// SPDX-FileCopyrightText: Copyright (c) 2024-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
// SPDX-License-Identifier: Apache-2.0

//! Tokio runtime metrics and event-loop canary

use once_cell::sync::{Lazy, OnceCell};
use prometheus::{Counter, Gauge, Histogram, HistogramOpts, IntCounterVec, IntGaugeVec, Opts};
use std::sync::atomic::{AtomicU64, Ordering};
use std::time::{Duration, Instant};
use tokio::runtime::Handle;
use tokio_util::sync::CancellationToken;

use super::prometheus_names::{frontend_perf, name_prefix, tokio_perf as names};
use crate::MetricsRegistry;

fn tokio_metric_name(suffix: &str) -> String {
    format!("{}_{}", name_prefix::TOKIO, suffix)
}

// --- Tokio runtime gauges/counters (updated every 1s by collector) ---

pub static TOKIO_GLOBAL_QUEUE_DEPTH: Lazy<Gauge> = Lazy::new(|| {
    Gauge::new(
        tokio_metric_name(names::GLOBAL_QUEUE_DEPTH),
        "Number of tasks in the runtime global queue",
    )
    .expect("tokio global_queue_depth gauge")
});

pub static TOKIO_BUDGET_FORCED_YIELD_TOTAL: Lazy<Counter> = Lazy::new(|| {
    Counter::new(
        tokio_metric_name(names::BUDGET_FORCED_YIELD_TOTAL),
        "Number of times tasks were forced to yield after exhausting budget",
    )
    .expect("tokio budget_forced_yield_total counter")
});

pub static TOKIO_BLOCKING_THREADS: Lazy<Gauge> = Lazy::new(|| {
    Gauge::new(
        tokio_metric_name(names::BLOCKING_THREADS),
        "Number of blocking threads",
    )
    .expect("tokio blocking_threads gauge")
});

pub static TOKIO_BLOCKING_IDLE_THREADS: Lazy<Gauge> = Lazy::new(|| {
    Gauge::new(
        tokio_metric_name(names::BLOCKING_IDLE_THREADS),
        "Number of idle blocking threads",
    )
    .expect("tokio blocking_idle_threads gauge")
});

pub static TOKIO_BLOCKING_QUEUE_DEPTH: Lazy<Gauge> = Lazy::new(|| {
    Gauge::new(
        tokio_metric_name(names::BLOCKING_QUEUE_DEPTH),
        "Number of tasks in the blocking thread pool queue",
    )
    .expect("tokio blocking_queue_depth gauge")
});

pub static TOKIO_ALIVE_TASKS: Lazy<Gauge> = Lazy::new(|| {
    Gauge::new(
        tokio_metric_name(names::ALIVE_TASKS),
        "Number of alive tasks in the runtime",
    )
    .expect("tokio alive_tasks gauge")
});

// Per-worker metrics (GaugeVec/IntCounterVec with label "worker")
pub static TOKIO_WORKER_MEAN_POLL_TIME_NS: Lazy<IntGaugeVec> = Lazy::new(|| {
    IntGaugeVec::new(
        Opts::new(
            tokio_metric_name(names::WORKER_MEAN_POLL_TIME_NS),
            "Worker mean task poll time (nanoseconds)",
        ),
        &["worker"],
    )
    .expect("tokio worker_mean_poll_time_ns gauge vec")
});

pub static TOKIO_WORKER_BUSY_RATIO_VEC: Lazy<IntGaugeVec> = Lazy::new(|| {
    IntGaugeVec::new(
        Opts::new(
            tokio_metric_name(names::WORKER_BUSY_RATIO),
            "Worker busy ratio (0-1) as integer mill ratio; >950 = saturated",
        ),
        &["worker"],
    )
    .expect("tokio worker_busy_ratio vec")
});

pub static TOKIO_WORKER_PARK_COUNT_TOTAL: Lazy<IntCounterVec> = Lazy::new(|| {
    IntCounterVec::new(
        Opts::new(
            tokio_metric_name(names::WORKER_PARK_COUNT_TOTAL),
            "Total number of times worker has parked",
        ),
        &["worker"],
    )
    .expect("tokio worker_park_count_total")
});

pub static TOKIO_WORKER_LOCAL_QUEUE_DEPTH: Lazy<IntGaugeVec> = Lazy::new(|| {
    IntGaugeVec::new(
        Opts::new(
            tokio_metric_name(names::WORKER_LOCAL_QUEUE_DEPTH),
            "Number of tasks in worker local queue",
        ),
        &["worker"],
    )
    .expect("tokio worker_local_queue_depth")
});

pub static TOKIO_WORKER_STEAL_COUNT_TOTAL: Lazy<IntCounterVec> = Lazy::new(|| {
    IntCounterVec::new(
        Opts::new(
            tokio_metric_name(names::WORKER_STEAL_COUNT_TOTAL),
            "Total number of tasks stolen by worker",
        ),
        &["worker"],
    )
    .expect("tokio worker_steal_count_total")
});

pub static TOKIO_WORKER_OVERFLOW_COUNT_TOTAL: Lazy<IntCounterVec> = Lazy::new(|| {
    IntCounterVec::new(
        Opts::new(
            tokio_metric_name(names::WORKER_OVERFLOW_COUNT_TOTAL),
            "Total number of times worker local queue overflowed",
        ),
        &["worker"],
    )
    .expect("tokio worker_overflow_count_total")
});

// --- Event loop canary ---
pub static EVENT_LOOP_DELAY_SECONDS: Lazy<Histogram> = Lazy::new(|| {
    Histogram::with_opts(
        HistogramOpts::new(
            format!(
                "{}_{}",
                name_prefix::FRONTEND,
                frontend_perf::EVENT_LOOP_DELAY_SECONDS
            ),
            "Event loop delay canary: drift from 10ms sleep (seconds)",
        )
        .buckets(vec![
            0.0, 0.001, 0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1.0,
        ]),
    )
    .expect("event_loop_delay_seconds histogram")
});

pub static EVENT_LOOP_STALL_TOTAL: Lazy<Counter> = Lazy::new(|| {
    Counter::new(
        format!(
            "{}_{}",
            name_prefix::FRONTEND,
            frontend_perf::EVENT_LOOP_STALL_TOTAL
        ),
        "Number of event loop stalls (delay > 5ms)",
    )
    .expect("event_loop_stall_total counter")
});

/// Guards idempotency for the `MetricsRegistry` registration path.
static REGISTERED: OnceCell<()> = OnceCell::new();

/// Guards idempotency for the raw `prometheus::Registry` registration path.
/// Kept separate from `REGISTERED` so that calling `ensure_tokio_perf_metrics_registered`
/// first does not silently prevent the metrics from being registered in the prometheus registry.
static PROMETHEUS_REGISTERED: OnceCell<()> = OnceCell::new();

/// Register tokio perf and canary metrics with the given registry. Idempotent.
pub fn ensure_tokio_perf_metrics_registered(registry: &MetricsRegistry) {
    let _ = REGISTERED.get_or_init(|| {
        registry
            .add_metric(Box::new(TOKIO_GLOBAL_QUEUE_DEPTH.clone()))
            .ok();
        registry
            .add_metric(Box::new(TOKIO_BUDGET_FORCED_YIELD_TOTAL.clone()))
            .ok();
        registry
            .add_metric(Box::new(TOKIO_BLOCKING_THREADS.clone()))
            .ok();
        registry
            .add_metric(Box::new(TOKIO_BLOCKING_IDLE_THREADS.clone()))
            .ok();
        registry
            .add_metric(Box::new(TOKIO_BLOCKING_QUEUE_DEPTH.clone()))
            .ok();
        registry
            .add_metric(Box::new(TOKIO_ALIVE_TASKS.clone()))
            .ok();
        registry
            .add_metric(Box::new(TOKIO_WORKER_MEAN_POLL_TIME_NS.clone()))
            .ok();
        registry
            .add_metric(Box::new(TOKIO_WORKER_BUSY_RATIO_VEC.clone()))
            .ok();
        registry
            .add_metric(Box::new(TOKIO_WORKER_PARK_COUNT_TOTAL.clone()))
            .ok();
        registry
            .add_metric(Box::new(TOKIO_WORKER_LOCAL_QUEUE_DEPTH.clone()))
            .ok();
        registry
            .add_metric(Box::new(TOKIO_WORKER_STEAL_COUNT_TOTAL.clone()))
            .ok();
        registry
            .add_metric(Box::new(TOKIO_WORKER_OVERFLOW_COUNT_TOTAL.clone()))
            .ok();
        registry
            .add_metric(Box::new(EVENT_LOOP_DELAY_SECONDS.clone()))
            .ok();
        registry
            .add_metric(Box::new(EVENT_LOOP_STALL_TOTAL.clone()))
            .ok();
    });
}

/// Register tokio perf and canary metrics with a raw Prometheus registry.
pub fn ensure_tokio_perf_metrics_registered_prometheus(
    registry: &prometheus::Registry,
) -> Result<(), prometheus::Error> {
    if PROMETHEUS_REGISTERED.get().is_some() {
        return Ok(());
    }
    registry.register(Box::new(TOKIO_GLOBAL_QUEUE_DEPTH.clone()))?;
    registry.register(Box::new(TOKIO_BUDGET_FORCED_YIELD_TOTAL.clone()))?;
    registry.register(Box::new(TOKIO_BLOCKING_THREADS.clone()))?;
    registry.register(Box::new(TOKIO_BLOCKING_IDLE_THREADS.clone()))?;
    registry.register(Box::new(TOKIO_BLOCKING_QUEUE_DEPTH.clone()))?;
    registry.register(Box::new(TOKIO_ALIVE_TASKS.clone()))?;
    registry.register(Box::new(TOKIO_WORKER_MEAN_POLL_TIME_NS.clone()))?;
    registry.register(Box::new(TOKIO_WORKER_BUSY_RATIO_VEC.clone()))?;
    registry.register(Box::new(TOKIO_WORKER_PARK_COUNT_TOTAL.clone()))?;
    registry.register(Box::new(TOKIO_WORKER_LOCAL_QUEUE_DEPTH.clone()))?;
    registry.register(Box::new(TOKIO_WORKER_STEAL_COUNT_TOTAL.clone()))?;
    registry.register(Box::new(TOKIO_WORKER_OVERFLOW_COUNT_TOTAL.clone()))?;
    registry.register(Box::new(EVENT_LOOP_DELAY_SECONDS.clone()))?;
    registry.register(Box::new(EVENT_LOOP_STALL_TOTAL.clone()))?;
    let _ = PROMETHEUS_REGISTERED.set(());
    Ok(())
}

/// Run the tokio metrics collector (1s interval) and event-loop canary.
/// Spawn this on the runtime you want to monitor (e.g. primary handle).
/// The loop exits cleanly when `cancel` is triggered.
pub async fn tokio_metrics_and_canary_loop(cancel: CancellationToken) {
    let canary_interval = Duration::from_millis(10);
    let stall_threshold = Duration::from_millis(5);
    let collect_interval = Duration::from_secs(1);
    let mut next_collect = Instant::now() + collect_interval;
    let mut prev_counters = PrevWorkerCounters::new();
    loop {
        let start = Instant::now();
        tokio::select! {
            _ = tokio::time::sleep(canary_interval) => {}
            _ = cancel.cancelled() => {
                tracing::debug!("tokio metrics and canary loop shutting down");
                return;
            }
        }
        let delay = start.elapsed().saturating_sub(canary_interval);
        EVENT_LOOP_DELAY_SECONDS.observe(delay.as_secs_f64());
        if delay > stall_threshold {
            EVENT_LOOP_STALL_TOTAL.inc();
        }
        if Instant::now() >= next_collect {
            next_collect = Instant::now() + collect_interval;
            sample_tokio_metrics(&mut prev_counters);
        }
    }
}

static PREV_BUDGET_FORCED_YIELD: AtomicU64 = AtomicU64::new(0);

/// Per-worker previous samples for the monotonic _TOTAL counters.
/// Owned by the single `tokio_metrics_and_canary_loop` task — no locks needed.
struct PrevWorkerCounters {
    park: Vec<u64>,
    steal: Vec<u64>,
    overflow: Vec<u64>,
}

impl PrevWorkerCounters {
    fn new() -> Self {
        Self {
            park: Vec::new(),
            steal: Vec::new(),
            overflow: Vec::new(),
        }
    }

    fn ensure_capacity(&mut self, num_workers: usize) {
        if self.park.len() < num_workers {
            self.park.resize(num_workers, 0);
            self.steal.resize(num_workers, 0);
            self.overflow.resize(num_workers, 0);
        }
    }
}

fn sample_tokio_metrics(prev: &mut PrevWorkerCounters) {
    let metrics = Handle::current().metrics();

    TOKIO_GLOBAL_QUEUE_DEPTH.set(metrics.global_queue_depth() as f64);
    let budget = metrics.budget_forced_yield_count();
    let prev_budget = PREV_BUDGET_FORCED_YIELD.swap(budget, Ordering::Relaxed);
    TOKIO_BUDGET_FORCED_YIELD_TOTAL.inc_by((budget.saturating_sub(prev_budget)) as f64);
    TOKIO_BLOCKING_THREADS.set(metrics.num_blocking_threads() as f64);
    TOKIO_BLOCKING_IDLE_THREADS.set(metrics.num_idle_blocking_threads() as f64);
    TOKIO_BLOCKING_QUEUE_DEPTH.set(metrics.blocking_queue_depth() as f64);
    TOKIO_ALIVE_TASKS.set(metrics.num_alive_tasks() as f64);

    let num_workers = metrics.num_workers();
    prev.ensure_capacity(num_workers);

    for w in 0..num_workers {
        let worker_label = w.to_string();
        let mean_poll = metrics.worker_mean_poll_time(w);

        TOKIO_WORKER_MEAN_POLL_TIME_NS
            .with_label_values(&[&worker_label])
            .set(mean_poll.as_nanos() as i64);

        TOKIO_WORKER_LOCAL_QUEUE_DEPTH
            .with_label_values(&[&worker_label])
            .set(metrics.worker_local_queue_depth(w) as i64);

        // Monotonically increasing totals: track deltas so we use inc_by on a Counter.
        let park = metrics.worker_park_count(w);
        TOKIO_WORKER_PARK_COUNT_TOTAL
            .with_label_values(&[&worker_label])
            .inc_by(park.saturating_sub(prev.park[w]));
        prev.park[w] = park;

        let steal = metrics.worker_steal_count(w);
        TOKIO_WORKER_STEAL_COUNT_TOTAL
            .with_label_values(&[&worker_label])
            .inc_by(steal.saturating_sub(prev.steal[w]));
        prev.steal[w] = steal;

        let overflow = metrics.worker_overflow_count(w);
        TOKIO_WORKER_OVERFLOW_COUNT_TOTAL
            .with_label_values(&[&worker_label])
            .inc_by(overflow.saturating_sub(prev.overflow[w]));
        prev.overflow[w] = overflow;

        // Busy ratio: total_busy_duration over 1s interval -> ratio. We don't have delta here;
        // use mean_poll_time as proxy: if high, worker is busy. Store as 0-1000 (per mille).
        let busy_proxy = (mean_poll.as_secs_f64() / 0.001).min(1.0); // 1ms = saturated
        TOKIO_WORKER_BUSY_RATIO_VEC
            .with_label_values(&[&worker_label])
            .set((busy_proxy * 1000.0) as i64);
    }
}
