//! Performance metrics types and tracing layer for sync pipeline observability.
//!
//! Provides:
//! - [`StageTiming`]: Serializable timing/counter data for pipeline stages
//! - [`MetricsLayer`]: Custom tracing subscriber layer that captures span timing

use std::collections::HashMap;
use std::sync::{Arc, Mutex};
use std::time::Instant;

use serde::{Deserialize, Serialize};
use tracing::Subscriber;
use tracing::span::{Attributes, Id, Record};
use tracing_subscriber::layer::{Context, Layer};
use tracing_subscriber::registry::LookupSpan;

/// Returns true when value is zero (for serde `skip_serializing_if`).
fn is_zero(v: &usize) -> bool {
    *v == 0
}

/// Timing and counter data for a single pipeline stage.
///
/// Supports nested sub-stages for hierarchical timing breakdowns.
/// Fields with zero/empty values are omitted from JSON output to
/// keep robot-mode payloads compact.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct StageTiming {
    pub name: String,
    #[serde(skip_serializing_if = "Option::is_none", default)]
    pub project: Option<String>,
    pub elapsed_ms: u64,
    pub items_processed: usize,
    #[serde(skip_serializing_if = "is_zero", default)]
    pub items_skipped: usize,
    #[serde(skip_serializing_if = "is_zero", default)]
    pub errors: usize,
    #[serde(skip_serializing_if = "is_zero", default)]
    pub rate_limit_hits: usize,
    #[serde(skip_serializing_if = "is_zero", default)]
    pub retries: usize,
    #[serde(skip_serializing_if = "Vec::is_empty", default)]
    pub sub_stages: Vec<StageTiming>,
}

// ============================================================================
// MetricsLayer: custom tracing subscriber layer
// ============================================================================

/// Internal data tracked per open span.
struct SpanData {
    name: String,
    parent_id: Option<u64>,
    start: Instant,
    fields: HashMap<String, serde_json::Value>,
    rate_limit_hits: usize,
    retries: usize,
}

/// Completed span data with its original ID and parent ID.
struct CompletedSpan {
    id: u64,
    parent_id: Option<u64>,
    timing: StageTiming,
}

/// Custom tracing layer that captures span timing and structured fields.
///
/// Collects data from `#[instrument]` spans and materializes it into
/// a `Vec<StageTiming>` tree via [`extract_timings`].
///
/// Thread-safe via `Arc<Mutex<>>` — suitable for concurrent span operations.
#[derive(Debug, Clone)]
pub struct MetricsLayer {
    spans: Arc<Mutex<HashMap<u64, SpanData>>>,
    completed: Arc<Mutex<Vec<CompletedSpan>>>,
}

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

impl MetricsLayer {
    pub fn new() -> Self {
        Self {
            spans: Arc::new(Mutex::new(HashMap::new())),
            completed: Arc::new(Mutex::new(Vec::new())),
        }
    }

    /// Extract timing tree for a completed run.
    ///
    /// Returns the top-level stages with sub-stages nested.
    /// Call after the root span closes.
    pub fn extract_timings(&self) -> Vec<StageTiming> {
        let completed = self.completed.lock().unwrap_or_else(|e| e.into_inner());
        if completed.is_empty() {
            return Vec::new();
        }

        // Build children map: parent_id -> Vec<StageTiming>
        let mut children_map: HashMap<u64, Vec<StageTiming>> = HashMap::new();
        let mut roots = Vec::new();
        let mut id_to_timing: HashMap<u64, StageTiming> = HashMap::new();

        // First pass: collect all timings by ID
        for entry in completed.iter() {
            id_to_timing.insert(entry.id, entry.timing.clone());
        }

        // Second pass: process in reverse order (children close before parents)
        // to build the tree bottom-up
        for entry in completed.iter() {
            // Attach any children that were collected for this span
            if let Some(timing) = id_to_timing.get_mut(&entry.id)
                && let Some(children) = children_map.remove(&entry.id)
            {
                timing.sub_stages = children;
            }

            if let Some(parent_id) = entry.parent_id {
                // This is a child span — attach to parent's children
                if let Some(timing) = id_to_timing.remove(&entry.id) {
                    children_map.entry(parent_id).or_default().push(timing);
                }
            }
        }

        // Remaining entries in id_to_timing are roots
        for entry in completed.iter() {
            if entry.parent_id.is_none()
                && let Some(mut timing) = id_to_timing.remove(&entry.id)
            {
                if let Some(children) = children_map.remove(&entry.id) {
                    timing.sub_stages = children;
                }
                roots.push(timing);
            }
        }

        roots
    }
}

/// Visitor that extracts field values from span attributes.
struct FieldVisitor<'a>(&'a mut HashMap<String, serde_json::Value>);

impl tracing::field::Visit for FieldVisitor<'_> {
    fn record_debug(&mut self, field: &tracing::field::Field, value: &dyn std::fmt::Debug) {
        self.0.insert(
            field.name().to_string(),
            serde_json::Value::String(format!("{value:?}")),
        );
    }

    fn record_i64(&mut self, field: &tracing::field::Field, value: i64) {
        self.0.insert(
            field.name().to_string(),
            serde_json::Value::Number(value.into()),
        );
    }

    fn record_u64(&mut self, field: &tracing::field::Field, value: u64) {
        self.0.insert(
            field.name().to_string(),
            serde_json::Value::Number(value.into()),
        );
    }

    fn record_str(&mut self, field: &tracing::field::Field, value: &str) {
        self.0.insert(
            field.name().to_string(),
            serde_json::Value::String(value.to_string()),
        );
    }

    fn record_bool(&mut self, field: &tracing::field::Field, value: bool) {
        self.0
            .insert(field.name().to_string(), serde_json::Value::Bool(value));
    }
}

/// Visitor that extracts event fields for rate-limit/retry detection.
#[derive(Default)]
struct EventVisitor {
    status_code: Option<u64>,
    message: String,
}

impl tracing::field::Visit for EventVisitor {
    fn record_debug(&mut self, field: &tracing::field::Field, value: &dyn std::fmt::Debug) {
        if field.name() == "message" {
            self.message = format!("{value:?}");
        }
    }

    fn record_u64(&mut self, field: &tracing::field::Field, value: u64) {
        if field.name() == "status_code" {
            self.status_code = Some(value);
        }
    }

    fn record_str(&mut self, field: &tracing::field::Field, value: &str) {
        if field.name() == "message" {
            self.message = value.to_string();
        }
    }
}

impl<S> Layer<S> for MetricsLayer
where
    S: Subscriber + for<'a> LookupSpan<'a>,
{
    fn on_new_span(&self, attrs: &Attributes<'_>, id: &Id, ctx: Context<'_, S>) {
        let parent_id = ctx
            .span(id)
            .and_then(|s| s.parent().map(|p| p.id().into_u64()));

        let mut fields = HashMap::new();
        let mut visitor = FieldVisitor(&mut fields);
        attrs.record(&mut visitor);

        self.spans.lock().unwrap_or_else(|e| e.into_inner()).insert(
            id.into_u64(),
            SpanData {
                name: attrs.metadata().name().to_string(),
                parent_id,
                start: Instant::now(),
                fields,
                rate_limit_hits: 0,
                retries: 0,
            },
        );
    }

    fn on_record(&self, id: &Id, values: &Record<'_>, _ctx: Context<'_, S>) {
        if let Some(data) = self
            .spans
            .lock()
            .unwrap_or_else(|e| e.into_inner())
            .get_mut(&id.into_u64())
        {
            let mut visitor = FieldVisitor(&mut data.fields);
            values.record(&mut visitor);
        }
    }

    fn on_event(&self, event: &tracing::Event<'_>, ctx: Context<'_, S>) {
        // Count rate-limit and retry events on the current span
        if let Some(span_ref) = ctx.event_span(event) {
            let id = span_ref.id();
            if let Some(data) = self
                .spans
                .lock()
                .unwrap_or_else(|e| e.into_inner())
                .get_mut(&id.into_u64())
            {
                let mut visitor = EventVisitor::default();
                event.record(&mut visitor);

                if visitor.status_code == Some(429) {
                    data.rate_limit_hits += 1;
                }
                if visitor.message.contains("retrying") || visitor.message.contains("Retrying") {
                    data.retries += 1;
                }
            }
        }
    }

    fn on_close(&self, id: Id, _ctx: Context<'_, S>) {
        if let Some(data) = self
            .spans
            .lock()
            .unwrap_or_else(|e| e.into_inner())
            .remove(&id.into_u64())
        {
            let elapsed = data.start.elapsed();
            let timing = StageTiming {
                name: data.name,
                project: data
                    .fields
                    .get("project")
                    .and_then(|v| v.as_str())
                    .map(String::from),
                elapsed_ms: elapsed.as_millis() as u64,
                items_processed: data
                    .fields
                    .get("items_processed")
                    .and_then(|v| v.as_u64())
                    .unwrap_or(0) as usize,
                items_skipped: data
                    .fields
                    .get("items_skipped")
                    .and_then(|v| v.as_u64())
                    .unwrap_or(0) as usize,
                errors: data
                    .fields
                    .get("errors")
                    .and_then(|v| v.as_u64())
                    .unwrap_or(0) as usize,
                rate_limit_hits: data.rate_limit_hits,
                retries: data.retries,
                sub_stages: vec![],
            };
            self.completed
                .lock()
                .unwrap_or_else(|e| e.into_inner())
                .push(CompletedSpan {
                    id: id.into_u64(),
                    parent_id: data.parent_id,
                    timing,
                });
        }
    }
}

// Manual Debug impl since SpanData and CompletedSpan don't derive Debug
impl std::fmt::Debug for SpanData {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("SpanData")
            .field("name", &self.name)
            .field("parent_id", &self.parent_id)
            .field("fields", &self.fields.keys().collect::<Vec<_>>())
            .finish()
    }
}

impl std::fmt::Debug for CompletedSpan {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("CompletedSpan")
            .field("id", &self.id)
            .field("parent_id", &self.parent_id)
            .field("timing", &self.timing.name)
            .finish()
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use tracing_subscriber::layer::SubscriberExt;

    #[test]
    fn test_stage_timing_serialization() {
        let timing = StageTiming {
            name: "sync".to_string(),
            project: None,
            elapsed_ms: 1500,
            items_processed: 42,
            items_skipped: 3,
            errors: 1,
            rate_limit_hits: 2,
            retries: 5,
            sub_stages: vec![StageTiming {
                name: "ingest_issues".to_string(),
                project: Some("group/repo".to_string()),
                elapsed_ms: 800,
                items_processed: 30,
                items_skipped: 0,
                errors: 0,
                rate_limit_hits: 0,
                retries: 0,
                sub_stages: vec![],
            }],
        };

        let json = serde_json::to_value(&timing).unwrap();
        assert_eq!(json["name"], "sync");
        assert_eq!(json["elapsed_ms"], 1500);
        assert_eq!(json["items_processed"], 42);
        assert_eq!(json["items_skipped"], 3);
        assert_eq!(json["errors"], 1);
        assert_eq!(json["rate_limit_hits"], 2);
        assert_eq!(json["retries"], 5);

        // Sub-stage present
        let sub = &json["sub_stages"][0];
        assert_eq!(sub["name"], "ingest_issues");
        assert_eq!(sub["project"], "group/repo");
        assert_eq!(sub["elapsed_ms"], 800);
    }

    #[test]
    fn test_stage_timing_zero_fields_omitted() {
        let timing = StageTiming {
            name: "embed".to_string(),
            project: None,
            elapsed_ms: 500,
            items_processed: 10,
            items_skipped: 0,
            errors: 0,
            rate_limit_hits: 0,
            retries: 0,
            sub_stages: vec![],
        };

        let json = serde_json::to_value(&timing).unwrap();
        let obj = json.as_object().unwrap();

        // Zero fields must be absent
        assert!(!obj.contains_key("items_skipped"));
        assert!(!obj.contains_key("errors"));
        assert!(!obj.contains_key("rate_limit_hits"));
        assert!(!obj.contains_key("retries"));
        assert!(!obj.contains_key("sub_stages"));
        assert!(!obj.contains_key("project"));

        // Required fields always present
        assert!(obj.contains_key("name"));
        assert!(obj.contains_key("elapsed_ms"));
        assert!(obj.contains_key("items_processed"));
    }

    #[test]
    fn test_stage_timing_empty_sub_stages() {
        let timing = StageTiming {
            name: "docs".to_string(),
            project: None,
            elapsed_ms: 200,
            items_processed: 5,
            items_skipped: 0,
            errors: 0,
            rate_limit_hits: 0,
            retries: 0,
            sub_stages: vec![],
        };

        let json_str = serde_json::to_string(&timing).unwrap();
        assert!(!json_str.contains("sub_stages"));
    }

    #[test]
    fn test_stage_timing_debug_clone() {
        let timing = StageTiming {
            name: "test".to_string(),
            project: None,
            elapsed_ms: 100,
            items_processed: 1,
            items_skipped: 0,
            errors: 0,
            rate_limit_hits: 0,
            retries: 0,
            sub_stages: vec![],
        };

        let cloned = timing.clone();
        assert_eq!(cloned.name, "test");

        let debug_str = format!("{timing:?}");
        assert!(debug_str.contains("test"));
    }

    #[test]
    fn test_stage_timing_nested_sub_stages() {
        let timing = StageTiming {
            name: "sync".to_string(),
            project: None,
            elapsed_ms: 3000,
            items_processed: 100,
            items_skipped: 0,
            errors: 0,
            rate_limit_hits: 0,
            retries: 0,
            sub_stages: vec![StageTiming {
                name: "ingest".to_string(),
                project: None,
                elapsed_ms: 2000,
                items_processed: 80,
                items_skipped: 0,
                errors: 0,
                rate_limit_hits: 0,
                retries: 0,
                sub_stages: vec![StageTiming {
                    name: "discussions".to_string(),
                    project: Some("a/b".to_string()),
                    elapsed_ms: 1000,
                    items_processed: 50,
                    items_skipped: 0,
                    errors: 0,
                    rate_limit_hits: 0,
                    retries: 0,
                    sub_stages: vec![],
                }],
            }],
        };

        let json = serde_json::to_value(&timing).unwrap();
        let nested = &json["sub_stages"][0]["sub_stages"][0];
        assert_eq!(nested["name"], "discussions");
        assert_eq!(nested["project"], "a/b");
    }

    #[test]
    fn test_rate_limit_fields_omitted_when_zero() {
        let timing = StageTiming {
            name: "ingest".to_string(),
            project: None,
            elapsed_ms: 100,
            items_processed: 5,
            items_skipped: 0,
            errors: 0,
            rate_limit_hits: 0,
            retries: 0,
            sub_stages: vec![],
        };

        let json_str = serde_json::to_string(&timing).unwrap();
        assert!(!json_str.contains("rate_limit_hits"));
        assert!(!json_str.contains("retries"));
    }

    #[test]
    fn test_rate_limit_fields_present_when_nonzero() {
        let timing = StageTiming {
            name: "ingest".to_string(),
            project: None,
            elapsed_ms: 100,
            items_processed: 5,
            items_skipped: 0,
            errors: 0,
            rate_limit_hits: 3,
            retries: 7,
            sub_stages: vec![],
        };

        let json = serde_json::to_value(&timing).unwrap();
        assert_eq!(json["rate_limit_hits"], 3);
        assert_eq!(json["retries"], 7);
    }

    #[test]
    fn test_metrics_layer_single_span() {
        let metrics = MetricsLayer::new();
        let subscriber = tracing_subscriber::registry().with(metrics.clone());

        tracing::subscriber::with_default(subscriber, || {
            let span = tracing::info_span!("test_stage");
            let _guard = span.enter();
            // Simulate work
        });

        let timings = metrics.extract_timings();
        assert_eq!(timings.len(), 1);
        assert_eq!(timings[0].name, "test_stage");
        assert!(timings[0].elapsed_ms < 100); // Should be near-instant
    }

    #[test]
    fn test_metrics_layer_nested_spans() {
        let metrics = MetricsLayer::new();
        let subscriber = tracing_subscriber::registry().with(metrics.clone());

        tracing::subscriber::with_default(subscriber, || {
            let parent = tracing::info_span!("parent");
            let _parent_guard = parent.enter();
            {
                let child = tracing::info_span!("child");
                let _child_guard = child.enter();
            }
        });

        let timings = metrics.extract_timings();
        assert_eq!(timings.len(), 1);
        assert_eq!(timings[0].name, "parent");
        assert_eq!(timings[0].sub_stages.len(), 1);
        assert_eq!(timings[0].sub_stages[0].name, "child");
    }

    #[test]
    fn test_metrics_layer_parallel_spans() {
        let metrics = MetricsLayer::new();
        let subscriber = tracing_subscriber::registry().with(metrics.clone());

        tracing::subscriber::with_default(subscriber, || {
            let parent = tracing::info_span!("parent");
            let _parent_guard = parent.enter();
            {
                let child_a = tracing::info_span!("child_a");
                let _a = child_a.enter();
            }
            {
                let child_b = tracing::info_span!("child_b");
                let _b = child_b.enter();
            }
        });

        let timings = metrics.extract_timings();
        assert_eq!(timings.len(), 1);
        assert_eq!(timings[0].sub_stages.len(), 2);

        let names: Vec<&str> = timings[0]
            .sub_stages
            .iter()
            .map(|s| s.name.as_str())
            .collect();
        assert!(names.contains(&"child_a"));
        assert!(names.contains(&"child_b"));
    }

    #[test]
    fn test_metrics_layer_empty() {
        let metrics = MetricsLayer::new();
        let timings = metrics.extract_timings();
        assert!(timings.is_empty());
    }
}