// API Gateway & Service Mesh - Rust Implementation
// Production-grade API gateway with Axum + Envoy integration

use axum::{
    Router, Json, extract::{Path, State},
    http::{StatusCode, HeaderMap},
    middleware,
};
use serde::{Deserialize, Serialize};
use std::sync::Arc;
use tokio::sync::RwLock;
use tower::ServiceBuilder;
use tower_http::{
    trace::TraceLayer,
    compression::CompressionLayer,
    limit::RequestBodyLimitLayer,
};

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct RouteConfig {
    pub path: String,
    pub method: String,
    pub backend_service: String,
    pub rate_limit_rps: Option<u32>,
    pub circuit_breaker: Option<CircuitBreakerConfig>,
    pub retry_policy: Option<RetryConfig>,
    pub timeout_seconds: u64,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct CircuitBreakerConfig {
    pub failure_threshold: u32,
    pub timeout_seconds: u64,
    pub half_open_requests: u32,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct RetryConfig {
    pub max_attempts: u32,
    pub backoff_multiplier: f64,
    pub initial_delay_ms: u64,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ServiceMeshConfig {
    pub mesh_type: String, // istio, linkerd, consul
    pub mtls_mode: String, // STRICT, PERMISSIVE, DISABLE
    pub sidecar_port: u16,
    pub control_plane_endpoint: String,
}

#[derive(Debug, Clone)]
pub struct GatewayState {
    pub routes: Arc<RwLock<Vec<RouteConfig>>>,
    pub mesh_config: Arc<ServiceMeshConfig>,
    pub health_checker: Arc<HealthChecker>,
    pub metrics: Arc<MetricsCollector>,
}

pub struct HealthChecker {
    pub interval_seconds: u64,
    pub failure_threshold: u32,
    pub success_threshold: u32,
}

pub struct MetricsCollector {
    pub request_counter: Arc<RwLock<u64>>,
    pub error_counter: Arc<RwLock<u64>>,
    pub latency_histogram: Arc<RwLock<Vec<u64>>>,
}

impl GatewayState {
    pub fn new(mesh_config: ServiceMeshConfig) -> Self {
        Self {
            routes: Arc::new(RwLock::new(Vec::new())),
            mesh_config: Arc::new(mesh_config),
            health_checker: Arc::new(HealthChecker {
                interval_seconds: 30,
                failure_threshold: 3,
                success_threshold: 2,
            }),
            metrics: Arc::new(MetricsCollector {
                request_counter: Arc::new(RwLock::new(0)),
                error_counter: Arc::new(RwLock::new(0)),
                latency_histogram: Arc::new(RwLock::new(Vec::new())),
            }),
        }
    }

    pub async fn add_route(&self, route: RouteConfig) {
        let mut routes = self.routes.write().await;
        routes.push(route);
    }

    pub async fn find_route(&self, path: &str, method: &str) -> Option<RouteConfig> {
        let routes = self.routes.read().await;
        routes.iter()
            .find(|r| r.path == path && r.method == method)
            .cloned()
    }
}

pub async fn proxy_request(
    State(state): State<Arc<GatewayState>>,
    Path((path, method)): Path<(String, String)>,
    headers: HeaderMap,
    body: String,
) -> Result<Json<serde_json::Value>, (StatusCode, String)> {
    // Increment request counter
    {
        let mut counter = state.metrics.request_counter.write().await;
        *counter += 1;
    }

    // Find matching route
    let route = state.find_route(&path, &method).await
        .ok_or((StatusCode::NOT_FOUND, "Route not found".to_string()))?;

    // Apply rate limiting
    if let Some(rate_limit) = route.rate_limit_rps {
        if !check_rate_limit(rate_limit).await {
            return Err((StatusCode::TOO_MANY_REQUESTS, "Rate limit exceeded".to_string()));
        }
    }

    // Check circuit breaker
    if let Some(cb_config) = &route.circuit_breaker {
        if !check_circuit_breaker(&route.backend_service, cb_config).await {
            return Err((StatusCode::SERVICE_UNAVAILABLE, "Circuit breaker open".to_string()));
        }
    }

    // Forward to backend with retry logic
    let response = forward_with_retry(
        &route.backend_service,
        &method,
        headers,
        body,
        route.retry_policy.as_ref(),
        route.timeout_seconds,
    ).await?;

    Ok(Json(response))
}

async fn check_rate_limit(rps: u32) -> bool {
    // Implement token bucket or sliding window rate limiter
    true // Placeholder
}

async fn check_circuit_breaker(service: &str, config: &CircuitBreakerConfig) -> bool {
    // Implement circuit breaker state machine
    true // Placeholder
}

async fn forward_with_retry(
    backend: &str,
    method: &str,
    headers: HeaderMap,
    body: String,
    retry_config: Option<&RetryConfig>,
    timeout: u64,
) -> Result<serde_json::Value, (StatusCode, String)> {
    let retry_config = retry_config.cloned().unwrap_or(RetryConfig {
        max_attempts: 1,
        backoff_multiplier: 1.0,
        initial_delay_ms: 0,
    });

    let mut attempts = 0;
    let mut delay_ms = retry_config.initial_delay_ms;

    loop {
        attempts += 1;

        match execute_backend_request(backend, method, &headers, &body, timeout).await {
            Ok(response) => return Ok(response),
            Err(e) if attempts >= retry_config.max_attempts => {
                return Err((StatusCode::BAD_GATEWAY, format!("Backend failed: {}", e)));
            }
            Err(_) => {
                tokio::time::sleep(tokio::time::Duration::from_millis(delay_ms)).await;
                delay_ms = (delay_ms as f64 * retry_config.backoff_multiplier) as u64;
            }
        }
    }
}

async fn execute_backend_request(
    backend: &str,
    method: &str,
    headers: &HeaderMap,
    body: &str,
    timeout: u64,
) -> Result<serde_json::Value, String> {
    // Implement actual HTTP client request with timeout
    // Add mTLS if service mesh requires it
    Ok(serde_json::json!({"status": "success"}))
}

pub async fn health_check(
    State(state): State<Arc<GatewayState>>,
) -> Result<Json<serde_json::Value>, StatusCode> {
    let request_count = *state.metrics.request_counter.read().await;
    let error_count = *state.metrics.error_counter.read().await;

    Ok(Json(serde_json::json!({
        "status": "healthy",
        "mesh_type": state.mesh_config.mesh_type,
        "mtls_mode": state.mesh_config.mtls_mode,
        "requests": request_count,
        "errors": error_count,
        "error_rate": if request_count > 0 {
            (error_count as f64 / request_count as f64) * 100.0
        } else {
            0.0
        }
    })))
}

pub fn create_router(state: Arc<GatewayState>) -> Router {
    Router::new()
        .route("/proxy/:path/:method", axum::routing::any(proxy_request))
        .route("/health", axum::routing::get(health_check))
        .layer(
            ServiceBuilder::new()
                .layer(TraceLayer::new_for_http())
                .layer(CompressionLayer::new())
                .layer(RequestBodyLimitLayer::new(10 * 1024 * 1024)) // 10MB limit
        )
        .with_state(state)
}

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

    #[tokio::test]
    async fn test_route_registration() {
        let mesh_config = ServiceMeshConfig {
            mesh_type: "istio".to_string(),
            mtls_mode: "STRICT".to_string(),
            sidecar_port: 15001,
            control_plane_endpoint: "istiod.istio-system:15012".to_string(),
        };

        let state = GatewayState::new(mesh_config);

        let route = RouteConfig {
            path: "/api/v1/users".to_string(),
            method: "GET".to_string(),
            backend_service: "users-service:8080".to_string(),
            rate_limit_rps: Some(100),
            circuit_breaker: Some(CircuitBreakerConfig {
                failure_threshold: 5,
                timeout_seconds: 30,
                half_open_requests: 3,
            }),
            retry_policy: Some(RetryConfig {
                max_attempts: 3,
                backoff_multiplier: 2.0,
                initial_delay_ms: 100,
            }),
            timeout_seconds: 5,
        };

        state.add_route(route).await;

        let found = state.find_route("/api/v1/users", "GET").await;
        assert!(found.is_some());
        assert_eq!(found.unwrap().backend_service, "users-service:8080");
    }

    #[tokio::test]
    async fn test_circuit_breaker() {
        let config = CircuitBreakerConfig {
            failure_threshold: 5,
            timeout_seconds: 30,
            half_open_requests: 3,
        };

        let result = check_circuit_breaker("test-service", &config).await;
        assert!(result);
    }
}
