// SPDX-License-Identifier: AGPL-3.0-only

//! Dedicated server for Synvael.
//!
//! The server handles the authoritative game simulation, including world management, physics, and combat.

/// A bounded LRU cache of regenerated chunk baselines, shared across the worker pool.
pub mod chunk_cache;
/// Per-connection chunk-streaming state: desired-set tracking and delivery.
pub mod client_stream;
/// Entity components describing players and other world-streaming anchors.
pub mod player;
/// On-disk persistence: region files and the atomic durability layer.
pub mod save;
/// Authoritative chunk storage and generation logic for the server.
pub mod world_server;

use std::collections::{HashMap, HashSet};
use std::fs;
use std::net::{Ipv4Addr, SocketAddr};
use std::time::{Duration, Instant};

use anyhow::Context;
use bevy_ecs::prelude::{Query, ResMut, Schedule, With, World};
use glam::Vec3;
use shared::generator::{VoxelGenerator, WorldGenConfig};
use shared::world::{Chunk, ChunkPos, EntityPos};
use tracing::{debug, info, warn};

use client_stream::ClientStream;
use net::{NetworkServer, ServerEvent};
use player::{Player, Position, ViewDistance};
use world_server::{ServerWorld, cylinder_chunks};

/// Nominal simulation rate, in ticks per second. Sole source of truth for both the loop's target period and the advisory rate advertised to clients in the handshake.
// TODO: make configurable through server configs once the real tick scheduler lands.
const TICK_RATE_HZ: u16 = 20;

/// Target wall-clock period of one simulation tick, derived from [`TICK_RATE_HZ`].
const TICK_PERIOD: Duration = Duration::from_millis(1000 / TICK_RATE_HZ as u64);

/// Streaming system: loads and unloads chunks so that the resident set matches the union of the cylinders around every player anchor.
fn stream_chunks(
    anchors: Query<(&Position, &ViewDistance), With<Player>>,
    mut world: ResMut<ServerWorld>,
) {
    // Desired set is the union of every anchor's cylinder; a chunk survives as long as it lies within any one player's view.
    let mut desired = HashSet::new();
    for (position, view) in &anchors {
        cylinder_chunks(position.0.chunk, view.0, &mut desired);
    }

    let stats = world.reconcile(&desired);
    info!(
        loaded = stats.loaded,
        unloaded = stats.unloaded,
        resident = stats.resident,
        in_flight = stats.in_flight,
        "streaming reconcile"
    );
}

fn main() -> anyhow::Result<()> {
    tracing_subscriber::fmt()
        .with_env_filter(
            tracing_subscriber::EnvFilter::try_from_default_env()
                .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new("debug")),
        )
        .init();

    info!("Starting Synvael server");

    let config_str = fs::read_to_string("assets/data/worldgen/default.json")
        .context("reading worldgen config assets/data/worldgen/default.json")?;

    let worldgen_config: WorldGenConfig =
        serde_json::from_str(&config_str).context("parsing worldgen config as JSON")?;

    info!("Successfully loaded world configuration");
    debug!("Base height: {}", worldgen_config.base_height);
    debug!("Noise scale: {}", worldgen_config.noise_scale);
    debug!("Surface block: {}", worldgen_config.surface_block.0);
    debug!("Subsurface block: {}", worldgen_config.subsurface_block.0);
    debug!("Stone block: {}", worldgen_config.stone_block.0);

    let seed = 4_813_530;

    let generator = VoxelGenerator::new(worldgen_config, seed);

    // The region directory holds the `.region` save files for this world
    // TODO: resolve it per named world under a shared save root.
    let region_dir = std::path::PathBuf::from("saves/default/region");

    // Number of chunk baselines the worker pool retains before evicting the least-recently-used entry
    // TODO: make this configurable through server configs
    let cache_capacity =
        std::num::NonZeroUsize::new(4_096).context("chunk cache capacity is non-zero")?;

    let mut world = World::new();
    world.insert_resource(ServerWorld::new(generator, region_dir, cache_capacity));

    // Spawn a single dummy player anchor at the world origin.
    world.spawn((
        Player,
        Position(EntityPos::new(ChunkPos::new(0, 0, 0), Vec3::ZERO)),
        ViewDistance(4),
    ));

    // A schedule is one tick's worth of systems; running it advances the world.
    let mut schedule = Schedule::default();
    schedule.add_systems(stream_chunks);

    // Loading phase: dispatch the initial region and wait for the worker pool to finish before granting control.
    info!("Streaming initial region");
    loop {
        schedule.run(&mut world);

        let server_world = world.resource::<ServerWorld>();
        let resident = server_world.loaded_count();
        let in_flight = server_world.in_flight_count();
        // Loading progress is simply the resident fraction of all known chunks.
        let total = resident + in_flight;
        debug!(resident, in_flight, total, "loading progress");

        // The region is ready once at least one chunk has been generated and none remain in flight.
        if server_world.streaming_idle() && resident > 0 {
            break;
        }
    }
    info!("Initial region ready; granting player control");

    // Spawn the networking thread and bind the QUIC endpoint. The synchronous simulation loop below communicates with it only by draining events.
    let bind = SocketAddr::from((Ipv4Addr::LOCALHOST, net::DEFAULT_PORT));
    let (network, local_addr) =
        NetworkServer::spawn(bind, env!("CARGO_PKG_VERSION").to_owned(), TICK_RATE_HZ)
            .context("spawning network server")?;
    info!(%local_addr, "network endpoint listening");

    run_simulation(&mut world, &network)
}

/// Runs the authoritative simulation loop forever, at the fixed cadence given by [`TICK_PERIOD`].
fn run_simulation(world: &mut World, network: &NetworkServer) -> ! {
    // Chunk diffs are computed against an all-air baseline so each delivered payload is self-contained: the client renders only server-owned content and has no generator to reconstruct a worldgen baseline. Allocated once and shared across every delivery.
    let empty_baseline = Chunk::default();

    // Per-connection streaming state, keyed by the stable session id the network thread assigns.
    let mut clients: HashMap<u64, ClientStream> = HashMap::new();

    loop {
        let tick_start = Instant::now();

        // Fold network events into per-client subscription state.
        for event in network.poll_events() {
            match event {
                ServerEvent::ClientConnected { id, hello, chunks } => {
                    info!(id, name = %hello.player_identity.display_name, "client connected");
                    clients.insert(id, ClientStream::new(chunks));
                }
                ServerEvent::ClientDisconnected { id, reason } => {
                    info!(id, %reason, "client disconnected");
                    clients.remove(&id);
                }
                ServerEvent::ChunkSubscribe { id, request } => {
                    if let Some(client) = clients.get_mut(&id) {
                        let (added, drops) = client.resubscribe(request.center, request.radius);
                        info!(
                            id,
                            added, drops, "client {id}: +{added} chunks, -{drops} drops"
                        );
                    } else {
                        warn!(id, "chunk subscribe from unknown session");
                    }
                }
            }
        }

        // Reconcile the resident world to the union of every client's desired set. A chunk survives as long as any connected client wants it; when no client is connected the union is empty and the world drains.
        let mut desired = HashSet::new();
        for client in clients.values() {
            desired.extend(client.desired().iter().copied());
        }
        world.resource_mut::<ServerWorld>().reconcile(&desired);

        // Deliver newly-resident chunks to each client. Loads dispatched above may not be resident this tick; `flush` retries on later ticks until the worker pool returns them.
        let server_world = world.resource::<ServerWorld>();
        for (id, client) in &mut clients {
            let delivered = client.flush(server_world, &empty_baseline);
            if delivered > 0 {
                debug!(id, delivered, "delivered resident chunks");
            }
        }

        // Sleep only the unused remainder of the tick's budget, so the period stays [`TICK_PERIOD`] rather than growing with the cost of the work above. A tick that overruns its budget does not sleep at all; the overrun is reported because it is the signal that the server is falling behind its nominal rate.
        let elapsed = tick_start.elapsed();
        if elapsed > TICK_PERIOD {
            warn!(
                elapsed_ms = elapsed.as_secs_f32() * 1000.0,
                budget_ms = TICK_PERIOD.as_secs_f32() * 1000.0,
                "tick overran its budget"
            );
        } else {
            // `saturating_sub` cannot underflow here (the branch already establishes `elapsed <= TICK_PERIOD`) and is used because `Duration` subtraction panics on overflow.
            std::thread::sleep(TICK_PERIOD.saturating_sub(elapsed));
        }
    }
}