// Copyright 2024 The NativeLink Authors. All rights reserved.

//

// Licensed under the Apache License, Version 2.0 (the "License");

// you may not use this file except in compliance with the License.

// You may obtain a copy of the License at

//

//    http://www.apache.org/licenses/LICENSE-2.0

//

// Unless required by applicable law or agreed to in writing, software

// distributed under the License is distributed on an "AS IS" BASIS,

// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

// See the License for the specific language governing permissions and

// limitations under the License.

use std::borrow::Cow;

use std::collections::HashMap;

use std::fmt::Debug;

use std::marker::PhantomData;

use std::sync::Arc;

use parking_lot::Mutex;

use tracing::span::Attributes;

use tracing::subscriber::Interest;

use tracing::{Event, Id, Metadata, Subscriber};

use tracing_subscriber::layer::Context;

use tracing_subscriber::registry::SpanRef;

use tracing_subscriber::Layer;

use crate::metrics_collection::{

    CollectedMetricChildren, CollectedMetricPrimitive, CollectedMetrics, RootMetricCollectedMetrics,

};

use crate::metrics_visitors::{MetricDataVisitor, SpanFields};

/// The layer that is given to `tracing` to collect metrics.

/// The output of the metrics will be populated in the `root_collected_metrics`

/// field.

pub struct MetricsCollectorLayer<S> {

    spans: Mutex<HashMap<Id, SpanFields>>,

    root_collected_metrics: Arc<Mutex<RootMetricCollectedMetrics>>,

    _subscriber: PhantomData<S>,

}

impl<S> MetricsCollectorLayer<S> {

    /// Creates a new `MetricsCollectorLayer` and returns it along with the

    /// `root_collected_metrics` that will be populated with the collected metrics.

    pub fn new() -> (Self, Arc<Mutex<RootMetricCollectedMetrics>>) {

        let root_collected_metrics = Arc::new(Mutex::new(RootMetricCollectedMetrics::default()));

        (

            MetricsCollectorLayer {

                spans: Mutex::new(HashMap::new()),

                root_collected_metrics: root_collected_metrics.clone(),

                _subscriber: PhantomData,

            },

            root_collected_metrics,

        )

    }

}

impl<S> Layer<S> for MetricsCollectorLayer<S>

where

    S: Subscriber + for<'a> tracing_subscriber::registry::LookupSpan<'a> + Debug,

{

    fn enabled(&self, metadata: &Metadata<'_>, _ctx: Context<'_, S>) -> bool {

        metadata.target() == "nativelink_metric"

    }

    fn on_new_span(&self, attrs: &Attributes<'_>, id: &Id, _ctx: Context<'_, S>) {

        let mut span_fields = SpanFields {

            name: Cow::Borrowed(attrs.metadata().name()),

        };

        // Store the current metadata values map representing the current span.

        // We need to 'snapshot' the current span, because when a more recent

        // span (such as the one being initialized) updates, these values will

        // be overwritten.

        attrs.values().record(&mut span_fields);

        self.spans.lock().insert(id.clone(), span_fields);

    }

    fn on_event(&self, event: &Event<'_>, ctx: Context<'_, S>) {

        let mut event_visitor = MetricDataVisitor::default();

        // First, we populate the MetricDataVisitor we are interested

        // in from the event.

        event.record(&mut event_visitor);

        // This represents the field we are concerned with updating or

        // initializing.

        let name = event_visitor.name.clone();

        let mut root_collected_metrics = self.root_collected_metrics.lock();

        let collected_component = &mut **root_collected_metrics;

        // Find out which span we are currently in and retrieve its metadata.

        // It is possible to not be in a span in the tracing library.

        // If we are not in a span, we assume you want your metrics published

        // in the root of the collected metrics.

        if let Some(
current_span6
) = ctx.lookup_current() {

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            let mut known_spans = self.spans.lock();

            // By default tracing starts you at the bottom of the span tree,

            // but we want to start at the root of the tree and walk down,

            // so invert it.

            let span_iter = current_span.scope().from_root();

            // Find the layer in our output struct we are going to populate

            // the data into.

            let collected_component =

                find_component(span_iter, &mut known_spans, collected_component);

            // Get the new value from the event and update it in the component.

            let primitive = CollectedMetricPrimitive::from(event_visitor);

            collected_component.insert(name, CollectedMetrics::Primitive(primitive));

        } else {

            let primitive = CollectedMetricPrimitive::from(event_visitor);

            collected_component.insert(name, CollectedMetrics::Primitive(primitive));

        }

    }

    fn register_callsite(&self, _metadata: &'static Metadata<'static>) -> Interest {

        Interest::always()

    }

}

fn find_component<'a, 'b, S, I>(

    mut iter: I,

    known_spans: &'a mut HashMap<Id, SpanFields>,

    mut collected_component: &'a mut CollectedMetricChildren,

) -> &'a mut CollectedMetricChildren

where

    S: Subscriber + for<'c> tracing_subscriber::registry::LookupSpan<'c> + Debug,

    I: Iterator<Item = SpanRef<'b, S>>,

{

    let Some(
span8
) = iter.next() else {

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        // Once there are no more nested spans, we have reached a leaf field.

        return collected_component;

    };

    let span_fields = known_spans.get(&span.id()).expect("Span not found");

    // LayerMap<Name, Either<LayerMap, Primitive>>

    // This is a hashmap of the existing data for the layer

    let collected_metric = collected_component

        .entry(span_fields.name.to_string())

        .or_insert_with(CollectedMetrics::new_component);

    collected_component = match collected_metric {

        CollectedMetrics::Component(component) => &mut **component,

        CollectedMetrics::Primitive(_) => panic!("Expected to be component"),

    };

    // DFS the iterator of keys and return the first leaf found matching the name query.

    find_component(iter, known_spans, collected_component)

}