//! HTTP surface metrics: request counts by route/method/status, and per-route
//! latency.
//!
//! WHY THIS EXISTS: the engine publishes a rich DOMAIN surface (search latency,
//! WAL fsync, quorum timeouts, replication lag) but nothing about HTTP. There
//! was no way to answer "how many requests did we serve" or "what is our error
//! rate" from metrics at all — a production cluster could serve 401s or 503s
//! indefinitely with every existing gauge looking healthy.
//!
//! CARDINALITY IS THE WHOLE DESIGN CONSTRAINT. A naive `path` label explodes:
//! `/items/12345` is a distinct series per entity. Two defences:
//!
//! 1. The route label is axum's [`MatchedPath`] — the route TEMPLATE
//!    (`/cluster/shards/{id}/transfer`), not the concrete path. Bounded by the
//!    router's route count, whatever the traffic.
//! 2. A hard [`MAX_SERIES`] cap. Anything past it folds into an explicit
//!    overflow bucket, so a future router change (or an unmatched-path flood)
//!    degrades the labels rather than the process.
//!
//! Unmatched requests carry no `MatchedPath`, so they are attributed to a single
//! constant [`UNMATCHED_ROUTE`] rather than their (attacker-controlled) path.

use std::collections::HashMap;
use std::sync::Mutex;
use std::time::Instant;

use axum::extract::MatchedPath;
use axum::extract::Request;
use axum::middleware::Next;
use axum::response::Response;
use tidaldb::{LatencyHistogram, QUERY_LATENCY_BOUNDS};

/// Route label for a request that matched no route (404s, and anything rejected
/// before routing). A single constant, never the caller-supplied path — that
/// path is attacker-controlled and would be an unbounded label.
const UNMATCHED_ROUTE: &str = "<unmatched>";

/// Route label used once [`MAX_SERIES`] distinct keys exist.
const OVERFLOW_ROUTE: &str = "<over-cap>";

/// Hard ceiling on distinct (route, method, status) keys.
///
/// The template-based route label already bounds this to roughly
/// routes × methods × observed-statuses (~250 for the cluster router). The cap
/// exists so a future refactor that accidentally admits a dynamic route label
/// cannot grow this map without bound; it degrades to [`OVERFLOW_ROUTE`].
const MAX_SERIES: usize = 512;

/// One counted series: a route template, method, and HTTP status.
#[derive(PartialEq, Eq, Hash, Clone)]
struct Key {
    route: Box<str>,
    method: &'static str,
    status: u16,
}

/// HTTP request counters and per-route latency for one server process.
///
/// Registered with the engine's metrics listener via
/// `MetricsState::set_extra_renderer`, so these series appear on the SAME
/// scrape target as the engine's own — one target per node, not two.
pub struct HttpMetrics {
    /// Counts keyed by (route, method, status).
    ///
    /// `std::sync::Mutex<HashMap>` rather than a lock-free structure or
    /// `parking_lot`, deliberately. The critical section is one hash and one
    /// `u64` increment (tens of nanoseconds) against requests that already do
    /// WAL fsyncs and vector searches (hundreds of microseconds and up), and the
    /// in-flight cap at `router::MAX_CONCURRENCY` bounds the contention ceiling.
    /// `std::sync` is also what the rest of this workspace uses — introducing
    /// `parking_lot` for one module would plant a second convention. Poisoning
    /// is handled, never unwrapped: losing a metric sample must not fail a
    /// request that already succeeded.
    counts: Mutex<HashMap<Key, u64>>,
    /// Per-route end-to-end latency, in microseconds.
    latency: Mutex<HashMap<Box<str>, LatencyHistogram>>,
}

/// The process-wide HTTP metrics.
///
/// A process serves one HTTP surface through one `/metrics` listener, so a
/// single instance is the honest model — and it keeps the recording layer from
/// having to be threaded through all three router builders (standalone,
/// single-process cluster, multi-process region) and their call sites.
static GLOBAL: std::sync::LazyLock<std::sync::Arc<HttpMetrics>> =
    std::sync::LazyLock::new(|| std::sync::Arc::new(HttpMetrics::new()));

/// Handle to the process-wide HTTP metrics.
#[must_use]
pub fn global() -> std::sync::Arc<HttpMetrics> {
    std::sync::Arc::clone(&GLOBAL)
}

/// Publish this process's HTTP series on the engine's existing `/metrics`
/// listener.
///
/// Call once after opening the database. Returns `false` if a renderer was
/// already registered (the engine ignores the second call rather than
/// double-rendering).
pub fn publish_to(metrics: &tidaldb::MetricsState) -> bool {
    metrics.set_extra_renderer(Box::new(|out| global().render_into(out)))
}

impl std::fmt::Debug for HttpMetrics {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        let series = self.counts.lock().map_or(0, |m| m.len());
        f.debug_struct("HttpMetrics")
            .field("series", &series)
            .finish_non_exhaustive()
    }
}

impl Default for HttpMetrics {
    fn default() -> Self {
        Self::new()
    }
}

impl HttpMetrics {
    #[must_use]
    pub fn new() -> Self {
        Self {
            counts: Mutex::new(HashMap::new()),
            latency: Mutex::new(HashMap::new()),
        }
    }

    /// Record one completed request.
    ///
    /// `route` should be the matched TEMPLATE. Poisoned locks are ignored rather
    /// than propagated: losing a metric sample must never fail a request that
    /// already succeeded.
    pub fn record(&self, route: &str, method: &'static str, status: u16, elapsed_us: u64) {
        if let Ok(mut counts) = self.counts.lock() {
            let key = Key {
                route: route.into(),
                method,
                status,
            };
            // Only admit a NEW key while under the cap; existing keys always
            // increment, so an established series never stops counting because
            // the map happens to be full.
            if let Some(hit) = counts.get_mut(&key) {
                *hit += 1;
            } else if counts.len() < MAX_SERIES {
                counts.insert(key, 1);
            } else {
                *counts
                    .entry(Key {
                        route: OVERFLOW_ROUTE.into(),
                        method,
                        status,
                    })
                    .or_insert(0) += 1;
            }
        }

        if let Ok(mut latency) = self.latency.lock() {
            if let Some(hist) = latency.get(route) {
                hist.observe(elapsed_us);
            } else if latency.len() < MAX_SERIES {
                let hist = LatencyHistogram::new(QUERY_LATENCY_BOUNDS);
                hist.observe(elapsed_us);
                latency.insert(route.into(), hist);
            }
        }
    }

    /// Append this process's HTTP series in Prometheus text format.
    ///
    /// Label values are escaped: a route template is repo-controlled, but
    /// emitting an unescaped label is the kind of thing that silently corrupts a
    /// whole scrape, so it is not left to chance.
    pub fn render_into(&self, out: &mut String) {
        use std::fmt::Write;

        if let Ok(counts) = self.counts.lock() {
            let _ = write!(
                out,
                "\n# HELP tidaldb_http_requests_total Total HTTP requests served, by matched route \
                 template, method, and response status.\n\
                 # TYPE tidaldb_http_requests_total counter\n"
            );
            // Sorted so a diff of two scrapes is readable and the output is
            // deterministic for tests.
            let mut rows: Vec<_> = counts.iter().collect();
            rows.sort_unstable_by(|(a, _), (b, _)| {
                (&a.route, a.method, a.status).cmp(&(&b.route, b.method, b.status))
            });
            for (key, count) in rows {
                let _ = writeln!(
                    out,
                    "tidaldb_http_requests_total{{route=\"{}\",method=\"{}\",status=\"{}\"}} {count}",
                    escape_label(&key.route),
                    key.method,
                    key.status
                );
            }
        }

        if let Ok(latency) = self.latency.lock() {
            let mut routes: Vec<_> = latency.iter().collect();
            routes.sort_unstable_by(|(a, _), (b, _)| a.cmp(b));
            for (route, hist) in routes {
                out.push_str(&hist.render_prometheus_labeled(
                    "tidaldb_http_request_duration_us",
                    "HTTP request end-to-end latency in microseconds, by matched route template.",
                    &format!("route=\"{}\"", escape_label(route)),
                ));
            }
        }
    }
}

/// Escape a Prometheus label value per the exposition format: backslash, double
/// quote, and newline.
fn escape_label(raw: &str) -> String {
    if !raw.contains(['\\', '"', '\n']) {
        return raw.to_string();
    }
    let mut out = String::with_capacity(raw.len() + 8);
    for c in raw.chars() {
        match c {
            '\\' => out.push_str("\\\\"),
            '"' => out.push_str("\\\""),
            '\n' => out.push_str("\\n"),
            other => out.push(other),
        }
    }
    out
}

/// Map a method to a `'static` label, so the common ones cost no allocation and
/// an exotic one cannot become an unbounded label value.
fn method_label(method: &axum::http::Method) -> &'static str {
    match *method {
        axum::http::Method::GET => "GET",
        axum::http::Method::POST => "POST",
        axum::http::Method::PUT => "PUT",
        axum::http::Method::DELETE => "DELETE",
        axum::http::Method::PATCH => "PATCH",
        axum::http::Method::HEAD => "HEAD",
        axum::http::Method::OPTIONS => "OPTIONS",
        _ => "OTHER",
    }
}

/// Middleware recording every request into `metrics`.
///
/// Applied as the OUTERMOST layer on each router so it observes the response
/// actually returned to the client — including the 401/403 from the auth gates,
/// the 408 from the timeout layer, and the 429 from the rate limiter. A layer
/// applied further in would miss exactly the failures worth counting.
pub async fn track(metrics: std::sync::Arc<HttpMetrics>, req: Request, next: Next) -> Response {
    // The template, not the concrete path. Absent for unmatched requests.
    let route = req
        .extensions()
        .get::<MatchedPath>()
        .map_or(UNMATCHED_ROUTE, |m| m.as_str())
        .to_string();
    let method = method_label(req.method());
    let started = Instant::now();
    let response = next.run(req).await;
    let elapsed_us = u64::try_from(started.elapsed().as_micros()).unwrap_or(u64::MAX);
    metrics.record(&route, method, response.status().as_u16(), elapsed_us);
    response
}

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

    #[test]
    fn counts_are_keyed_by_route_method_and_status() {
        let m = HttpMetrics::new();
        m.record("/items", "POST", 201, 10);
        m.record("/items", "POST", 201, 20);
        m.record("/items", "POST", 401, 5);
        m.record("/search", "GET", 200, 30);

        let mut out = String::new();
        m.render_into(&mut out);
        assert!(out.contains(
            "tidaldb_http_requests_total{route=\"/items\",method=\"POST\",status=\"201\"} 2"
        ));
        assert!(out.contains(
            "tidaldb_http_requests_total{route=\"/items\",method=\"POST\",status=\"401\"} 1"
        ));
        assert!(out.contains(
            "tidaldb_http_requests_total{route=\"/search\",method=\"GET\",status=\"200\"} 1"
        ));
    }

    /// The error surface is the reason this module exists, so prove a 5xx is
    /// distinguishable from a success on the same route.
    #[test]
    fn errors_are_countable_separately_from_successes() {
        let m = HttpMetrics::new();
        for _ in 0..7 {
            m.record("/feed", "GET", 200, 100);
        }
        m.record("/feed", "GET", 503, 100);

        let mut out = String::new();
        m.render_into(&mut out);
        assert!(out.contains("route=\"/feed\",method=\"GET\",status=\"200\"} 7"));
        assert!(out.contains("route=\"/feed\",method=\"GET\",status=\"503\"} 1"));
    }

    #[test]
    fn latency_histogram_is_rendered_per_route() {
        let m = HttpMetrics::new();
        m.record("/search", "GET", 200, 1_500);
        let mut out = String::new();
        m.render_into(&mut out);
        assert!(out.contains("tidaldb_http_request_duration_us"));
        assert!(out.contains("route=\"/search\""));
        // A histogram must carry bucket + count series, not just a gauge.
        assert!(out.contains("_bucket"), "expected le buckets: {out}");
        assert!(out.contains("_count"));
    }

    /// Cardinality is the failure mode this design exists to prevent: past the
    /// cap, NEW keys must fold into one overflow bucket instead of growing the
    /// map, while already-tracked series keep counting.
    #[test]
    fn series_are_capped_and_existing_keys_keep_counting() {
        let m = HttpMetrics::new();
        for i in 0..MAX_SERIES {
            m.record(&format!("/r{i}"), "GET", 200, 1);
        }
        // Established key still increments at the cap.
        m.record("/r0", "GET", 200, 1);
        // A brand new key does NOT add a series.
        m.record("/brand-new", "GET", 200, 1);

        let counts = m.counts.lock().unwrap();
        assert!(
            counts.len() <= MAX_SERIES + 1,
            "map grew past the cap (+1 overflow bucket): {}",
            counts.len()
        );
        assert_eq!(
            *counts
                .get(&Key {
                    route: "/r0".into(),
                    method: "GET",
                    status: 200
                })
                .unwrap(),
            2
        );
        assert!(
            counts.keys().any(|k| &*k.route == OVERFLOW_ROUTE),
            "over-cap requests must be attributed to the overflow bucket"
        );
        assert!(
            !counts.keys().any(|k| &*k.route == "/brand-new"),
            "a new route past the cap must NOT create its own series"
        );
    }

    #[test]
    fn label_values_are_escaped() {
        assert_eq!(escape_label("/items"), "/items");
        assert_eq!(escape_label("a\"b"), "a\\\"b");
        assert_eq!(escape_label("a\\b"), "a\\\\b");
        assert_eq!(escape_label("a\nb"), "a\\nb");
    }

    #[test]
    fn exotic_methods_collapse_to_a_bounded_label() {
        assert_eq!(method_label(&axum::http::Method::GET), "GET");
        assert_eq!(method_label(&axum::http::Method::TRACE), "OTHER");
    }
}