use std::collections::{HashMap, VecDeque};
use std::sync::atomic::{AtomicU64, Ordering::Relaxed};
use std::sync::{Arc, Mutex, OnceLock};

use crate::providers::Usage;

const WINDOW: usize = 100;
const DEGRADE_THRESHOLD: usize = 3;
const DEGRADE_WINDOW: usize = 10;

#[derive(Default)]
struct ProviderStat {
    model: String,
    tokens_in: u64,
    tokens_out: u64,
    cost: f64,
    calls: u64,
    last_latency_ms: u64,
    latencies: VecDeque<u64>,
    recent_results: VecDeque<bool>,
}

pub struct Metrics {
    tokens_in: AtomicU64,
    tokens_out: AtomicU64,
    turns: AtomicU64,
    tool_calls: AtomicU64,
    per_tool: Mutex<HashMap<String, u64>>,
    turn_latencies: Mutex<VecDeque<u64>>,
    providers: Mutex<HashMap<String, ProviderStat>>,
}

#[derive(serde::Serialize)]
pub struct MetricsSnapshot {
    pub tokens_in: u64,
    pub tokens_out: u64,
    pub cost: f64,
    pub turns: u64,
    pub tool_calls: u64,
    pub turn_latency_p95_ms: u64,
    pub per_tool: HashMap<String, u64>,
    pub providers: Vec<ProviderSnapshot>,
}

#[derive(serde::Serialize)]
pub struct ProviderSnapshot {
    pub name: String,
    pub model: String,
    pub status: String,
    pub latency_ms: u64,
    pub latencies: Vec<u64>,
    pub tokens_in: u64,
    pub tokens_out: u64,
    pub cost: f64,
}

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

impl Metrics {
    pub fn new() -> Self {
        Self {
            tokens_in: AtomicU64::new(0),
            tokens_out: AtomicU64::new(0),
            turns: AtomicU64::new(0),
            tool_calls: AtomicU64::new(0),
            per_tool: Mutex::new(HashMap::new()),
            turn_latencies: Mutex::new(VecDeque::new()),
            providers: Mutex::new(HashMap::new()),
        }
    }

    pub fn record_turn(&self, usage: Option<&Usage>, latency_ms: u64) {
        self.turns.fetch_add(1, Relaxed);
        if let Some(u) = usage {
            self.tokens_in
                .fetch_add(u64::from(u.prompt_tokens), Relaxed);
            self.tokens_out
                .fetch_add(u64::from(u.completion_tokens), Relaxed);
        }
        let mut q = self
            .turn_latencies
            .lock()
            .unwrap_or_else(|e| e.into_inner());
        q.push_back(latency_ms);
        while q.len() > WINDOW {
            q.pop_front();
        }
    }

    pub fn record_tool_call(&self, name: &str, _success: bool) {
        self.tool_calls.fetch_add(1, Relaxed);
        *self
            .per_tool
            .lock()
            .unwrap_or_else(|e| e.into_inner())
            .entry(name.to_string())
            .or_insert(0) += 1;
    }

    pub fn record_provider(
        &self,
        name: &str,
        model: &str,
        cost: Option<f64>,
        latency_ms: u64,
        is_error: bool,
    ) {
        let mut map = self.providers.lock().unwrap_or_else(|e| e.into_inner());
        let stat = map.entry(name.to_string()).or_insert_with(|| ProviderStat {
            model: model.to_string(),
            ..Default::default()
        });
        stat.model = model.to_string();
        stat.calls += 1;
        stat.last_latency_ms = latency_ms;
        stat.latencies.push_back(latency_ms);
        while stat.latencies.len() > WINDOW {
            stat.latencies.pop_front();
        }
        stat.recent_results.push_back(is_error);
        while stat.recent_results.len() > DEGRADE_WINDOW {
            stat.recent_results.pop_front();
        }
        if let Some(c) = cost {
            stat.cost += c;
        }
    }

    pub fn record_provider_tokens(&self, name: &str, usage: &Usage) {
        let mut map = self.providers.lock().unwrap_or_else(|e| e.into_inner());
        let stat = map.entry(name.to_string()).or_default();
        stat.tokens_in += u64::from(usage.prompt_tokens);
        stat.tokens_out += u64::from(usage.completion_tokens);
    }

    pub fn tool_call_count(&self, name: &str) -> u64 {
        *self
            .per_tool
            .lock()
            .unwrap_or_else(|e| e.into_inner())
            .get(name)
            .unwrap_or(&0)
    }

    pub fn snapshot(&self) -> MetricsSnapshot {
        let latencies = self
            .turn_latencies
            .lock()
            .unwrap_or_else(|e| e.into_inner());
        let p95 = percentile_95(&latencies);
        let providers = self.providers.lock().unwrap_or_else(|e| e.into_inner());
        let mut cost = 0.0;
        let provider_snaps = providers
            .iter()
            .map(|(name, s)| {
                cost += s.cost;
                let errors = s.recent_results.iter().filter(|e| **e).count();
                ProviderSnapshot {
                    name: name.clone(),
                    model: s.model.clone(),
                    status: if s.recent_results.len() >= DEGRADE_WINDOW
                        && errors >= DEGRADE_THRESHOLD
                    {
                        "degraded".to_string()
                    } else {
                        "ok".to_string()
                    },
                    latency_ms: s.last_latency_ms,
                    latencies: s.latencies.iter().copied().collect(),
                    tokens_in: s.tokens_in,
                    tokens_out: s.tokens_out,
                    cost: s.cost,
                }
            })
            .collect();
        MetricsSnapshot {
            tokens_in: self.tokens_in.load(Relaxed),
            tokens_out: self.tokens_out.load(Relaxed),
            cost,
            turns: self.turns.load(Relaxed),
            tool_calls: self.tool_calls.load(Relaxed),
            turn_latency_p95_ms: p95,
            per_tool: self
                .per_tool
                .lock()
                .unwrap_or_else(|e| e.into_inner())
                .clone(),
            providers: provider_snaps,
        }
    }
}

fn percentile_95(values: &VecDeque<u64>) -> u64 {
    if values.is_empty() {
        return 0;
    }
    let mut sorted: Vec<u64> = values.iter().copied().collect();
    sorted.sort_unstable();
    let idx = ((sorted.len() as f64 * 0.95).ceil() as usize)
        .saturating_sub(1)
        .min(sorted.len() - 1);
    sorted[idx]
}

static GLOBAL: OnceLock<Arc<Metrics>> = OnceLock::new();

pub fn global_metrics() -> Arc<Metrics> {
    GLOBAL.get_or_init(|| Arc::new(Metrics::new())).clone()
}

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

    #[test]
    fn records_turns_tokens_and_p95() {
        let m = Metrics::new();
        for i in 1..=100u64 {
            m.record_turn(
                Some(&Usage {
                    prompt_tokens: 10,
                    completion_tokens: 20,
                    total_tokens: 30,
                    ..Default::default()
                }),
                i,
            );
        }
        let s = m.snapshot();
        assert_eq!(s.turns, 100);
        assert_eq!(s.tokens_in, 1000);
        assert_eq!(s.tokens_out, 2000);
        assert!(s.turn_latency_p95_ms >= 95);
    }

    #[test]
    fn records_per_tool_counts() {
        let m = Metrics::new();
        m.record_tool_call("bash", true);
        m.record_tool_call("bash", true);
        m.record_tool_call("read_file", false);
        let s = m.snapshot();
        assert_eq!(s.tool_calls, 3);
        assert_eq!(s.per_tool.get("bash"), Some(&2));
        assert_eq!(s.per_tool.get("read_file"), Some(&1));
    }

    #[test]
    fn records_provider_tokens_and_cost() {
        let m = Metrics::new();
        m.record_provider("openai", "gpt-4o", Some(0.05), 120, false);
        m.record_provider_tokens(
            "openai",
            &Usage {
                prompt_tokens: 100,
                completion_tokens: 50,
                total_tokens: 150,
                ..Default::default()
            },
        );
        let s = m.snapshot();
        assert_eq!(s.cost, 0.05);
        let p = s.providers.iter().find(|p| p.name == "openai").unwrap();
        assert_eq!(p.tokens_in, 100);
        assert_eq!(p.tokens_out, 50);
        assert_eq!(p.latency_ms, 120);
        assert_eq!(p.status, "ok");
    }

    #[test]
    fn derives_provider_status() {
        let m = Metrics::new();
        for _ in 0..7 {
            m.record_provider("openai", "gpt-4o", None, 100, false);
        }
        for _ in 0..3 {
            m.record_provider("openai", "gpt-4o", None, 100, true);
        }
        let s = m.snapshot();
        let p = s.providers.iter().find(|p| p.name == "openai").unwrap();
        assert_eq!(p.status, "degraded");
    }

    #[test]
    fn tool_call_count_accessor() {
        let m = Metrics::new();
        assert_eq!(m.tool_call_count("bash"), 0);
        m.record_tool_call("bash", true);
        assert_eq!(m.tool_call_count("bash"), 1);
    }

    #[test]
    fn global_metrics_returns_same_instance() {
        let a = global_metrics();
        let b = global_metrics();
        assert!(Arc::ptr_eq(&a, &b));
    }
}