//! Anwesen daemon application: Hydra supervisor tree per [[ADR-004 Hydra as
//! Process Runtime]] and [ANW-17](https://crvrs.youtrack.cloud/issue/ANW-17).
//!
//! ```text
//! RootSupervisor (one_for_one)
//!   -- vault_scanner       (permanent; startup walk + overflow recovery)
//!   -- filesystem_watcher  (permanent; restart on inotify error)
//!   -- index_writer        (permanent; owns the Tantivy IndexWriter)
//!   -- http_server         (permanent; restart on bind loss)
//! ```
//!
//! Only `vault_scanner` has a real body in this commit: it runs a startup
//! walk via [`crate::vault::scan`] and accepts a `RescanNow` cast that
//! re-runs the same walk (for the inotify-overflow recovery path described
//! in [[ADR-003 Filesystem Change Tracking]]). The other three roles are
//! idle stubs that subsequent issues (ANW-12, ANW-13, ANW-16) replace with
//! real implementations.
//!
//! [`RestartCounters`] is the shared, lock-free snapshot consumed by
//! [ANW-8](https://crvrs.youtrack.cloud/issue/ANW-8) when wiring the
//! `/health` endpoint.

use std::collections::BTreeMap;
use std::net::SocketAddr;
use std::path::PathBuf;
use std::sync::Arc;
use std::sync::atomic::{AtomicBool, AtomicU32, Ordering};
use std::time::Duration;

use hydra::{
    Application, ApplicationConfig, ChildSpec, Dest, ExitReason, From as HydraFrom, GenServer,
    GenServerOptions, Pid, SupervisionStrategy, Supervisor, SupervisorOptions,
};
use notify::{RecursiveMode, Watcher};
use serde::{Deserialize, Serialize};
use tokio::sync::mpsc;
use tokio::task::JoinHandle;

use crate::http::{self as http_layer, HttpState};
use crate::index::NoteIndex;
use crate::store::NoteStore;
use crate::vault::{self, Note};
use crate::watcher::run_debouncer;

pub(crate) const INDEX_WRITER_NAME: &str = "index_writer";
pub(crate) const VAULT_SCANNER_NAME: &str = "vault_scanner";
pub(crate) const FILESYSTEM_WATCHER_NAME: &str = "filesystem_watcher";
pub(crate) const HTTP_SERVER_NAME: &str = "http_server";

/// Default debounce window for the filesystem watcher. Per [[ADR-003
/// Filesystem Change Tracking]] -- 100 ms is the documented target, tunable
/// once real save patterns are observed.
const WATCH_DEBOUNCE_WINDOW: Duration = Duration::from_millis(100);

/// Snapshot of per-process restart counters. Each role increments its own
/// counter on every `init` *after the first*, so the count represents
/// supervisor-initiated restarts rather than the initial start.
#[derive(Debug, Default)]
pub struct RestartCounters {
    pub vault_scanner: RoleCounter,
    pub filesystem_watcher: RoleCounter,
    pub index_writer: RoleCounter,
    pub http_server: RoleCounter,
}

#[derive(Debug, Default)]
pub struct RoleCounter {
    started: AtomicBool,
    restarts: AtomicU32,
}

impl RoleCounter {
    /// Record an `init` call. Returns the current restart count.
    fn record_init(&self) -> u32 {
        if self.started.swap(true, Ordering::AcqRel) {
            self.restarts.fetch_add(1, Ordering::Relaxed) + 1
        } else {
            0
        }
    }

    pub fn get(&self) -> u32 {
        self.restarts.load(Ordering::Relaxed)
    }
}

impl RestartCounters {
    #[must_use]
    pub fn new() -> Arc<Self> {
        Arc::new(Self::default())
    }

    /// Snapshot the counters with the role names from [[ADR-004 Hydra as
    /// Process Runtime]]'s "Open questions / Closed in v1" section -- these
    /// keys are part of the `/health` contract ([ANW-8]).
    #[must_use]
    pub fn snapshot(&self) -> BTreeMap<&'static str, u32> {
        let mut out = BTreeMap::new();
        out.insert("vault_scanner", self.vault_scanner.get());
        out.insert("filesystem_watcher", self.filesystem_watcher.get());
        out.insert("index_writer", self.index_writer.get());
        out.insert("http_server", self.http_server.get());
        out
    }
}

/// Daemon application: a Hydra [`Application`] that links a `one_for_one`
/// supervisor over the four process roles.
pub struct Anwesen {
    pub vault: PathBuf,
    pub bind: SocketAddr,
    pub counters: Arc<RestartCounters>,
    /// Shared in-memory note store. The scanner populates it, the watcher
    /// keeps it current via the writer's batches, and the HTTP layer reads
    /// from it on every request.
    pub store: Arc<NoteStore>,
}

impl Anwesen {
    #[must_use]
    pub fn new(vault: PathBuf, bind: SocketAddr) -> Self {
        Self {
            vault,
            bind,
            counters: RestartCounters::new(),
            store: NoteStore::new(),
        }
    }
}

impl Application for Anwesen {
    fn config() -> ApplicationConfig {
        // We install our own tracing-subscriber in `main::init_logging`, so
        // tell Hydra not to install a second one. Keep the panic hook off
        // too -- panics surface through `tracing` via our subscriber.
        ApplicationConfig::new()
            .with_tracing_subscribe(false)
            .with_tracing_panics(false)
            .with_graceful_shutdown(true)
    }

    async fn start(&self) -> Result<Pid, ExitReason> {
        // Startup order matters: `vault_scanner` casts a `Rebuild` to the
        // `index_writer` name at the end of its init. Hydra start_link is
        // synchronous, so we put `index_writer` first to guarantee it is
        // registered and in its message loop before `vault_scanner` runs.
        // The `one_for_one` strategy makes the order irrelevant for restart
        // semantics, only for the cold-start handshake.
        let children = [
            IndexWriter {
                counters: self.counters.clone(),
                store: self.store.clone(),
            }
            .child_spec(),
            VaultScanner {
                vault: self.vault.clone(),
                counters: self.counters.clone(),
            }
            .child_spec(),
            FilesystemWatcher {
                vault: self.vault.clone(),
                counters: self.counters.clone(),
            }
            .child_spec(),
            HttpServer {
                bind: self.bind,
                counters: self.counters.clone(),
                store: self.store.clone(),
            }
            .child_spec(),
        ];

        Supervisor::with_children(children)
            .strategy(SupervisionStrategy::OneForOne)
            .start_link(SupervisorOptions::new().name("anwesen_root"))
            .await
    }
}

// -- vault_scanner ----------------------------------------------------------

#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum VaultScannerMessage {
    /// Re-run the startup walk. Sent by `filesystem_watcher` on inotify
    /// overflow recovery per [[ADR-003 Filesystem Change Tracking]].
    RescanNow,
}

fn call_not_supported<M>(role: &str) -> Result<Option<M>, ExitReason> {
    // Each role's GenServer trait shares one `Message` type for both call and
    // cast (Hydra's API shape). To keep a stray `Foo::call(...)` from hanging
    // forever waiting on a reply, every `handle_call` returns this error.
    // Sie flagged the silent-Ok(None) pattern in ANW-17 review; pinning it
    // here in ANW-12 before ANW-16 wires the watcher-to-scanner call.
    Err(ExitReason::from(format!(
        "{role} does not handle synchronous calls; use cast"
    )))
}

#[derive(Clone)]
pub struct VaultScanner {
    vault: PathBuf,
    counters: Arc<RestartCounters>,
}

impl VaultScanner {
    fn child_spec(self) -> ChildSpec {
        let counters = self.counters.clone();
        let vault = self.vault.clone();
        ChildSpec::new(VAULT_SCANNER_NAME).start(move || {
            VaultScanner {
                vault: vault.clone(),
                counters: counters.clone(),
            }
            .start_link(GenServerOptions::new().name(VAULT_SCANNER_NAME))
        })
    }

    fn run_walk(&self) {
        let result = vault::scan(&self.vault);
        tracing::info!(
            notes = result.notes.len(),
            issues = result.issues.len(),
            vault = %self.vault.display(),
            "vault_scanner: walk complete"
        );
        for issue in &result.issues {
            tracing::warn!(
                path = %issue.path.display(),
                kind = %issue.kind,
                "vault_scanner: skipped file"
            );
        }
        // Push the fresh record set to the index writer. The cast is
        // address-by-name so we don't have to thread the index_writer Pid
        // through child specs.
        IndexWriterState::cast(
            Dest::from(INDEX_WRITER_NAME),
            IndexWriterMessage::Rebuild(result.notes),
        );
    }
}

impl GenServer for VaultScanner {
    type Message = VaultScannerMessage;

    async fn init(&mut self) -> Result<(), ExitReason> {
        let restart = self.counters.vault_scanner.record_init();
        tracing::info!(restart, "vault_scanner: init");
        self.run_walk();
        Ok(())
    }

    async fn handle_cast(&mut self, message: Self::Message) -> Result<(), ExitReason> {
        match message {
            VaultScannerMessage::RescanNow => {
                tracing::info!("vault_scanner: rescan_now received");
                self.run_walk();
            }
        }
        Ok(())
    }

    async fn handle_call(
        &mut self,
        _message: Self::Message,
        _from: HydraFrom,
    ) -> Result<Option<Self::Message>, ExitReason> {
        call_not_supported("vault_scanner")
    }
}

// -- filesystem_watcher -----------------------------------------------------

/// The watcher accepts no inbound messages today -- its work is the
/// `notify` stream plus the debouncer task. The single `Noop` variant
/// satisfies Hydra's `Receivable` bound; once a real call/cast surface is
/// useful (e.g. for tests) it can be added.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum FilesystemWatcherMessage {
    Noop,
}

#[derive(Clone)]
pub struct FilesystemWatcher {
    vault: PathBuf,
    counters: Arc<RestartCounters>,
}

impl FilesystemWatcher {
    fn child_spec(self) -> ChildSpec {
        let counters = self.counters.clone();
        let vault = self.vault.clone();
        ChildSpec::new(FILESYSTEM_WATCHER_NAME).start(move || {
            FilesystemWatcherState {
                vault: vault.clone(),
                counters: counters.clone(),
                watcher: None,
                debouncer: None,
            }
            .start_link(GenServerOptions::new().name(FILESYSTEM_WATCHER_NAME))
        })
    }
}

/// Runtime state for the watcher process. Holds the live
/// [`notify::RecommendedWatcher`] (must outlive event delivery) and the
/// [`JoinHandle`] for the debouncer task; both are torn down when the
/// process is dropped on restart.
struct FilesystemWatcherState {
    vault: PathBuf,
    counters: Arc<RestartCounters>,
    watcher: Option<notify::RecommendedWatcher>,
    debouncer: Option<JoinHandle<()>>,
}

impl Drop for FilesystemWatcherState {
    fn drop(&mut self) {
        if let Some(h) = self.debouncer.take() {
            h.abort();
        }
        // `watcher` drops on its own, which is enough to stop the inotify
        // binding; the debouncer task then sees the channel close.
    }
}

impl GenServer for FilesystemWatcherState {
    type Message = FilesystemWatcherMessage;

    async fn init(&mut self) -> Result<(), ExitReason> {
        let restart = self.counters.filesystem_watcher.record_init();

        let (tx, rx) = mpsc::unbounded_channel::<notify::Result<notify::Event>>();
        let mut watcher = notify::recommended_watcher(move |res| {
            // Send is non-blocking on an unbounded channel; ignore the
            // SendError that arises only when the receiver has been dropped
            // (process shutdown).
            let _ = tx.send(res);
        })
        .map_err(|e| ExitReason::from(format!("filesystem_watcher: recommended_watcher: {e}")))?;
        watcher
            .watch(&self.vault, RecursiveMode::Recursive)
            .map_err(|e| ExitReason::from(format!("filesystem_watcher: watch: {e}")))?;

        let handle = tokio::spawn(run_debouncer(rx, self.vault.clone(), WATCH_DEBOUNCE_WINDOW));

        self.watcher = Some(watcher);
        self.debouncer = Some(handle);

        tracing::info!(
            restart,
            vault = %self.vault.display(),
            debounce = ?WATCH_DEBOUNCE_WINDOW,
            "filesystem_watcher: init"
        );
        Ok(())
    }

    async fn handle_cast(&mut self, _message: Self::Message) -> Result<(), ExitReason> {
        Ok(())
    }

    async fn handle_call(
        &mut self,
        _message: Self::Message,
        _from: HydraFrom,
    ) -> Result<Option<Self::Message>, ExitReason> {
        call_not_supported("filesystem_watcher")
    }
}

// -- index_writer -----------------------------------------------------------

/// Messages handled by the [`IndexWriter`] `GenServer`. All variants are casts
/// (one-way fire-and-forget); calls return an error from `handle_call`.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum IndexWriterMessage {
    /// Discard the current index and reindex the given notes. Sent by
    /// `vault_scanner` at startup and after `rescan_now`.
    Rebuild(Vec<Note>),
    /// Apply one debounce-window's worth of upserts and deletes in a single
    /// Tantivy commit. Sent by `filesystem_watcher`. Picks up sie's ANW-12
    /// follow-up #3 (commit batching) before ANW-16's watcher can turn
    /// per-save events into a hot commit loop.
    Batch(IndexBatch),
    /// Insert-or-replace one note. Retained for direct callers; the watcher
    /// uses `Batch`.
    Upsert(Box<Note>),
    /// Drop one note from the index. Retained for direct callers; the
    /// watcher uses `Batch`.
    Delete(String),
}

/// One batched index update. Deletes apply first so a "delete-then-upsert"
/// sequence is unambiguous; per-path coalescing in
/// [`crate::watcher::coalesce`] already keeps at most one action per path.
#[derive(Debug, Clone, Default, Serialize, Deserialize)]
pub struct IndexBatch {
    pub upserts: Vec<Note>,
    pub deletes: Vec<String>,
}

#[derive(Clone)]
pub struct IndexWriter {
    counters: Arc<RestartCounters>,
    store: Arc<NoteStore>,
}

impl IndexWriter {
    fn child_spec(self) -> ChildSpec {
        let counters = self.counters.clone();
        let store = self.store.clone();
        ChildSpec::new(INDEX_WRITER_NAME).start(move || {
            IndexWriterState {
                counters: counters.clone(),
                store: store.clone(),
                index: None,
            }
            .start_link(GenServerOptions::new().name(INDEX_WRITER_NAME))
        })
    }
}

/// Runtime state for the [`IndexWriter`] process. Held in a separate struct
/// so the `Clone`-friendly child-spec form (which doesn't carry the live
/// `NoteIndex`) stays simple. Mirrors every write into the shared
/// [`NoteStore`] so the HTTP layer can serve read-one and listing
/// responses without consulting the index.
pub(crate) struct IndexWriterState {
    counters: Arc<RestartCounters>,
    store: Arc<NoteStore>,
    /// Created lazily in `init` so a Tantivy construction failure surfaces
    /// as an `ExitReason` and triggers a supervisor restart, rather than
    /// poisoning the child spec.
    index: Option<NoteIndex>,
}

impl GenServer for IndexWriterState {
    type Message = IndexWriterMessage;

    async fn init(&mut self) -> Result<(), ExitReason> {
        let restart = self.counters.index_writer.record_init();
        let index = NoteIndex::new()
            .map_err(|e| ExitReason::from(format!("index_writer: NoteIndex::new failed: {e}")))?;
        self.index = Some(index);

        if restart > 0 {
            // Writer-only restart: a fresh empty index has come up while the
            // watcher keeps streaming batches at it. Ask `vault_scanner` for
            // a full walk via the same `rescan_now` path the inotify-overflow
            // recovery uses. Per kaa's pin (ADR-004 amendment 2026-05-14),
            // rescan upserts are idempotent on path so in-flight watcher
            // batches converge with the rescan.
            VaultScanner::cast(
                Dest::from(VAULT_SCANNER_NAME),
                VaultScannerMessage::RescanNow,
            );
            tracing::info!(
                restart,
                "index_writer: init -- rescan_now dispatched to vault_scanner"
            );
        } else {
            tracing::info!(restart, "index_writer: init");
        }
        Ok(())
    }

    async fn handle_cast(&mut self, message: Self::Message) -> Result<(), ExitReason> {
        let Some(index) = self.index.as_mut() else {
            return Err(ExitReason::from(
                "index_writer: handle_cast invoked before init",
            ));
        };
        match message {
            IndexWriterMessage::Rebuild(notes) => {
                let count = notes.len();
                if let Err(e) = index.rebuild(&notes) {
                    tracing::error!(error = %e, "index_writer: rebuild failed");
                    return Err(ExitReason::from(format!("rebuild: {e}")));
                }
                self.store.replace(notes);
                tracing::info!(notes = count, "index_writer: rebuilt");
            }
            IndexWriterMessage::Batch(batch) => {
                let (u, d) = (batch.upserts.len(), batch.deletes.len());
                if let Err(e) = index.apply_batch(&batch.upserts, &batch.deletes) {
                    tracing::error!(error = %e, "index_writer: batch apply failed");
                    return Err(ExitReason::from(format!("batch: {e}")));
                }
                self.store.apply_batch(batch.upserts, &batch.deletes);
                tracing::info!(upserts = u, deletes = d, "index_writer: batch applied");
            }
            IndexWriterMessage::Upsert(note) => {
                let path = note.path.clone();
                if let Err(e) = index.upsert(&note) {
                    tracing::error!(%path, error = %e, "index_writer: upsert failed");
                    return Err(ExitReason::from(format!("upsert {path}: {e}")));
                }
                self.store.upsert(*note);
                tracing::debug!(%path, "index_writer: upserted");
            }
            IndexWriterMessage::Delete(path) => {
                if let Err(e) = index.delete(&path) {
                    tracing::error!(%path, error = %e, "index_writer: delete failed");
                    return Err(ExitReason::from(format!("delete {path}: {e}")));
                }
                self.store.delete(&path);
                tracing::debug!(%path, "index_writer: deleted");
            }
        }
        Ok(())
    }

    async fn handle_call(
        &mut self,
        _message: Self::Message,
        _from: HydraFrom,
    ) -> Result<Option<Self::Message>, ExitReason> {
        call_not_supported("index_writer")
    }
}

// -- http_server ------------------------------------------------------------

/// The HTTP server has no inbound message protocol; the single `Noop`
/// variant satisfies Hydra's `Receivable` bound.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum HttpServerMessage {
    Noop,
}

#[derive(Clone)]
pub struct HttpServer {
    bind: SocketAddr,
    counters: Arc<RestartCounters>,
    store: Arc<NoteStore>,
}

impl HttpServer {
    fn child_spec(self) -> ChildSpec {
        let bind = self.bind;
        let counters = self.counters.clone();
        let store = self.store.clone();
        ChildSpec::new(HTTP_SERVER_NAME).start(move || {
            HttpServerState {
                bind,
                counters: counters.clone(),
                store: store.clone(),
                server: None,
            }
            .start_link(GenServerOptions::new().name(HTTP_SERVER_NAME))
        })
    }
}

struct HttpServerState {
    bind: SocketAddr,
    counters: Arc<RestartCounters>,
    store: Arc<NoteStore>,
    server: Option<JoinHandle<()>>,
}

impl Drop for HttpServerState {
    fn drop(&mut self) {
        if let Some(h) = self.server.take() {
            h.abort();
        }
    }
}

impl GenServer for HttpServerState {
    type Message = HttpServerMessage;

    async fn init(&mut self) -> Result<(), ExitReason> {
        let restart = self.counters.http_server.record_init();

        let listener = tokio::net::TcpListener::bind(self.bind)
            .await
            .map_err(|e| ExitReason::from(format!("http_server: bind {}: {e}", self.bind)))?;

        let router = http_layer::router(HttpState {
            store: self.store.clone(),
        });
        let server = tokio::spawn(async move {
            if let Err(e) = axum::serve(listener, router).await {
                tracing::error!(error = %e, "http_server: serve loop exited with error");
            }
        });
        self.server = Some(server);

        tracing::info!(restart, bind = %self.bind, "http_server: init");
        Ok(())
    }

    async fn handle_cast(&mut self, _message: Self::Message) -> Result<(), ExitReason> {
        Ok(())
    }

    async fn handle_call(
        &mut self,
        _message: Self::Message,
        _from: HydraFrom,
    ) -> Result<Option<Self::Message>, ExitReason> {
        call_not_supported("http_server")
    }
}

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

    #[test]
    fn role_counter_first_init_is_zero() {
        let c = RoleCounter::default();
        assert_eq!(c.record_init(), 0);
        assert_eq!(c.get(), 0);
    }

    #[test]
    fn role_counter_counts_restarts_only() {
        let c = RoleCounter::default();
        assert_eq!(c.record_init(), 0); // first start
        assert_eq!(c.record_init(), 1); // first restart
        assert_eq!(c.record_init(), 2); // second restart
        assert_eq!(c.get(), 2);
    }

    #[test]
    fn snapshot_keys_match_health_contract() {
        let counters = RestartCounters::new();
        let snap = counters.snapshot();
        // Keys are part of the /health payload contract per ADR-004.
        assert_eq!(
            snap.keys().copied().collect::<Vec<_>>(),
            vec![
                "filesystem_watcher",
                "http_server",
                "index_writer",
                "vault_scanner"
            ]
        );
        assert!(snap.values().all(|v| *v == 0));
    }

    #[test]
    fn app_config_disables_hydra_tracing_subscribe() {
        // We install our own tracing-subscriber; Hydra installing a second
        // one would race and double-format every event.
        let cfg = Anwesen::config();
        // Field is pub(crate) inside hydra; we cannot assert directly. Instead
        // we exercise the builder path so a future Hydra change that flips the
        // default trips a compile-level reminder here.
        let _ = cfg;
    }
}