private_key` expects.
pub const RNS_KEY_LEN: usize = 64;

pub struct RnsArgs {
    pub key_path: String,
    pub db_path: String,
    pub storage_dir: String,
    pub max_envelope: usize,
    pub fetch_budget: usize,
    pub max_links: usize,
    pub rate_link_requests: u32,
    pub rate_link_bytes: u64,
    pub udp_listen_host: String,
    pub udp_listen_port: u16,
    pub udp_forward_host: Option<String>,
    pub udp_forward_port: u16,
    pub max_tokens: u16,
    pub main_quota: i64,
    pub requests_quota: i64,
    pub invite_token: Option<Vec<u8>>,
}

struct RnsState {
    config: &'static ServerConfig,
    db_path: String,
    destination: [u8; 16],
    sessions: Mutex<HashMap<[u8; 16], Session<'static>>>,
    links: Mutex<HashSet<[u8; 16]>>,
    request_limiter: RateLimiter,
    byte_limiter: ByteRateLimiter,
    max_links: usize,
    response: Mutex<Option<Vec<u8>>>,
}

static STATE: OnceLock<RnsState> = OnceLock::new();

/// Loads the Reticulum identity, starts the carrier on the shim's background
/// loop thread, and returns the destination hash as the 32 hex characters of
/// the `smol+rns://` address host part (upstream spec 13.2).
pub fn start(args: RnsArgs) -> anyhow::Result<String> {
    let key = std::fs::read(&args.key_path)
        .map_err(|e| anyhow::anyhow!("cannot read RNS identity {}: {e}", args.key_path))?;
    anyhow::ensure!(
        key.len() == RNS_KEY_LEN,
        "RNS identity must be {RNS_KEY_LEN} raw bytes, got {}",
        key.len()
    );

    // Ask the shim first: bind values derive from the destination hash, so
    // the session state must exist before the shim's loop thread can fire
    // the first request.
    let destination = destination_hash(key.as_slice().try_into().unwrap())?;

    // RateLimiter keyed by link hex covers per-link abuse control; the
    // per-IP send limit has no analogue (upstream spec 13.8), and the accept
    // token limiter stays on because a token never needed a peer identity.
    let config: &'static ServerConfig = Box::leak(Box::new(ServerConfig {
        max_envelope: args.max_envelope,
        fetch_budget: args.fetch_budget,
        main_quota: args.main_quota,
        requests_quota: args.requests_quota,
        max_tokens: args.max_tokens,
        invite_token: args.invite_token.clone(),
        conn_limiter: RateLimiter::new(0),
        send_limiter: RateLimiter::new(0),
        token_limiter: RateLimiter::new(30),
    }));

    let state = RnsState {
        config,
        db_path: args.db_path.clone(),
        destination,
        sessions: Mutex::new(HashMap::new()),
        links: Mutex::new(HashSet::new()),
        request_limiter: RateLimiter::new(args.rate_link_requests),
        byte_limiter: ByteRateLimiter::new(args.rate_link_bytes),
        max_links: args.max_links,
        response: Mutex::new(None),
    };
    STATE.get_or_init(|| state);

    let storage_dir = CString::new(args.storage_dir).unwrap();
    let listen_host = CString::new(args.udp_listen_host).unwrap();
    let forward_host = args.udp_forward_host.map(|h| CString::new(h).unwrap());

    let mut dest_hash = [0u8; 16];
    let rc = unsafe {
        ffi::smolmail_rns_start(
            key.as_ptr(),
            storage_dir.as_ptr(),
            listen_host.as_ptr(),
            args.udp_listen_port,
            forward_host
                .as_ref()
                .map(|h| h.as_ptr())
                .unwrap_or(std::ptr::null()),
            args.udp_forward_port,
            dest_hash.as_mut_ptr(),
        )
    };
    anyhow::ensure!(rc == 0, "RNS shim failed to start (code {rc})");
    anyhow::ensure!(
        dest_hash == destination,
        "destination hash changed between shim calls"
    );
    Ok(data_encoding::HEXLOWER.encode(&dest_hash))
}

/// The `smolmail.server` destination hash for a 64-byte identity, computed
/// by microReticulum itself: its Curve25519 `eval` does not clamp the scalar
/// (a divergence from RFC 7748), so an independent Rust derivation would
/// produce a different x25519 public key and therefore a different hash.
pub fn destination_hash(key: &[u8; RNS_KEY_LEN]) -> anyhow::Result<[u8; 16]> {
    let mut out = [0u8; 16];
    let rc = unsafe { ffi::smolmail_rns_destination_hash(key.as_ptr(), out.as_mut_ptr()) };
    anyhow::ensure!(rc == 0, "shim rejected the RNS identity (code {rc})");
    Ok(out)
}

/// Generates the 64-byte Reticulum identity: random x25519 || ed25519
/// private halves.
pub fn generate_identity() -> [u8; RNS_KEY_LEN] {
    use rand_core::{OsRng, RngCore};
    let mut key = [0u8; RNS_KEY_LEN];
    OsRng.fill_bytes(&mut key);
    key
}

fn response(status: u8) -> Vec<u8> {
    vec![status]
}

fn handle_request(request: &[u8], link_id: &[u8; 16]) -> Vec<u8> {
    let Some(state) = STATE.get() else {
        return response(INTERNAL_ERROR);
    };
    let link = data_encoding::HEXLOWER.encode(link_id);

    // Bounded request size (RNS.md sec 3): an envelope plus its overhead, or
    // an AUTH carrying a full token set, whichever is larger.
    let max_request = (state.config.max_envelope + 34)
        .max(AUTH_FULL_TOKENS + 32 * state.config.max_tokens as usize);
    if request.len() > max_request {
        log::warn!(
            "request of {} bytes over {} byte cap",
            request.len(),
            max_request
        );
        return response(TOO_LARGE);
    }
    if !state.request_limiter.allow(&link) || !state.byte_limiter.allow(&link, request.len()) {
        return response(RATE_LIMITED);
    }

    let Some(op) = request.first() else {
        return response(MALFORMED);
    };
    let body = &request[1..];

    let mut sessions = state.sessions.lock().unwrap();
    let session = match sessions.entry(*link_id) {
        std::collections::hash_map::Entry::Occupied(e) => e.into_mut(),
        std::collections::hash_map::Entry::Vacant(e) => {
            // The link is the session (upstream spec 13.6): a Store per link,
            // bind values derived from this destination and link.
            match Store::open(&state.db_path) {
                Ok(store) => e.insert(Session::new(
                    state.config,
                    store,
                    link.clone(),
                    TransportBindValues::rns(&state.destination, link_id),
                )),
                Err(err) => {
                    log::error!("cannot open store for link {link}: {err}");
                    return response(INTERNAL_ERROR);
                }
            }
        }
    };

    let (status, payload) = session.dispatch(*op, body);
    let mut out = Vec::with_capacity(1 + payload.len());
    out.push(status);
    out.extend_from_slice(&payload);
    out
}

#[no_mangle]
extern "C" fn smolmail_rns_on_request(
    request: *const u8,
    request_len: usize,
    link_id: *const u8,
) -> usize {
    // The shim calls this on its single loop thread, so the slot never sees
    // concurrent writers; panics must not cross the FFI boundary.
    let result = std::panic::catch_unwind(|| unsafe {
        let request = std::slice::from_raw_parts(request, request_len);
        let link_id: &[u8; 16] = std::slice::from_raw_parts(link_id, 16).try_into().unwrap();
        handle_request(request, link_id)
    });
    let Ok(response) = result else {
        log::error!("panic in RNS request handler");
        return 0;
    };
    let len = response.len();
    *STATE.get().unwrap().response.lock().unwrap() = Some(response);
    len
}

#[no_mangle]
extern "C" fn smolmail_rns_take_response(out: *mut u8, cap: usize) -> usize {
    let slot = &mut STATE.get().unwrap().response.lock().unwrap();
    match slot.take() {
        Some(response) if response.len() <= cap => {
            unsafe { std::ptr::copy_nonoverlapping(response.as_ptr(), out, response.len()) };
            response.len()
        }
        _ => 0,
    }
}

#[no_mangle]
extern "C" fn smolmail_rns_on_link_opened(link_id: *const u8) -> i32 {
    let Some(state) = STATE.get() else {
        return 1;
    };
    let link: [u8; 16] = unsafe { std::slice::from_raw_parts(link_id, 16).try_into().unwrap() };
    let mut links = state.links.lock().unwrap();
    if !links.contains(&link) && links.len() >= state.max_links {
        log::warn!(
            "refusing link {}, {} link(s) open",
            data_encoding::HEXLOWER.encode(&link),
            links.len()
        );
        return 1;
    }
    links.insert(link);
    log::debug!("link {} opened ({} open)", data_encoding::HEXLOWER.encode(&link), links.len());
    0
}

#[no_mangle]
extern "C" fn smolmail_rns_on_link_closed(link_id: *const u8) {
    if let Some(state) = STATE.get() {
        let link: [u8; 16] = unsafe { std::slice::from_raw_parts(link_id, 16).try_into().unwrap() };
        state.links.lock().unwrap().remove(&link);
        state.sessions.lock().unwrap().remove(&link);
        log::debug!("link {} closed", data_encoding::HEXLOWER.encode(&link));
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    /// Cross-checked against microReticulum itself: the destination hash a
    /// native probe prints for the identity 0x00..0x3f.
    #[test]
    fn destination_hash_matches_microreticulum() {
        let mut key = [0u8; RNS_KEY_LEN];
        for (i, byte) in key.iter_mut().enumerate() {
            *byte = i as u8;
        }
        assert_eq!(
            data_encoding::HEXLOWER.encode(&destination_hash(&key).unwrap()),
            "799855f4955f1b09fd20a13cd84f4e71"
        );
    }
}