//! Which runtime, which provider and which model form — decided once per
//! device, and the only crate that knows the answer (docs/inference.md).
//!
//! Consumers ask for a session by [`Role`] and get `ort`'s `Session` back;
//! what built it — tract on one core, ONNX Runtime's CPU pool, a TensorRT
//! engine, the Hexagon — is this crate's business and shows up in
//! [`status`] for the settings row and nowhere else.
//!
//! The shape follows §3 of the spec: `ort` links nothing (`alternative-backend`),
//! and the first call hands it an API table from either a `libonnxruntime`
//! found on disk or from `tract`. That choice is once per process, because
//! `ort::set_api` is; everything after it — which provider, whether an engine
//! has been compiled yet — is per session and may change between two calls.

use std::collections::{BTreeSet, HashMap};
use std::path::{Path, PathBuf};
use std::sync::{Arc, Mutex, MutexGuard, OnceLock};
use std::time::{Duration, Instant};

use serde::{Deserialize, Serialize};

mod api;
mod engines;
mod probe;
mod session;

pub use api::Runtime;
pub use ort::session::Session;

/// What a model is for. The role fixes the precision rule (§7): an embedder
/// runs in f32 on every rung, a detector may run in fp16 or int8.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum Role {
    Detector,
    Embedder,
    Segmenter,
    Scene,
    /// The dense landmark model behind the eye reading (docs/faces.md §7c).
    Landmarks,
    /// The eye-state and sunglasses classifiers, a few hundred kilobytes.
    EyeClassifier,
}

/// Which numeric form of a model a session was built from.
///
/// `Int8` is a different network from `F32` for a detector — it finds a
/// different set of faces — which is why [`form_suffix`] exists and why a
/// caller appends it to `model_id`.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum Form {
    F32,
    Int8,
}

/// A rung of the ladder (§2). Ordered: a user override names the highest rung
/// the probe may take, and a compiling rung falls back to the one below it
/// until its engine exists.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord, Serialize, Deserialize)]
pub enum Rung {
    /// ONNX Runtime's CPU provider, or tract when no runtime file was found.
    Cpu,
    /// NVIDIA, through the CUDA provider. Desktop only.
    Cuda,
    /// NVIDIA, through a TensorRT engine compiled on this device. Desktop only.
    TensorRt,
    /// Qualcomm's Hexagon NPU through QNN, int8 models only. Android only.
    Hexagon,
}

impl Rung {
    pub fn label(self) -> &'static str {
        match self {
            Rung::Cpu => "CPU",
            Rung::Cuda => "CUDA",
            Rung::TensorRt => "TensorRT",
            Rung::Hexagon => "Hexagon NPU",
        }
    }

    /// The rung a request lands on while this one's engine is still being
    /// compiled (§6 step 2).
    fn fallback(self) -> Rung {
        match self {
            Rung::TensorRt => Rung::Cuda,
            Rung::Hexagon | Rung::Cuda | Rung::Cpu => Rung::Cpu,
        }
    }

    /// Whether a session on this rung needs an engine built first.
    fn compiles(self) -> bool {
        matches!(self, Rung::TensorRt | Rung::Hexagon)
    }

    /// The model form this rung wants for a role.
    fn form(self, role: Role) -> Form {
        match (self, role) {
            (Rung::Hexagon, Role::Embedder) => Form::F32,
            (Rung::Hexagon, _) => Form::Int8,
            _ => Form::F32,
        }
    }
}

/// How long a session outlives its last use unless [`Config::decay`] says
/// otherwise: long enough for the next click, short enough that a session's
/// GPU or NPU memory does not sit under the develop view for long.
pub const DEFAULT_DECAY: Duration = Duration::from_secs(30);

/// What [`init`] is told once, at launch.
#[derive(Clone, Debug, Default)]
pub struct Config {
    /// Where to look for `libonnxruntime`, in order. An empty path means "the
    /// bare library name through the system loader", which is how the APK's
    /// own copy is found on Android.
    pub runtime_dirs: Vec<PathBuf>,
    /// Probe cache and compiled engines (§4, §5). Disposable.
    pub cache_dir: PathBuf,
    /// The canonical model files on this device, so engines can be compiled
    /// ahead of the first request for them.
    pub models: Vec<(Role, PathBuf)>,
    /// Models compiled into the binary, for the same reason.
    pub embedded: Vec<(Role, &'static [u8])>,
    /// The highest rung the user allows; `None` is "the best that works".
    pub ceiling: Option<Rung>,
    /// ONNX Runtime's intra-op pool; 0 picks from the core count.
    pub threads: usize,
    /// How long an unused session stays loaded. Zero means the default.
    pub decay: Duration,
}

/// One line for the settings row, and the numbers behind the progress row.
#[derive(Clone, Debug)]
pub struct Status {
    pub runtime: Runtime,
    /// The rung selected, or the floor while the probe is still running.
    pub rung: Rung,
    /// Why — "probe passed", or the failure that demoted the rung above.
    pub reason: String,
    pub probing: bool,
    /// Engines compiled and engines wanted, for a compiling rung; `(0, 0)`
    /// otherwise.
    pub engines: (usize, usize),
}

impl Status {
    /// "Hexagon NPU · int8 · ONNX Runtime 1.29" — the settings row's text.
    pub fn line(&self) -> String {
        let form = match self.rung {
            Rung::Hexagon => " · int8",
            Rung::TensorRt => " · fp16",
            _ => "",
        };
        format!("{}{} · {}", self.rung.label(), form, self.runtime.label())
    }
}

/// A model the caller can run, whatever is or is not loaded right now.
///
/// Holds the bytes, not a session. [`Model::acquire`] finds the loaded copy
/// in the registry — shared with every other holder of the same model —
/// or loads one, and every acquire refreshes the copy's last-used time.
/// The reaper unloads anything idle for [`Config::decay`]; a scan that runs
/// the detector on every image never lets it go idle, a click in the
/// develop view lets the segmenter go after a quiet spell, and a handle
/// used again after that simply loads again. Nobody states a policy.
///
/// The registry key includes the rung, so a reload after a compiled engine
/// has landed moves up to it by itself (§6 step 4).
pub struct Model {
    role: Role,
    form: Form,
    bytes: Arc<[u8]>,
}

/// A loaded session, held for one `run` and its output decoding.
pub struct Acquired {
    entry: Arc<Loaded>,
}

struct Loaded {
    rung: Rung,
    session: Mutex<Session>,
    last_used: Mutex<Instant>,
}

impl Model {
    /// The loaded session, loading it if the reaper took it. Lock it for
    /// one run; a scan and a develop click can want the same detector at
    /// once, and the second waits on the first.
    pub fn acquire(&self) -> Result<Acquired, Error> {
        acquire(self.role, self.form, &self.bytes)
    }

    pub fn form(&self) -> Form {
        self.form
    }
}

impl Acquired {
    pub fn lock(&self) -> MutexGuard<'_, Session> {
        self.entry.session.lock().unwrap_or_else(|e| e.into_inner())
    }

    /// Where this session runs.
    pub fn rung(&self) -> Rung {
        self.entry.rung
    }
}

impl Drop for Acquired {
    fn drop(&mut self) {
        // The clock starts when the use ends, not when it began: a long run
        // is not idle time.
        *self.entry.last_used.lock().unwrap() = Instant::now();
    }
}

type Registry = HashMap<String, Arc<Loaded>>;

static REGISTRY: OnceLock<Mutex<Registry>> = OnceLock::new();

fn registry() -> &'static Mutex<Registry> {
    REGISTRY.get_or_init(|| {
        std::thread::Builder::new()
            .name("inference-reaper".into())
            .spawn(|| loop {
                std::thread::sleep(Duration::from_secs(5));
                release_idle();
            })
            .expect("spawn inference reaper");
        Mutex::new(HashMap::new())
    })
}

fn acquire(role: Role, form: Form, bytes: &Arc<[u8]>) -> Result<Acquired, Error> {
    api::ensure_installed();
    let (rung, cfg) = {
        let s = state().lock().unwrap();
        let selected = current_rung(&s);
        (
            effective_rung(&s, selected, role, form, bytes),
            s.config.clone(),
        )
    };
    let key = format!("{role:?}:{}", engines::key(rung, bytes));

    if let Some(entry) = registry().lock().unwrap().get(&key).cloned() {
        *entry.last_used.lock().unwrap() = Instant::now();
        return Ok(Acquired { entry });
    }

    // Built outside the registry lock: a TensorRT engine load is long enough
    // that another role's acquire should not wait on it.
    let session = session::build(rung, role, bytes, &cfg, false)?;
    log::debug!("inference: {role:?} loaded on {}", rung.label());
    let entry = Arc::new(Loaded {
        rung,
        session: Mutex::new(session),
        last_used: Mutex::new(Instant::now()),
    });
    let mut reg = registry().lock().unwrap();
    // Two acquires raced; keep the first, drop this one.
    let entry = reg.entry(key).or_insert_with(|| entry.clone()).clone();
    Ok(Acquired { entry })
}

/// Unload every session idle for longer than the decay. The reaper does
/// this every five seconds. A session in use survives until its run ends:
/// the `Acquired` holds it, the registry merely forgets it.
pub fn release_idle() {
    let decay = match state().lock().unwrap().config.decay {
        Duration::ZERO => DEFAULT_DECAY,
        d => d,
    };
    let now = Instant::now();
    registry()
        .lock()
        .unwrap()
        .retain(|_, e| now.duration_since(*e.last_used.lock().unwrap()) < decay);
}

/// Unload every session now, decay or not — what a low-memory signal
/// asks for. Sessions mid-run finish first.
pub fn release_all() {
    registry().lock().unwrap().clear();
}

/// Unload every session of `role` now — "I am done segmenting".
pub fn unload(role: Role) {
    let prefix = format!("{role:?}:");
    registry()
        .lock()
        .unwrap()
        .retain(|k, _| !k.starts_with(&prefix));
}

/// How many sessions are loaded, for the settings row and the tests.
pub fn loaded() -> usize {
    registry().lock().unwrap().len()
}

#[derive(Debug, thiserror::Error)]
pub enum Error {
    #[error(transparent)]
    Inference(#[from] ort::Error),
    #[error("reading model: {0}")]
    Io(#[from] std::io::Error),
}

/// What the probe writes and the next launch reads (§4 step 3).
#[derive(Clone, Debug, Default, Serialize, Deserialize)]
struct Cache {
    /// Runtime, driver, hardware and model identity; any change re-probes.
    fingerprint: String,
    rung: Option<Rung>,
    reason: String,
    /// Model hashes whose engine exists on disk, per compiling rung.
    compiled: BTreeSet<String>,
    /// Rungs that failed under this fingerprint, and why. Not retried until
    /// the fingerprint changes: a wedged driver must not cost every launch
    /// thirty seconds.
    failed: Vec<(Rung, String)>,
}

struct State {
    config: Config,
    cache: Cache,
    probing: bool,
    wanted: usize,
}

static STATE: OnceLock<Mutex<State>> = OnceLock::new();

fn state() -> &'static Mutex<State> {
    STATE.get_or_init(|| {
        Mutex::new(State {
            config: Config::default(),
            cache: Cache::default(),
            probing: false,
            wanted: 0,
        })
    })
}

/// Choose the runtime and start the probe. Idempotent; the first call wins.
///
/// Returns at once: the probe and any engine compilation run on their own
/// low-priority thread, and every request meanwhile is served by the floor
/// (§4). Never blocks the first frame.
pub fn init(config: Config) {
    let runtime = api::install(&config.runtime_dirs);
    {
        let mut s = state().lock().unwrap();
        if s.probing || s.cache.rung.is_some() {
            return;
        }
        s.config = config;
        s.probing = true;
    }
    log::info!("inference: runtime {}", runtime.label());
    std::thread::Builder::new()
        .name("inference-probe".into())
        .spawn(move || {
            probe::run(runtime);
            engines::run();
        })
        .expect("spawn inference probe");
}

/// Make sure `ort` has an API table, for code that drives `ort` directly.
/// [`open`] does this itself; only the M1 probe example needs it by name.
pub fn ensure_runtime() {
    api::ensure_installed();
}

/// The line for the settings row.
pub fn status() -> Status {
    let s = state().lock().unwrap();
    let rung = current_rung(&s);
    Status {
        runtime: api::runtime(),
        rung,
        reason: s.cache.reason.clone(),
        probing: s.probing,
        engines: if rung.compiles() {
            (s.cache.compiled.len(), s.wanted)
        } else {
            (0, 0)
        },
    }
}

fn current_rung(s: &State) -> Rung {
    if s.probing {
        Rung::Cpu
    } else {
        s.cache.rung.unwrap_or(Rung::Cpu)
    }
}

/// The file to load for `role` under the current selection, and its form.
///
/// A rung that wants int8 gets the `.int8.onnx` sibling of the canonical file
/// if it exists; otherwise the canonical file, on the rung's fallback. A
/// caller adds [`form_suffix`] to the `model_id` it records.
pub fn resolve_model(role: Role, canonical: &Path) -> (PathBuf, Form) {
    let rung = current_rung(&state().lock().unwrap());
    if rung.form(role) == Form::Int8 {
        let sibling = int8_sibling(canonical);
        if sibling.is_file() {
            return (sibling, Form::Int8);
        }
    }
    (canonical.to_path_buf(), Form::F32)
}

fn int8_sibling(canonical: &Path) -> PathBuf {
    let stem = canonical
        .file_stem()
        .map(|s| s.to_string_lossy().into_owned())
        .unwrap_or_default();
    canonical.with_file_name(format!("{stem}.int8.onnx"))
}

/// What a form appends to a detector's `model_id` (§7).
pub fn form_suffix(form: Form) -> &'static str {
    match form {
        Form::F32 => "",
        Form::Int8 => "_i8",
    }
}

/// A handle on the model `bytes` in `role`.
///
/// Loads it once here, so a graph the runtime rejects fails at
/// construction and not on the first image; what happens to that session
/// afterwards is the registry's business (see [`Model`]).
///
/// Works without [`init`] — a test, or the examples — by installing tract
/// and using the CPU rung, which is exactly what every consumer did before
/// this crate existed.
pub fn open(role: Role, form: Form, bytes: &[u8]) -> Result<Model, Error> {
    let bytes: Arc<[u8]> = Arc::from(bytes);
    acquire(role, form, &bytes)?;
    Ok(Model { role, form, bytes })
}

/// Where a request lands: the selected rung unless the role's precision rule,
/// the form on offer, or a missing engine says one lower (§6 step 4).
fn effective_rung(s: &State, selected: Rung, role: Role, form: Form, bytes: &[u8]) -> Rung {
    let mut rung = selected;
    if rung.form(role) != form {
        // The embedder on a Hexagon device, or an f32 detector where the int8
        // sibling was missing: neither can go to the NPU.
        rung = rung.fallback();
    }
    if rung.compiles() && !s.cache.compiled.contains(&engines::key(rung, bytes)) {
        rung = rung.fallback();
    }
    rung
}

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

    /// The registry is one per process, so these run one at a time.
    static SERIAL: Mutex<()> = Mutex::new(());
    fn serial() -> MutexGuard<'static, ()> {
        SERIAL.lock().unwrap_or_else(|e| e.into_inner())
    }

    /// The smallest shipped graph, if this checkout has the weights; a test
    /// suite that needs a research-licensed download is one that does not
    /// run in CI (docs/faces.md §3), so absence is a skip.
    fn probe_bytes() -> Option<Vec<u8>> {
        let path = concat!(
            env!("CARGO_MANIFEST_DIR"),
            "/../../models/face/scrfd_500m_640.onnx"
        );
        let bytes = std::fs::read(path).ok()?;
        (bytes.len() > 100_000).then_some(bytes)
    }

    #[test]
    fn two_handles_on_one_model_share_one_session() {
        let _serial = serial();
        let Some(bytes) = probe_bytes() else { return };
        release_all();
        let a = open(Role::Detector, Form::F32, &bytes).unwrap();
        let b = open(Role::Detector, Form::F32, &bytes).unwrap();
        assert_eq!(loaded(), 1);
        let (x, y) = (a.acquire().unwrap(), b.acquire().unwrap());
        assert!(Arc::ptr_eq(&x.entry, &y.entry));
    }

    #[test]
    fn a_released_model_reloads_on_its_next_use() {
        let _serial = serial();
        let Some(bytes) = probe_bytes() else { return };
        release_all();
        let model = open(Role::Detector, Form::F32, &bytes).unwrap();
        assert_eq!(loaded(), 1);
        release_all();
        assert_eq!(loaded(), 0);
        let acquired = model.acquire().unwrap();
        assert_eq!(loaded(), 1);
        assert_eq!(acquired.lock().inputs().len(), 1);
    }

    #[test]
    fn an_idle_session_decays_and_a_used_one_does_not() {
        let _serial = serial();
        let Some(bytes) = probe_bytes() else { return };
        release_all();
        state().lock().unwrap().config.decay = Duration::from_millis(50);
        let model = open(Role::Detector, Form::F32, &bytes).unwrap();
        // Used within the decay: stays.
        std::thread::sleep(Duration::from_millis(30));
        drop(model.acquire().unwrap());
        release_idle();
        assert_eq!(loaded(), 1);
        // Idle past it: goes.
        std::thread::sleep(Duration::from_millis(80));
        release_idle();
        assert_eq!(loaded(), 0);
        state().lock().unwrap().config.decay = Duration::ZERO;
    }

    #[test]
    fn unload_by_role_leaves_the_other_roles() {
        let _serial = serial();
        let Some(bytes) = probe_bytes() else { return };
        release_all();
        let _d = open(Role::Detector, Form::F32, &bytes).unwrap();
        let _s = open(Role::Segmenter, Form::F32, &bytes).unwrap();
        assert_eq!(loaded(), 2);
        unload(Role::Segmenter);
        assert_eq!(loaded(), 1);
    }

    #[test]
    fn the_status_line_reads_as_the_floor_before_init() {
        let s = status();
        assert_eq!(s.rung, Rung::Cpu);
        assert!(s.line().starts_with("CPU"), "{}", s.line());
    }
}