//! Records metrics in the Prometheus exposition format.
#![deny(missing_docs)]
#![cfg_attr(docsrs, feature(doc_cfg), deny(broken_intra_doc_links))]
use std::future::Future;

#[cfg(feature = "tokio-exporter")]
use hyper::{
    service::{make_service_fn, service_fn},
    {Body, Error as HyperError, Response, Server},
};
use metrics::{Key, Recorder, SetRecorderError, Unit};
use metrics_util::{
    parse_quantiles, CompositeKey, Handle, Histogram, MetricKind, Quantile, Registry,
};
use parking_lot::RwLock;
use std::collections::HashMap;
use std::io;
use std::iter::FromIterator;
use std::net::{IpAddr, Ipv4Addr, SocketAddr};
use std::sync::Arc;
#[cfg(feature = "tokio-exporter")]
use std::thread;
use std::time::SystemTime;
use thiserror::Error as ThisError;
#[cfg(feature = "tokio-exporter")]
use tokio::{pin, runtime, select};

type PrometheusRegistry = Registry<CompositeKey, Handle>;
type HdrHistogram = hdrhistogram::Histogram<u64>;

/// Errors that could occur while installing a Prometheus recorder/exporter.
#[derive(ThisError, Debug)]
pub enum Error {
    /// Creating the networking event loop did not succeed.
    #[error("failed to spawn Tokio runtime for endpoint: {0}")]
    Io(#[from] io::Error),

    /// Binding/listening to the given address did not succeed.
    #[cfg(feature = "tokio-exporter")]
    #[error("failed to bind to given listen address: {0}")]
    Hyper(#[from] HyperError),

    /// Installing the recorder did not succeed.
    #[error("failed to install exporter as global recorder: {0}")]
    Recorder(#[from] SetRecorderError),
}

#[derive(Clone)]
enum Distribution {
    /// A Prometheus histogram.
    ///
    /// Exposes "bucketed" values to Prometheus, counting the number of samples
    /// below a given threshold i.e. 100 requests faster than 20ms, 1000 requests
    /// faster than 50ms, etc.
    Histogram(Histogram),
    /// A Prometheus summary.
    ///
    /// Computes and exposes value quantiles directly to Prometheus i.e. 50% of
    /// requests were faster than 200ms, and 99% of requests were faster than
    /// 1000ms, etc.
    Summary(HdrHistogram, u64),
}

struct Snapshot {
    pub counters: HashMap<String, HashMap<Vec<String>, u64>>,
    pub gauges: HashMap<String, HashMap<Vec<String>, f64>>,
    pub distributions: HashMap<String, HashMap<Vec<String>, Distribution>>,
}

struct Inner {
    registry: PrometheusRegistry,
    distributions: RwLock<HashMap<String, HashMap<Vec<String>, Distribution>>>,
    quantiles: Vec<Quantile>,
    buckets: Vec<u64>,
    buckets_by_name: Option<HashMap<String, Vec<u64>>>,
    descriptions: RwLock<HashMap<String, &'static str>>,
}

impl Inner {
    pub fn registry(&self) -> &PrometheusRegistry {
        &self.registry
    }

    fn get_recent_metrics(&self) -> Snapshot {
        let metrics = self.registry.get_handles();

        let mut counters = HashMap::new();
        let mut gauges = HashMap::new();

        let mut sorted_overrides = self
            .buckets_by_name
            .as_ref()
            .map(|h| Vec::from_iter(h.iter()))
            .unwrap_or_else(|| vec![]);
        sorted_overrides.sort_by(|(a, _), (b, _)| b.len().cmp(&a.len()));

        for (key, handle) in metrics.into_iter() {
            let (kind, key) = key.into_parts();
            let (name, labels) = key_to_parts(key);

            match kind {
                MetricKind::Counter => {
                    let entry = counters
                        .entry(name)
                        .or_insert_with(|| HashMap::new())
                        .entry(labels)
                        .or_insert(0);

                    *entry = handle.read_counter();
                }
                MetricKind::Gauge => {
                    let entry = gauges
                        .entry(name)
                        .or_insert_with(|| HashMap::new())
                        .entry(labels)
                        .or_insert(0.0);

                    *entry = handle.read_gauge();
                }
                MetricKind::Histogram => {
                    let buckets = sorted_overrides
                        .iter()
                        .find(|(k, _)| name.ends_with(*k))
                        .map(|(_, buckets)| *buckets)
                        .unwrap_or(&self.buckets);

                    let mut wg = self.distributions.write();
                    let entry = wg
                        .entry(name.clone())
                        .or_insert_with(|| HashMap::new())
                        .entry(labels)
                        .or_insert_with(|| match buckets.is_empty() {
                            false => {
                                let histogram = Histogram::new(buckets)
                                    .expect("failed to create histogram with buckets defined");
                                Distribution::Histogram(histogram)
                            }
                            true => {
                                let summary =
                                    HdrHistogram::new(3).expect("failed to create histogram");
                                Distribution::Summary(summary, 0)
                            }
                        });

                    match entry {
                        Distribution::Histogram(histogram) => handle
                            .read_histogram_with_clear(|samples| histogram.record_many(samples)),
                        Distribution::Summary(summary, sum) => {
                            handle.read_histogram_with_clear(|samples| {
                                for sample in samples {
                                    let _ = summary.record(*sample);
                                    *sum += *sample;
                                }
                            })
                        }
                    }
                }
            }
        }

        let distributions = self.distributions.read().clone();

        Snapshot {
            counters,
            gauges,
            distributions,
        }
    }

    pub fn render(&self) -> String {
        let mut sorted_overrides = self
            .buckets_by_name
            .as_ref()
            .map(|h| Vec::from_iter(h.iter()))
            .unwrap_or_else(|| vec![]);
        sorted_overrides.sort_by(|(a, _), (b, _)| b.len().cmp(&a.len()));

        let Snapshot {
            mut counters,
            mut gauges,
            mut distributions,
        } = self.get_recent_metrics();

        let ts = SystemTime::now()
            .duration_since(SystemTime::UNIX_EPOCH)
            .map(|d| d.as_secs())
            .unwrap_or(0);

        let mut output = format!(
            "# metrics snapshot (ts={}) (prometheus exposition format)\n",
            ts
        );

        let descriptions = self.descriptions.read();

        for (name, mut by_labels) in counters.drain() {
            if let Some(desc) = descriptions.get(name.as_str()) {
                output.push_str("# HELP ");
                output.push_str(name.as_str());
                output.push_str(" ");
                output.push_str(desc);
                output.push_str("\n");
            }

            output.push_str("# TYPE ");
            output.push_str(name.as_str());
            output.push_str(" counter\n");
            for (labels, value) in by_labels.drain() {
                let full_name = render_labeled_name(&name, &labels);
                output.push_str(full_name.as_str());
                output.push_str(" ");
                output.push_str(value.to_string().as_str());
                output.push_str("\n");
            }
        }

        for (name, mut by_labels) in gauges.drain() {
            if let Some(desc) = descriptions.get(name.as_str()) {
                output.push_str("# HELP ");
                output.push_str(name.as_str());
                output.push_str(" ");
                output.push_str(desc);
                output.push_str("\n");
            }

            output.push_str("# TYPE ");
            output.push_str(name.as_str());
            output.push_str(" gauge\n");
            for (labels, value) in by_labels.drain() {
                let full_name = render_labeled_name(&name, &labels);
                output.push_str(full_name.as_str());
                output.push_str(" ");
                output.push_str(value.to_string().as_str());
                output.push_str("\n");
            }
        }

        let mut sorted_overrides = self
            .buckets_by_name
            .as_ref()
            .map(|h| Vec::from_iter(h.iter()))
            .unwrap_or_else(|| vec![]);
        sorted_overrides.sort_by(|(a, _), (b, _)| b.len().cmp(&a.len()));

        for (name, mut by_labels) in distributions.drain() {
            if let Some(desc) = descriptions.get(name.as_str()) {
                output.push_str("# HELP ");
                output.push_str(name.as_str());
                output.push_str(" ");
                output.push_str(desc);
                output.push_str("\n");
            }

            let has_buckets = sorted_overrides
                .iter()
                .any(|(k, _)| !self.buckets.is_empty() || name.ends_with(*k));

            output.push_str("# TYPE ");
            output.push_str(name.as_str());
            output.push_str(" ");
            output.push_str(if has_buckets { "histogram" } else { "summary" });
            output.push_str("\n");

            for (labels, distribution) in by_labels.drain() {
                let (sum, count) = match distribution {
                    Distribution::Summary(summary, sum) => {
                        for quantile in &self.quantiles {
                            let value = summary.value_at_quantile(quantile.value());
                            let mut labels = labels.clone();
                            labels.push(format!("quantile=\"{}\"", quantile.value()));
                            let full_name = render_labeled_name(&name, &labels);
                            output.push_str(full_name.as_str());
                            output.push_str(" ");
                            output.push_str(value.to_string().as_str());
                            output.push_str("\n");
                        }

                        (sum, summary.len())
                    }
                    Distribution::Histogram(histogram) => {
                        for (le, count) in histogram.buckets() {
                            let mut labels = labels.clone();
                            labels.push(format!("le=\"{}\"", le));
                            let bucket_name = format!("{}_bucket", name);
                            let full_name = render_labeled_name(&bucket_name, &labels);
                            output.push_str(full_name.as_str());
                            output.push_str(" ");
                            output.push_str(count.to_string().as_str());
                            output.push_str("\n");
                        }

                        let mut labels = labels.clone();
                        labels.push("le=\"+Inf\"".to_owned());
                        let bucket_name = format!("{}_bucket", name);
                        let full_name = render_labeled_name(&bucket_name, &labels);
                        output.push_str(full_name.as_str());
                        output.push_str(" ");
                        output.push_str(histogram.count().to_string().as_str());
                        output.push_str("\n");

                        (histogram.sum(), histogram.count())
                    }
                };

                let sum_name = format!("{}_sum", name);
                let full_sum_name = render_labeled_name(&sum_name, &labels);
                output.push_str(full_sum_name.as_str());
                output.push_str(" ");
                output.push_str(sum.to_string().as_str());
                output.push_str("\n");
                let count_name = format!("{}_count", name);
                let full_count_name = render_labeled_name(&count_name, &labels);
                output.push_str(full_count_name.as_str());
                output.push_str(" ");
                output.push_str(count.to_string().as_str());
                output.push_str("\n");
            }
        }

        output
    }
}

/// A Prometheus recorder.
///
/// This recorder should be composed with other recorders or installed globally via
/// [`metrics::set_boxed_recorder`].
///
///
pub struct PrometheusRecorder {
    inner: Arc<Inner>,
}

impl PrometheusRecorder {
    /// Gets a [`PrometheusHandle`] to this recorder.
    pub fn handle(&self) -> PrometheusHandle {
        PrometheusHandle {
            inner: self.inner.clone(),
        }
    }

    fn add_description_if_missing(&self, key: &Key, description: Option<&'static str>) {
        if let Some(description) = description {
            let mut descriptions = self.inner.descriptions.write();
            if !descriptions.contains_key(key.name().to_string().as_str()) {
                descriptions.insert(key.name().to_string(), description);
            }
        }
    }
}

/// Handle to [`PrometheusRecorder`].
///
/// Useful for exposing a scrape endpoint on an existing HTTP/HTTPS server.
pub struct PrometheusHandle {
    inner: Arc<Inner>,
}

impl PrometheusHandle {
    /// Returns the metrics in Prometheus accepted String format.
    pub fn render(&self) -> String {
        self.inner.render()
    }
}

/// Builder for creating and installing a Prometheus recorder/exporter.
pub struct PrometheusBuilder {
    listen_address: SocketAddr,
    quantiles: Vec<Quantile>,
    buckets: Vec<u64>,
    buckets_by_name: Option<HashMap<String, Vec<u64>>>,
}

impl PrometheusBuilder {
    /// Creates a new [`PrometheusBuilder`].
    pub fn new() -> Self {
        let quantiles = parse_quantiles(&[0.0, 0.5, 0.9, 0.95, 0.99, 0.999, 1.0]);

        Self {
            listen_address: SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)), 9000),
            quantiles,
            buckets: vec![],
            buckets_by_name: None,
        }
    }

    /// Sets the listen address for the Prometheus scrape endpoint.
    ///
    /// The HTTP listener that is spawned will respond to GET requests on any request path.
    ///
    /// Defaults to `127.0.0.1:9000`.
    pub fn listen_address(mut self, addr: impl Into<SocketAddr>) -> Self {
        self.listen_address = addr.into();
        self
    }

    /// Sets the quantiles to use when rendering histograms.
    ///
    /// Quantiles represent a scale of 0 to 1, where percentiles represent a scale of 1 to 100, so
    /// a quantile of 0.99 is the 99th percentile, and a quantile of 0.99 is the 99.9th percentile.
    ///
    /// By default, the quantiles will be set to: 0.0, 0.5, 0.9, 0.95, 0.99, 0.999, and 1.0. This means
    /// that all histograms will be exposed as Prometheus summaries.
    ///
    /// If buckets are set (via [`set_buckets`][Self::set_buckets] or
    /// [`set_buckets_for_metric`][Self::set_buckets_for_metric]) then all histograms will be exposed
    /// as summaries instead.
    pub fn set_quantiles(mut self, quantiles: &[f64]) -> Self {
        self.quantiles = parse_quantiles(quantiles);
        self
    }

    /// Sets the buckets to use when rendering histograms.
    ///
    /// Buckets values represent the higher bound of each buckets.  If buckets are set, then all
    /// histograms will be rendered as true Prometheus histograms, instead of summaries.
    pub fn set_buckets(mut self, values: &[u64]) -> Self {
        self.buckets = values.to_vec();
        self
    }

    /// Sets the buckets for a specific metric, overidding the default.
    ///
    /// The match is suffix-based, and the longest match found will be used.
    ///
    /// Buckets values represent the higher bound of each buckets.  If buckets are set, then any
    /// histograms that match will be rendered as true Prometheus histograms, instead of summaries.
    ///
    /// This option changes the observer's output of histogram-type metric into summaries.
    /// It only affects matching metrics if set_buckets was not used.
    pub fn set_buckets_for_metric(mut self, name: &str, values: &[u64]) -> Self {
        let buckets = self.buckets_by_name.get_or_insert_with(|| HashMap::new());
        buckets.insert(name.to_owned(), values.to_vec());
        self
    }

    /// Builds the recorder and exporter and installs them globally.
    ///
    /// An error will be returned if there's an issue with creating the HTTP server or with
    /// installing the recorder as the global recorder.
    #[cfg(feature = "tokio-exporter")]
    pub fn install(self) -> Result<(), Error> {
        let runtime = runtime::Builder::new_current_thread()
            .enable_all()
            .build()?;

        let (recorder, exporter) = {
            let _guard = runtime.enter();
            self.build_with_exporter()
        }?;
        metrics::set_boxed_recorder(Box::new(recorder))?;

        thread::Builder::new()
            .name("metrics-exporter-prometheus-http".to_string())
            .spawn(move || {
                runtime.block_on(async move {
                    pin!(exporter);
                    loop {
                        select! {
                            _ = &mut exporter => {}
                        }
                    }
                });
            })?;

        Ok(())
    }

    /// Builds the recorder and returns it.
    /// This function is only enabled when default features are not set.
    pub fn build(self) -> Result<PrometheusRecorder, Error> {
        let inner = Arc::new(Inner {
            registry: Registry::new(),
            distributions: RwLock::new(HashMap::new()),
            quantiles: self.quantiles.clone(),
            buckets: self.buckets.clone(),
            buckets_by_name: self.buckets_by_name,
            descriptions: RwLock::new(HashMap::new()),
        });

        let recorder = PrometheusRecorder { inner };

        Ok(recorder)
    }

    /// Builds the recorder and exporter and returns them both.
    ///
    /// In most cases, users should prefer to use [`PrometheusBuilder::install`] to create and
    /// install the recorder and exporter automatically for them.  If a caller is combining
    /// recorders, or needs to schedule the exporter to run in a particular way, this method
    /// provides the flexibility to do so.
    #[cfg(feature = "tokio-exporter")]
    pub fn build_with_exporter(
        self,
    ) -> Result<
        (
            PrometheusRecorder,
            impl Future<Output = Result<(), HyperError>> + Send + Sync + 'static,
        ),
        Error,
    > {
        let inner = Arc::new(Inner {
            registry: Registry::new(),
            distributions: RwLock::new(HashMap::new()),
            quantiles: self.quantiles.clone(),
            buckets: self.buckets.clone(),
            buckets_by_name: self.buckets_by_name.clone(),
            descriptions: RwLock::new(HashMap::new()),
        });

        let recorder = PrometheusRecorder {
            inner: inner.clone(),
        };

        let address = self.listen_address;
        let server = Server::try_bind(&address)?;

        let exporter = async move {
            let make_svc = make_service_fn(move |_| {
                let inner = inner.clone();

                async move {
                    Ok::<_, HyperError>(service_fn(move |_| {
                        let inner = inner.clone();

                        async move {
                            let output = inner.render();
                            Ok::<_, HyperError>(Response::new(Body::from(output)))
                        }
                    }))
                }
            });

            server.serve(make_svc).await
        };

        Ok((recorder, exporter))
    }
}

impl Recorder for PrometheusRecorder {
    fn register_counter(&self, key: Key, _unit: Option<Unit>, description: Option<&'static str>) {
        self.add_description_if_missing(&key, description);
        self.inner.registry().op(
            CompositeKey::new(MetricKind::Counter, key),
            |_| {},
            || Handle::counter(),
        );
    }

    fn register_gauge(&self, key: Key, _unit: Option<Unit>, description: Option<&'static str>) {
        self.add_description_if_missing(&key, description);
        self.inner.registry().op(
            CompositeKey::new(MetricKind::Gauge, key),
            |_| {},
            || Handle::gauge(),
        );
    }

    fn register_histogram(&self, key: Key, _unit: Option<Unit>, description: Option<&'static str>) {
        self.add_description_if_missing(&key, description);
        self.inner.registry().op(
            CompositeKey::new(MetricKind::Histogram, key),
            |_| {},
            || Handle::histogram(),
        );
    }

    fn increment_counter(&self, key: Key, value: u64) {
        self.inner.registry().op(
            CompositeKey::new(MetricKind::Counter, key),
            |h| h.increment_counter(value),
            || Handle::counter(),
        );
    }

    fn update_gauge(&self, key: Key, value: f64) {
        self.inner.registry().op(
            CompositeKey::new(MetricKind::Gauge, key),
            |h| h.update_gauge(value),
            || Handle::gauge(),
        );
    }

    fn record_histogram(&self, key: Key, value: u64) {
        self.inner.registry().op(
            CompositeKey::new(MetricKind::Histogram, key),
            |h| h.record_histogram(value),
            || Handle::histogram(),
        );
    }
}

fn key_to_parts(key: Key) -> (String, Vec<String>) {
    let name = key.name();
    let labels = key.labels();
    let sanitize = |c| c == '.' || c == '=' || c == '{' || c == '}' || c == '+' || c == '-';
    let name = name
        .parts()
        .map(|s| s.replace(sanitize, "_"))
        .collect::<Vec<_>>()
        .join("_");
    let labels = labels
        .into_iter()
        .map(|label| {
            let k = label.key();
            let v = label.value();
            format!(
                "{}=\"{}\"",
                k,
                v.replace("\\", "\\\\")
                    .replace("\"", "\\\"")
                    .replace("\n", "\\n")
            )
        })
        .collect();

    (name, labels)
}

fn render_labeled_name(name: &str, labels: &[String]) -> String {
    let mut output = name.to_string();
    if !labels.is_empty() {
        let joined = labels.join(",");
        output.push_str("{");
        output.push_str(&joined);
        output.push_str("}");
    }
    output
}