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

//! A dedicated thread that owns every open region file and answers load requests over a channel.

use std::collections::HashMap;
use std::collections::hash_map::Entry;
use std::path::{Path, PathBuf};
use std::thread::{self, JoinHandle};

use crossbeam_channel::{Receiver, Sender};
use shared::save::SaveError;
use shared::world::{ChunkData, ChunkPos};

use super::region_file::{RegionFile, region_coords, region_path};

/// A request sent to the save actor. Each variant carries its own one-shot reply channel.
pub enum SaveRequest {
    /// Reads the stored chunk at a position, replying with the saved modification if one exists.
    Read {
        /// The chunk position to look up.
        pos: ChunkPos,
        /// The one-shot channel the actor replies on: `Ok(Some(data))` for a saved modification, `Ok(None)` when the chunk was never modified, or `Err` on a save-layer failure.
        reply: Sender<Result<Option<ChunkData>, SaveError>>,
    },
}

/// A handle to the running save actor: the request sender plus the owning thread's join handle.
pub struct SaveActor {
    /// The sending end of the request channel; cloned into every worker so it can issue reads.
    request_tx: Sender<SaveRequest>,
    /// The actor thread handle, retained so it can be joined on shutdown.
    // * Retained ahead of a dedicated shutdown path; not yet read because the server has no graceful-stop sequence.
    #[expect(dead_code)]
    handle: JoinHandle<()>,
}

impl SaveActor {
    /// Spawns the actor thread, which owns the region files beneath `region_dir` for its lifetime.
    #[must_use]
    pub fn spawn(region_dir: PathBuf) -> Self {
        let (request_tx, request_rx) = crossbeam_channel::unbounded::<SaveRequest>();
        let handle = thread::spawn(move || actor_loop(&region_dir, &request_rx));
        Self { request_tx, handle }
    }

    /// Returns a fresh sender for a worker to issue requests through.
    #[must_use]
    pub fn sender(&self) -> Sender<SaveRequest> {
        self.request_tx.clone()
    }
}

/// The actor's run loop: it owns the region-file map and answers requests until every sender is dropped.
fn actor_loop(region_dir: &Path, request_rx: &Receiver<SaveRequest>) {
    // The actor is the sole owner of this map, so region files need no lock of their own.
    let mut regions: HashMap<(i32, i32), RegionFile> = HashMap::new();
    while let Ok(request) = request_rx.recv() {
        match request {
            SaveRequest::Read { pos, reply } => {
                let result = read_chunk(&mut regions, region_dir, pos);
                // A send error means the requesting worker has gone away; the reply is simply dropped.
                let _ = reply.send(result);
            }
        }
    }
}

/// Reads the stored chunk at `pos`, opening and caching its region file on first access.
fn read_chunk(
    regions: &mut HashMap<(i32, i32), RegionFile>,
    region_dir: &Path,
    pos: ChunkPos,
) -> Result<Option<ChunkData>, SaveError> {
    let key = region_coords(pos.x, pos.z);
    // The region file is opened once on first touch; every later read of a chunk in it hits the in-memory copy.
    let region = match regions.entry(key) {
        Entry::Occupied(entry) => entry.into_mut(),
        Entry::Vacant(entry) => {
            entry.insert(RegionFile::open(region_path(region_dir, pos.x, pos.z))?)
        }
    };
    region.read_chunk(pos)
}