//! Tiny per-IP token-bucket rate limiter.
//!
//! Currently used by `POST /v1/messages` to keep a single source from
//! filling everyone's mailbox until disk fills (the v0.1 spam concern
//! called out in messages.rs). Default rate: 60 messages/min per IP.
//!
//! Why not pull in `tower_governor` or `governor`? They're great
//! crates but each adds 10+ transitive deps for what's structurally
//! ~50 lines of code. We're already shipping nostr-sdk's dep tree;
//! restraint here keeps the build snappy.
//!
//! Client IP resolution priority:
//!   1. `CF-Connecting-IP` header — Cloudflare puts the real client
//!      IP here; we trust it because Cloudflare strips this header
//!      from anything that wasn't routed through our tunnel.
//!   2. `X-Forwarded-For` (first hop) — fallback for non-Cloudflare
//!      deployments.
//!   3. None — direct curl / loopback. Rate-limit by `0.0.0.0` so
//!      noisy test traffic still gets bucketed instead of bypassing.

use std::collections::HashMap;
use std::net::IpAddr;
use std::sync::Arc;
use std::time::{Duration, Instant};

use axum::http::HeaderMap;
use tokio::sync::Mutex;

/// One bucket per client. We store the residual token count + the
/// last refill timestamp; on each `try_acquire` we compute how many
/// tokens to add based on elapsed time, then either decrement or fail.
#[derive(Debug, Clone)]
struct Bucket {
    /// Tokens currently available.
    tokens: f64,
    /// Last time we refilled.
    last_refill: Instant,
    /// Most recent activity — used by the eviction sweep to drop
    /// long-cold buckets so the HashMap doesn't grow forever.
    last_seen: Instant,
}

#[derive(Debug, Clone, Copy)]
pub struct RateLimitConfig {
    /// Bucket capacity (max burst). Once exhausted, callers fail
    /// fast until enough time passes to refill ≥1 token.
    pub capacity: u32,
    /// Refill rate in tokens per second.
    pub refill_per_sec: f64,
    /// Buckets idle longer than this get evicted on next sweep so
    /// short-lived clients don't pile up in memory.
    pub idle_ttl: Duration,
}

impl Default for RateLimitConfig {
    fn default() -> Self {
        Self {
            capacity: 60,
            refill_per_sec: 1.0, // = 60/min steady-state
            idle_ttl: Duration::from_secs(15 * 60),
        }
    }
}

/// Process-shared rate limiter. Cheap to clone (Arc inside).
#[derive(Clone)]
pub struct RateLimiter {
    inner: Arc<Mutex<HashMap<IpAddr, Bucket>>>,
    config: RateLimitConfig,
}

impl RateLimiter {
    pub fn new(config: RateLimitConfig) -> Self {
        Self {
            inner: Arc::new(Mutex::new(HashMap::new())),
            config,
        }
    }

    /// Drain one token for `ip` if available. Returns `true` on
    /// success (caller may proceed) or `false` if rate-limited
    /// (caller should respond 429).
    pub async fn try_acquire(&self, ip: IpAddr) -> bool {
        let mut map = self.inner.lock().await;
        let now = Instant::now();
        let bucket = map.entry(ip).or_insert(Bucket {
            tokens: self.config.capacity as f64,
            last_refill: now,
            last_seen: now,
        });
        // Refill since last touch.
        let elapsed = now.saturating_duration_since(bucket.last_refill).as_secs_f64();
        bucket.tokens =
            (bucket.tokens + elapsed * self.config.refill_per_sec)
                .min(self.config.capacity as f64);
        bucket.last_refill = now;
        bucket.last_seen = now;
        if bucket.tokens >= 1.0 {
            bucket.tokens -= 1.0;
            true
        } else {
            false
        }
    }

    /// Periodically called by a background sweep to drop buckets
    /// for clients we haven't heard from in `idle_ttl`. Returns the
    /// number of buckets removed (diagnostic).
    pub async fn sweep(&self) -> usize {
        let now = Instant::now();
        let mut map = self.inner.lock().await;
        let before = map.len();
        map.retain(|_, b| now.saturating_duration_since(b.last_seen) < self.config.idle_ttl);
        before - map.len()
    }
}

/// Resolve the client IP from the request headers, with the
/// Cloudflare-first priority documented above. Falls back to
/// `0.0.0.0` if we can't extract anything sensible — that way
/// direct curl traffic still gets rate-limited as a single
/// "anonymous" client instead of bypassing entirely.
pub fn client_ip_from_headers(headers: &HeaderMap) -> IpAddr {
    if let Some(v) = headers.get("CF-Connecting-IP").and_then(|h| h.to_str().ok()) {
        if let Ok(ip) = v.trim().parse::<IpAddr>() {
            return ip;
        }
    }
    if let Some(v) = headers.get("X-Forwarded-For").and_then(|h| h.to_str().ok()) {
        // X-Forwarded-For is a comma-separated list; the leftmost
        // value is the original client.
        if let Some(first) = v.split(',').next() {
            if let Ok(ip) = first.trim().parse::<IpAddr>() {
                return ip;
            }
        }
    }
    "0.0.0.0".parse().expect("0.0.0.0 is a valid IpAddr")
}

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

    fn cfg_for_test() -> RateLimitConfig {
        RateLimitConfig {
            capacity: 3,
            refill_per_sec: 10.0,
            idle_ttl: Duration::from_secs(1),
        }
    }

    #[tokio::test]
    async fn within_capacity_succeeds() {
        let rl = RateLimiter::new(cfg_for_test());
        let ip: IpAddr = "1.2.3.4".parse().unwrap();
        for _ in 0..3 {
            assert!(rl.try_acquire(ip).await);
        }
    }

    #[tokio::test]
    async fn exhausting_capacity_fails_then_recovers() {
        let rl = RateLimiter::new(cfg_for_test());
        let ip: IpAddr = "1.2.3.4".parse().unwrap();
        for _ in 0..3 {
            assert!(rl.try_acquire(ip).await);
        }
        assert!(!rl.try_acquire(ip).await, "4th request should be rate-limited");
        // Refill rate is 10 tokens/sec → 1 token in 100ms.
        tokio::time::sleep(Duration::from_millis(150)).await;
        assert!(rl.try_acquire(ip).await);
    }

    #[tokio::test]
    async fn separate_ips_have_separate_buckets() {
        let rl = RateLimiter::new(cfg_for_test());
        let a: IpAddr = "1.2.3.4".parse().unwrap();
        let b: IpAddr = "5.6.7.8".parse().unwrap();
        for _ in 0..3 {
            assert!(rl.try_acquire(a).await);
        }
        assert!(rl.try_acquire(b).await, "different IP should still have full bucket");
    }

    #[test]
    fn cf_header_wins_over_xff() {
        let mut h = HeaderMap::new();
        h.insert("CF-Connecting-IP", "9.9.9.9".parse().unwrap());
        h.insert("X-Forwarded-For", "8.8.8.8, 7.7.7.7".parse().unwrap());
        assert_eq!(client_ip_from_headers(&h).to_string(), "9.9.9.9");
    }

    #[test]
    fn xff_first_hop() {
        let mut h = HeaderMap::new();
        h.insert("X-Forwarded-For", "8.8.8.8, 7.7.7.7".parse().unwrap());
        assert_eq!(client_ip_from_headers(&h).to_string(), "8.8.8.8");
    }

    #[test]
    fn fallback_when_no_headers() {
        let h = HeaderMap::new();
        assert_eq!(client_ip_from_headers(&h).to_string(), "0.0.0.0");
    }
}