//! Prometheus-compatible metrics for GitKS.
//!
//! Tracks:
//! - Request counts by gRPC method + status code
//! - Request duration histogram by method
//! - Active requests gauge
//! - Repository count
//! - Cache hits / misses
//! - Error counts by error type
//!
//! Exposes a `/metrics` HTTP endpoint on a configurable port (default 9100).

use dashmap::DashMap;
use std::sync::atomic::{AtomicU64, Ordering};
use std::sync::{Arc, OnceLock};
use std::time::{Duration, Instant};


struct MetricsInner {
    /// Counter: total requests by (method, status_code)
    /// Key: "method:status"
    request_count: DashMap<String, AtomicU64>,

    /// Histogram buckets for request duration (seconds).
    /// Each bucket: (method, le_bound_ms) → count
    duration_buckets: DashMap<String, AtomicU64>,

    /// Gauge: number of currently in-flight requests
    active_requests: AtomicU64,

    /// Gauge: total number of registered repositories
    repository_count: AtomicU64,

    /// Counter: cache hits
    cache_hits: AtomicU64,

    /// Counter: cache misses
    cache_misses: AtomicU64,

    /// Counter: errors by error kind
    error_count: DashMap<String, AtomicU64>,

    /// Start timestamp (seconds since Unix epoch)
    start_time: Instant,
}

static METRICS: OnceLock<Arc<MetricsInner>> = OnceLock::new();

fn metrics() -> &'static Arc<MetricsInner> {
    METRICS.get_or_init(|| {
        Arc::new(MetricsInner {
            request_count: DashMap::new(),
            duration_buckets: DashMap::new(),
            active_requests: AtomicU64::new(0),
            repository_count: AtomicU64::new(0),
            cache_hits: AtomicU64::new(0),
            cache_misses: AtomicU64::new(0),
            error_count: DashMap::new(),
            start_time: Instant::now(),
        })
    })
}


#[rustfmt::skip]
const DURATION_BUCKET_MS: &[u64] = &[
    5, 10, 25, 50, 100, 250, 500, 1_000,
    2_500, 5_000, 10_000, 30_000, 60_000, u64::MAX,
];

const BUCKET_INF: u64 = u64::MAX;

/// Record a request.
pub fn record_request(method: &str, status_code: &str, duration: Duration) {
    let m = metrics();
    let key = format!("{method}:{status_code}");
    m.request_count
        .entry(key)
        .or_insert_with(|| AtomicU64::new(0))
        .value()
        .fetch_add(1, Ordering::Relaxed);

    let duration_ms = duration.as_millis() as u64;
    for &bound_ms in DURATION_BUCKET_MS {
        if duration_ms <= bound_ms || bound_ms == BUCKET_INF {
            let bucket_key = format!("{method}:{bound_ms}");
            m.duration_buckets
                .entry(bucket_key)
                .or_insert_with(|| AtomicU64::new(0))
                .value()
                .fetch_add(1, Ordering::Relaxed);
        }
    }
}

/// Increment the active request gauge.
pub fn inc_active_requests() {
    metrics().active_requests.fetch_add(1, Ordering::Relaxed);
}

/// Decrement the active request gauge.
pub fn dec_active_requests() {
    metrics().active_requests.fetch_sub(1, Ordering::Relaxed);
}

/// Set the repository count.
pub fn set_repository_count(count: u64) {
    metrics()
        .repository_count
        .store(count, Ordering::Relaxed);
}

/// Record a cache hit.
pub fn inc_cache_hits(count: u64) {
    metrics().cache_hits.fetch_add(count, Ordering::Relaxed);
}

/// Record a cache miss.
pub fn inc_cache_misses(count: u64) {
    metrics()
        .cache_misses
        .fetch_add(count, Ordering::Relaxed);
}

/// Record an error by kind (e.g., "not_found", "internal", "invalid_argument").
pub fn inc_error(kind: &str) {
    metrics()
        .error_count
        .entry(kind.to_string())
        .or_insert_with(|| AtomicU64::new(0))
        .value()
        .fetch_add(1, Ordering::Relaxed);
}


/// Render all metrics in Prometheus text exposition format.
pub fn render_metrics() -> String {
    let m = metrics();
    let mut out = String::with_capacity(4096);

    // Header
    let uptime = m.start_time.elapsed().as_secs();
    out.push_str("# HELP gitks_uptime_seconds Time since gitks started\n");
    out.push_str("# TYPE gitks_uptime_seconds gauge\n");
    out.push_str(&format!("gitks_uptime_seconds {uptime}\n\n"));

    // Active requests
    let active = m.active_requests.load(Ordering::Relaxed);
    out.push_str("# HELP gitks_active_requests Currently in-flight requests\n");
    out.push_str("# TYPE gitks_active_requests gauge\n");
    out.push_str(&format!("gitks_active_requests {active}\n\n"));

    // Repository count
    let repos = m.repository_count.load(Ordering::Relaxed);
    out.push_str("# HELP gitks_repository_count Number of registered repositories\n");
    out.push_str("# TYPE gitks_repository_count gauge\n");
    out.push_str(&format!("gitks_repository_count {repos}\n\n"));

    // Request count
    out.push_str("# HELP gitks_requests_total Total gRPC requests by method and status\n");
    out.push_str("# TYPE gitks_requests_total counter\n");
    for entry in &m.request_count {
        let (method_and_status, count) = (entry.key(), entry.value());
        let count = count.load(Ordering::Relaxed);
        if let Some((method, status)) = method_and_status.rsplit_once(':') {
            out.push_str(
                &format!("gitks_requests_total{{method=\"{method}\",status=\"{status}\"}} {count}\n"),
            );
        }
    }
    out.push('\n');

    // Duration histogram
    out.push_str(
        "# HELP gitks_request_duration_milliseconds Request duration histogram in ms\n",
    );
    out.push_str("# TYPE gitks_request_duration_milliseconds histogram\n");
    for entry in &m.duration_buckets {
        let (method_and_bound, count) = (entry.key(), entry.value());
        let count = count.load(Ordering::Relaxed);
        if let Some((method, bound_str)) = method_and_bound.rsplit_once(':') {
            let bound = bound_str;
            let le = if bound_str.parse::<u64>() == Ok(BUCKET_INF) {
                "+Inf".to_string()
            } else {
                bound.to_string()
            };
            out.push_str(
                &format!("gitks_request_duration_milliseconds_bucket{{method=\"{method}\",le=\"{le}\"}} {count}\n"),
            );
        }
    }
    out.push('\n');

    // Cache
    let hits = m.cache_hits.load(Ordering::Relaxed);
    let misses = m.cache_misses.load(Ordering::Relaxed);
    out.push_str("# HELP gitks_cache_hits_total Cache hit count\n");
    out.push_str("# TYPE gitks_cache_hits_total counter\n");
    out.push_str(&format!("gitks_cache_hits_total {hits}\n\n"));
    out.push_str("# HELP gitks_cache_misses_total Cache miss count\n");
    out.push_str("# TYPE gitks_cache_misses_total counter\n");
    out.push_str(&format!("gitks_cache_misses_total {misses}\n\n"));

    // Errors
    out.push_str("# HELP gitks_errors_total Total errors by kind\n");
    out.push_str("# TYPE gitks_errors_total counter\n");
    for entry in &m.error_count {
        let (kind, count) = (entry.key(), entry.value());
        let count = count.load(Ordering::Relaxed);
        out.push_str(&format!("gitks_errors_total{{kind=\"{kind}\"}} {count}\n"));
    }
    out.push('\n');

    out
}


/// Start the metrics HTTP server on the given port.
/// Runs in a background task; returns the JoinHandle.
pub fn start_metrics_server(port: u16) -> tokio::task::JoinHandle<()> {
    tokio::spawn(async move {
        let listener = match tokio::net::TcpListener::bind(format!("0.0.0.0:{port}")).await {
            Ok(l) => l,
            Err(e) => {
                tracing::error!(port, error = %e, "failed to bind metrics server");
                return;
            }
        };
        tracing::info!(port, "metrics HTTP server started");

        loop {
            match listener.accept().await {
                Ok((socket, peer)) => {
                    tracing::debug!(%peer, "metrics request");
                    tokio::spawn(handle_metrics_connection(socket));
                }
                Err(e) => {
                    tracing::error!(error = %e, "metrics accept error");
                }
            }
        }
    })
}

async fn handle_metrics_connection(mut socket: tokio::net::TcpStream) {
    use tokio::io::{AsyncReadExt, AsyncWriteExt};

    let mut buf = [0u8; 4096];
    let _ = tokio::time::timeout(Duration::from_secs(5), socket.read(&mut buf)).await;

    let body = render_metrics();
    let response = format!(
        "HTTP/1.1 200 OK\r\nContent-Type: text/plain; version=0.0.4\r\nContent-Length: {}\r\nConnection: close\r\n\r\n{}",
        body.len(),
        body
    );

    let _ = tokio::time::timeout(Duration::from_secs(5), socket.write_all(response.as_bytes()))
        .await;
    let _ = socket.shutdown().await;
}


/// A guard that records metrics on drop.
///
/// Usage in handlers:
/// ```ignore
/// let m = crate::metrics::RequestMetrics::new("Service/Method");
/// // ... handle request ...
/// m.record("ok"); // on success
/// // m.record("internal"); // or on error, with tonic error kind
/// ```
pub struct RequestMetrics {
    method: &'static str,
    start: Instant,
}

impl RequestMetrics {
    pub fn new(method: &'static str) -> Self {
        inc_active_requests();
        Self {
            method,
            start: Instant::now(),
        }
    }

    /// Record the outcome. Idempotent — safe to call before each return.
    pub fn record(&self, status: &str) {
        let duration = self.start.elapsed();
        record_request(self.method, status, duration);
    }
}

impl Drop for RequestMetrics {
    fn drop(&mut self) {
        dec_active_requests();
    }
}

/// Convenience: record an error from a tonic Status.
pub fn record_rpc_error(m: &RequestMetrics, status: &tonic::Status) {
    let kind = status.code().description();
    inc_error(kind);
    m.record(kind);
}